Natural selection in nature and in the laboratory The action of selection is studied not only in laboratory experiments, but also in the course of long-term observations in nature. The first approach allows you to control the environmental conditions, highlighting from the countless real life

Natural selection I see no limit to the activity of this force, slowly and beautifully adapting each form to the most complex life relationships. C. Darwin Wasps, butterflies and Darwinism In the previous chapters we have repeatedly talked about natural selection. This and

9. Natural selection is the main driving force of evolution Remember! What types of selection do you know? Name the forms of natural selection known to you.

Natural selection - to be stronger than your animal nature It is especially important for us that this commandant forces the body to follow instincts with his strength. (Do not miss this moment!) That is, it is the commandant (his strength) that determines the animal principle in the body. And in terms of physics

A discussion of the phenomena and processes of variability and heredity shows that these factors are of great evolutionary importance. However, they are not leading. Natural selection plays a major role in evolution.

Hereditary variability, in itself, does not determine the "fate" of its carriers. As an example, we refer to the following facts. Arctic fox (Alopex) occurs in two hereditary forms. Some individuals have a hereditarily determined ability to acquire white wool by winter. Such foxes are called white. Other foxes do not have this ability. These are the so-called blue foxes.

It was shown that the second form dominates over the first in this quality, i.e. that the ability to turn white for the winter turns out to be a recessive property, and the preservation of a dark color in winter is dominant. These facts do not determine the evolution of the arctic fox.

In the conditions of the continental tundra and on the islands connected by ice to the mainland, the white fox dominates, constituting 96-97% of the total number. Blue foxes are relatively rare here. On the contrary, the blue fox dominates on the Commander Islands. The following explanation of these relationships was proposed (Paramonov, 1929). Within the mainland tundra, continuous snow cover prevails and food sources are very limited. Therefore, there is strong competition for food both between arctic foxes, and between the latter and other predators penetrating the tundra (fox, wolf, on the border with crooked forest - wolverine). Under these conditions, the white protective coloration gives clear advantages, which is the reason for the dominance of the white fox within the mainland tundra. There are other relations on the Commander Islands (Bering Sea), where the blue fox dominates. There is no continuous and so long-term snow cover here, food is plentiful, and interspecific competition is weaker. Obviously, these differences in environmental conditions also determine the numerical ratios between both forms of the Arctic fox, regardless of the dominance or recessiveness of their color. The evolution of the arctic fox is determined, therefore, not only by hereditary factors, but to a much greater extent by its relationship to the environment, that is, by the struggle for existence and, consequently, by natural selection. This factor, which is of decisive evolutionary importance, must be considered in more detail.

Struggle for existence

Natural selection is a complex factor that arises directly from the relationship between an organism and its biotic and abiotic environment. The form of these relations is based on the fact that the organism is an independent system, and the environment is another system. Both of these systems develop on the basis of completely different laws, and each organism has to deal with always fluctuating and changing environmental conditions. The rates of these fluctuations and changes are always much higher than the rates of changes in the organism, and the directions of changes in the environment and the variability of organisms are independent of each other.

Therefore, any organism always only relatively corresponds to the conditions of the environment, of which it is itself a component. From here arises that form of the relationship of organisms to the environment, which Darwin called the struggle for existence. The body really has to deal with physical and chemical environmental factors. Thus the struggle for existence is, as Engels pointed out, normal state and an inevitable sign of the existence of any living form.

However, what has been said above does not yet determine in any way the evolutionary significance of the struggle for existence, and the consequences of it that Darwin was interested in, namely, natural selection, do not follow from the relations described. If we imagine any single living form that exists under given conditions and struggles for existence with the physical and chemical factors of the environment, then no evolutionary consequences will follow from such relations. They arise only because in reality in a given environment there always exists a certain number of biologically unequal living forms.

Biological inequality, as already explained, stems from variability and its consequences - genotypic heterogeneity, why different individuals correspond to the environment to varying degrees. Therefore, the success of each of them in the struggle of life is different. This is where the inevitable death of the “less adapted” and the survival of the “more adapted” arise, and, consequently, the evolutionary improvement of living forms.

Thus, it is not the relationship of each individual living form with the environment that is of primary importance, but the success or failure in the life struggle in comparison with the success or failure in it of other individuals, who are always biologically unequal, i.e., having different chances of survival. Naturally, competition arises between individuals, as if “competition” in the struggle for life.

The main forms of the struggle for existence

Competition comes in two main forms.

We have to distinguish between indirect competition, when individuals do not directly fight among themselves, but with different success use the same means of subsistence or resist adverse conditions, and direct competition, when two forms actively collide with each other.

For clarification indirect struggle, we use the following example. Beketova (1896). Of the two hares, writes Beketov, pursued by a greyhound dog, the one that is faster and leaves the greyhound will win, but from the point of view of Darwinists, hares, running away from persecution, fought among themselves in the sense that they turned out to be biologically unequal in relation to another environmental factor - a pursuing predator. There was, therefore, indirect competition between them. The latter is a very common form of the struggle for existence.

Let's take another example. Bison have long lived in Belovezhskaya Pushcha. Subsequently, red deer were brought into the forests of Pushcha, which multiplied here in large numbers. Deer willingly eat the leaves and bark of young trees. As a result, they largely destroyed the deciduous young growth, and coniferous young growth appeared in the same places where this last one used to be. The general landscape of the Forest has thus changed. In places where deciduous forest used to grow, there was a lot of moisture, streams and springs; with the destruction of dense deciduous thickets, the amount of moisture, streams and springs decreased. Landscape change affected the general condition of the bison herd. Bison, firstly, have lost wood food, which they willingly eat. Secondly, the destruction of deciduous thickets deprived bison of comfortable shelters during calving and during the hot part of the day. Thirdly, the drying up of reservoirs has reduced the number of watering places. Therefore, the concentration of bison during watering in a few water bodies has led to a large spread of fascioliasis (Fasciola hepatica - hepatic dirot) and to more frequent death of animals, especially young animals. As a result of the described relationships, the population of the bison herd began to decrease (Kulagin, 1919). Bison were "defeated in the struggle for existence." It is quite obvious that the form of competition between deer and bison is indirect.

Somewhat different relationships are observed in cases straight competition, when, for example, one species actively crowds out another. For example, according to Formozov (Paramonov, 1929), the fox replaces the arctic fox everywhere on the Kola Peninsula. In Australia, the wild dog dingo displaces the local carnivorous marsupials. In Canada, the coyotes that have entered here are crowding out the foxes. Dergunov (1928) observed fierce competition during nesting due to hollows between kestrels, shelducks and jackdaws, and the kestrel displaced both of them. In the steppe zone of Europe and Asia, the saker falcon replaces the peregrine falcon, although there are suitable nesting grounds for the latter. Similar relationships are observed between plants. The author of these lines, together with S. N. Yaguzhinsky, carried out (at the Bolshevskaya Biological Station, near Moscow) the following experiment. The site, overgrown with wild cereals, was cleared and sown with seeds of cultivated plants. Approximately at a distance of 30 meters from this area there was a plot sown with clover. Not a single one remained on the trial site the next year. cultivated plant. However, the grass cover did not regenerate, despite the fact that the platform itself was carved into it. All of it turned out to be covered with clover, although the clover grew at a distance of 30 meters from it. Of course, seeds of both clover and cereals fell on the site, but clover replaced the cereals. A sharp clover square stood out against a green cereal background.

If we can thus distinguish between the two indicated forms of competition, then we should keep in mind that in a natural setting, direct and indirect competition are intertwined and their separation is conditional.

Even in classical examples of direct competition in life, elements of indirect competition are always woven into it, expressed in varying degrees of adaptation of competing forms to given environmental conditions. As an example confirming what has been said, consider the relationship between two species of rats - the pasyuk (Rattus norvegicus) and the black rat (Rattus rattus). At the beginning of the 18th century, the black rat dominated Europe. However, it seems that around 1827 pasyuk entered Europe and quickly spread within European Russia. Around 1730, pasyuk was brought by ship from the East Indies to England, and from there it penetrated the continent of Western Europe. Relationships between these species are usually determined by direct competition. Pasyuk actively displaces the black rat by attacking it. Its superiority, according to Brauner (1906), is determined by the following reasons.

1. Pasyuk is bigger and stronger. It is slightly taller and longer than a black rat. His legs are thicker, his back is wider. Pasyuk bones are stronger, muscle attachment points are more pronounced, which indicates greater muscle development.

2. Pasyuk swims well and stays on the water 3-4 times longer than a black rat.

3. Pasyuki are always the attacking side and are very aggressive, while the black rat only defends itself. There are known cases of Pasyukov attacks even on humans.

4. Pasyuks have a highly developed herd instinct, and in fights with a black rat they help each other, while black rats often fight alone.

Thus, a number of advantages determine the outcome of the struggle, which, as can be seen from the foregoing, has the character of direct competition between these species. As a result, the distribution area of ​​the black rat was greatly reduced and broke up within the European part of the USSR into four isolated areas (Kuznetsov). This reduction and fragmentation of the range is evidence of the depressive state of the species.

However, these relationships cannot be generalized. Thus, according to Brauner (1906) and Gamaleya (1903), the following ratios were found in the port of Odessa: 93.3% accounted for 24,116 burned pasyukov rats, Indian (black subspecies) - only 3 specimens. However, on the ships of foreign navigation and Caucasian, which arrived in the port of Odessa, the relationship was different: out of 735 pieces of Egyptian (black) - 76%; typical blacks - 15.5%, red (black subspecies) - 55 pieces, pasyukov - only two specimens. Pasyuki, Gamaleya points out, were only in the port of Odessa. Brauner points out that in Egypt, the pasyuk does not seem to supplant the black rat (its variety, that is, the Egyptian rat) as easily as in Europe. Indeed, on the North African coast, for example, both species are present, and the data on rats on steamboats (see above) positively indicate that competition between the two species on African coasts has a different outcome. Even Trussard (1905) reported that on the African coast the black rat penetrates south into the desert zone, where there are no pasyuki. Thus, if in Europe pasyuki dominate, then in Africa the relationship is different.

These facts show that the outcome of competition is not determined only by the physical advantages of one species over another, and that it also depends on other factors - adaptability to the environment in the broad sense of the word. Thus, indirect and direct competition, as a rule, are intertwined into one whole and can differ quite conditionally.

Here it must be emphasized that in the struggle for existence, undoubtedly, the "Malthusian" factor, i.e., overpopulation, also has a completely definite significance. Although not the main factor, overpopulation makes the struggle for existence more intense. Its intensity increases sharply. This proposition can be easily proved by the following facts. If, for example, a species enters new habitats, or is brought here by man, then in a number of cases it is observed that it begins to multiply vigorously and rapidly increase in numbers. Observations show that these phenomena are associated with the absence of competitors and enemies in new habitats, which reduced the abundance of this species in its former habitat.

As we can see, the indirect and direct struggle for existence are intertwined into a complex whole. Therefore, the vulgar understanding of it as a direct struggle in the form of direct physical fights between organisms is the furthest away from the true meaning of this term. On the contrary, the struggle for existence must be understood in the broadest sense, i.e., as a form of direct and indirect relationship of each specific organism to the biotic and abiotic factors of the environment, arising as a result of the relativity of the adaptability of any living form to any conditions and components of the environment, as well as due to overpopulation and competition, which determines the extermination of the unfit and the survival of the fit.

Complicated relationships in the struggle for existence

The examples above dealt with direct relationships between two species. In reality, these relationships are much more complicated. Any species lives in a certain area, which, first of all, has a certain physico-chemical, climatic and landscape characteristics. The average temperatures prevailing in the area, the amount of precipitation, the number of clear days per year, the nature and degree of insolation, the prevailing winds, the chemical composition of the soil, its physical structure, color and shape earth's surface, its relief, the absence or richness of water basins - all these and other factors, taken together, are part of the characteristics of a particular type of habitat, or station.

Stations are, for example, solonchak steppe, feather grass steppe, rocky desert, sandy desert, forest-steppe, deciduous forest, mixed (taiga) forest, coniferous forest, tundra. For small aquatic or even microscopic organisms, the stations will be, for example: shell sand, elodea thickets, zoster thickets, bottom detritus, muddy bottom, open water spaces, the surface of underwater rocks, etc.

These examples already show that stations are formed under the influence not only of physicochemical factors, but that organisms also participate in their formation (for example, the station of a deciduous forest). But even animal organisms impose their stamp on stations, and their activity also determines its character. All organisms inhabiting a given station are in complex relationships and are adapted to its conditions.

The totality of life forms of a given station, which are in interdependent and interdependent relationships, constitutes the historically established ecological system of life forms (species) or biocenosis.

The figure shows the complex "food chains" that link the life forms of the prairie biocenosis. Arrows go from prey to predator. A change in the number of one of the life forms entails a number of changes in the biocenosis. If the wolves, for example, exterminated the bison, then they begin to eat mice, becoming competitors of the coyote, which switches to predominant feeding on ground squirrels. A decrease in the number of ground squirrels leads to an increase in the number of insects - a factor that affects vegetation, and at the same time favorable for insectivorous forms, etc.

From what has been said, it is clear that life forms, directly or indirectly, are affected by changes in the biocenosis. It is easy to understand that the loss of one of the members of the biocenosis can lead to radical changes in it. In fact, this is how it happens. The biocenosis in time changes in its composition and develops into a new biocenosis. This change in the composition of the biocenosis is called succession. Succession perfectly shows the presence of a struggle for existence in the biocenosis and its influence on the species composition.

Let's look at some examples. The systematic driving of livestock to certain pastures leads to the development of slaughter. In the cereal steppe, its first stage is the destruction of the dead plant litter, which accumulates from year to year, and the exposure of the soil. Such bald spots are occupied by annual plants of an alien element. Due to the deterioration of the water permeability of the soil compacted by slaughter, the growth of grasses is reduced. At the second stage, feather grasses and tyrsy noticeably decrease in number, fescue is temporarily retained, and wormwood, chamomile and bulbous thin-legged become the predominant forms. Later, feather grasses and tyrsy completely disappear, fescue decreases in number, and dominance passes to sagebrush, etc. In general, tough grassy vegetation is replaced by more juicy semi-desert dry-lovers. This change favors the steppe rodents, whose numbers are increasing in the slaughter zones. On the other hand, slaughter affects the entomofauna (insects). Geophilic (soil-loving) forms typical of desert stations appear, for example, the steppe conic is replaced by prusik, etc. (Formozov). As you can see, under the influence of one factor - cattle slaughter - a complete succession occurred, the entire composition of the biocenosis changed. The new hydrological regime of the soil made the former plant forms unsuitable for the new conditions, and other forms took their place, which entailed a number of changes in the fauna. Some forms replace others.

A remarkable feature of these relations is the fact that a certain biocenosis, developing, prepares to replace it with others. So, for example, the deposition of plant residues on a grassy swamp leads to an increase in the surface of the swamp. Instead of a hollow, a convex relief is formed. The inflow of water decreases, and on the site of a grassy (sedge) swamp, sphagnum develops with sparse higher vegetation represented by swamp sheuchzeria (Scheuchzeria palustris). This complex (Sphagnum + Scheuchzeria) becomes denser and favorable conditions are created for the attachment of the third form - cotton grass (Eriophorum yaginatum). At the same time, the sphagnum cover turns out to be represented by another species (instead of Sph. medium - Sph. Inseni). The continuing rise of the sphagnum carpet favors the emergence of pine. Thus, each biocenosis prepares its own death (Sukachev, 1922).

The phenomenon of successions demonstrates the phenomenon of the struggle for existence in the biocenosis.

Fluctuations in the number of species, as a manifestation of the struggle for existence

Another important fact that testifies to the struggle for existence is the fluctuations in the number of the species in annual cycles.

This fact has been studied in relation to a number of forms - harmful rodents, commercial fauna, etc.

The figure shows that years of numerical depression are replaced by years of numerical growth, and fluctuations in numbers are approximately rhythmic. Let us consider this phenomenon of "waves of life", which is closely connected with the struggle for existence.

a. Reasons for the rhythmic fluctuations in numbers. It was found that the rhythm of numerical fluctuations is different for different species. For example, for mouse-like rodents, it is on average ten years (Vinogradov, 1934), for arctic foxes, 2-4 years, for squirrels, in the northern forests of Eurasia and America, 8-11 years, etc. Years of numerical depression are followed by years of upsurge . Obviously, the reasons for the nature of rhythm depend in part on the specific features of each biological species. Thus, S. A. Severtsov (1941) indicates that each species is characterized by a certain typical coefficient of individual mortality. Since the fecundity of each species is, on average, typical for it, a specific curve of increase in abundance arises from this. The lower the growth rate of producers, the slower the increase in numbers goes (Severtsov, 1941). Consequently, the increase in numbers (reproduction) occurs in relation to each species to a certain extent naturally. It lasts for some time, during which the population density of the species gradually increases, and the maximum of this density, again for different forms different. So, for mice it is 5 mil. pieces per sq. a mile, and for hares 1000 per sq. mile (Severtsov, 1941). Upon reaching a higher population density, the action of a number of unfavorable eliminating factors arises. At the same time, different forms have a different combination of eliminating factors that most affect them. For rodents, epizootics arising from close contact between individuals during mass reproduction are of the greatest importance. In ungulates, epizootics and climatic depressions are of great importance. However, for bison, for example, the deterioration of climatic conditions has little effect (resistance against them), and epizootics, on the contrary, have a great elimination value. On the contrary, wild boars do not suffer from epizootics, etc. (Severtsov, 1941). Consequently, species specificity is clearly visible from this side, as the reason for the rhythmicity of oscillations. The foregoing is also confirmed by the fact that in omnivorous forms (euryphages) the rhythm of population fluctuations is less pronounced than in forms attached to monotonous food (stenophages). For example, in omnivorous foxes, the variability of food conditions does not cause sharp fluctuations in numbers (Naumov, 1938). On the contrary, seed yield is essential for squirrels. coniferous trees(Formozov, Naumov and Kiris, 1934), and fluctuations in its numbers are significant.

Finally, we point out that each species has a specific biotic potential, by which Chapman (1928) understands the hereditarily determined degree of resistance of the species in the struggle for existence, determined by the potential for reproduction and the potential for survival in fluctuating environmental conditions.

Thus, for each species, there is certainly an approximately correct rhythm of numerical fluctuations, determined by its biotic potential.

However, the importance of this factor should not be overestimated. The "internal" causes of the rhythmicity of numerical fluctuations, appearing when different species are compared, are covered by "external" causes, i.e., ecological conditions within each individual species. For example, in foxes living in the forest, fluctuations in numbers are not great, but in the steppe and desert regions they are more noticeable (Naumov, 1938). For the squirrel, the rhythm of numerical fluctuations in the conditions of the northern forests of Eurasia and America, as indicated, is 8-11 years, in the middle latitudes 7 years, and in the southern parts of its range - 5 years (Naumov, 1938).

These data prove that under different conditions the struggle for existence has a different intensity and that it is not determined only by the "internal" features of the species. For insects, it was generally not possible to establish the correct rhythms of numerical fluctuations, as can be seen from the following data for the outskirts of Moscow (Kulagin, 1932).

Ultimately, the question in all cases is covered by the relationship between the species and the environment.

b. Elements of the biotic potential of a species. As has been pointed out, the biotic potential of a species is a complex whole, consisting of the potential for reproduction and the potential for survival. Let us consider these elements of the biotic potential separately.

Breeding potential primarily depends on the fertility of the species. The latter is determined by the number of cubs in a litter and the number of litters per year. These factors lead to a huge increase in the number of offspring. For example, the breeding rate of a sparrow is such that, assuming that all the offspring survived, one pair of sparrows in ten years would give a population of 257,716,983,696 individuals. The offspring of one pair of fruit flies, producing an average of 30 clutches of 40 eggs each year, would cover the entire earth in one year with a layer of a million miles thick. Under the same conditions, one individual of the hop aphid would give offspring in the summer of 1022 individuals. One female gamma cutworm can theoretically produce 125,000 caterpillars over the summer, etc.

However, the breeding potential of a species depends not only on fertility. Of great importance is also the age of the first fruiting of the female. As S. A. Severtsov (1941) points out, with an equal number of cubs, a species in which females reach sexual maturity at an earlier age and in which the period between two births is shorter will multiply faster.

Further, the life span of individuals of a species is of great importance, a value, on average, specific to each species (S. A. Severtsov, 1941). Without dwelling on this issue in detail, we will only point out that species with very low fecundity can nevertheless have a high breeding potential if they are characterized by long individual life spans. A classic example of this genus would be Darwin's references to the reproduction of elephants. Despite the exceptional slowness of their reproduction, theoretical calculations show "that in the period 740-750 years, about nineteen million living elephants could have been obtained from one pair" (Darwin). Finally, it should be emphasized that the potential for reproduction also depends on the conditions for the development of offspring and, in particular, on the forms of care for offspring. Without dwelling on the description of the phenomenon itself, which has a very different character in different groups of animals, we only emphasize that care for offspring increases the potential for reproduction. Therefore, as a rule, in forms with low fecundity, a strong development of adaptations to protect offspring is observed. And vice versa, the absence or weak expression of such adaptations, as a rule, is compensated by high fecundity. Thus, the breeding potential is determined by a number of factors: fecundity, the number of litters per year, life expectancy, and adaptations to protect offspring.

Survival potential is a value of a different order and is determined by the degree of adaptation of individuals of the species to the conditions of their habitat. This fitness, as we already know, is relative, why numerous environmental factors affect the population of the species in an eliminating (exterminating) way, moderating the effect of the breeding potential. What exactly are the factors that moderate reproduction? Let's briefly dwell on them.

Of great importance are, first of all, climatic factors especially temperature and rainfall. For each species, there is a certain optimum of climatic factors, under which the survival rate increases, and the number of the species increases in accordance with its breeding potential. Naturally, in years close to optimal conditions, the curve of the "wave of life" rises, and vice versa - deviations from the optimum, in one direction or another, reduce reproduction. Let's give some examples.

In the winter of 1928, in the vicinity of Leningrad, mass freezing of the wintering pupae of the cabbage white was observed, and in the winter of 1924/25, the caterpillars of the winter scoop. It has been experimentally established that, for example, rearing pupae of a winter dog at T° +22.5° C increases the fecundity of butterflies hatched from them to a maximum (1500-2000 eggs). However, fluctuations to one side or the other from this optimum reduce fertility. So, at T° = +10-12°C, the fecundity of butterflies drops to 50%. In warm-blooded animals, in view of their ability to regulate heat, the temperature factor has a lesser effect. However, temperature changes still affect, for example, the rate of development of the gonads. An increase in T° to a certain limit accelerates the formation of the sex glands, however, its further increase has an inhibitory effect.

Climatic factors affect not only fertility, but also the number of individuals of the species. For example, in very severe winters, an increase in the percentage of death of animals is observed. Interesting data on the death of birds in the harsh winter of 1939/1940 are reported by Dementiev and Shimbireva (1941). Gray partridges, for example, have almost completely died out in places, or have sharply decreased in numbers. Observed mass death coots, many waterfowl, owls (in Ukraine), sparrows, bullfinches, tap dances, siskins, crossbills, etc.

The eliminating effect of climatic factors is thus dual in nature (direct and indirect), affecting, for example, nutrition (amount of food) and resistance to diseases (weakening of the body).

Next to the climatic one should put soil or edaphic factors. In dry years, the soil is more or less deprived of moisture, and this phenomenon has a moderating effect on the reproduction of many insects, the larval stages of which are biologically associated with the soil. Freezing of the soil in winter also destroys many forms.

The predator has a great moderating effect on reproduction. In some cases, it is almost decisive. For example, the vedalia ladybug (Vedalia cardinalis) cuts off the reproduction of mealybugs from the genus Icerya with great speed due to the voracity of both the larvae of this beetle and the adult form. One vedalia larva can destroy over 200 mealybug larvae in its lifetime. Some ground beetles are also a powerful exterminating factor. Observations on the ground beetle Carabus nemoralis showed the amazing voracity of this predatory beetle. For example, one female, at the time of capture, weighed 550 mg, and after 2.5 hours of food had a weight equal to 1005 mg, and her abdomen was swollen and protruded from under the elytra. Insect reproduction is also moderated by birds and mammals. Insectivorous birds are of great importance in this regard. In one forestry, it was found that tits destroyed up to 74% of all wintering caterpillars of goldentail butterflies during the winter. The destruction of mouse-like rodents by birds of prey and mammals is also significant. Therefore, for example, the destruction of the steppe polecat (Putorius eversmanni) causes an increase in the number of rodents.

In places of concentration of rodents, predators are also concentrated, contributing to a decrease in the number of the former. These relationships are characterized by an interesting feature. The first to be destroyed are rodents living in more open habitats. In habitats that are most conducive to experience, the death of rodents is less, and they are not destroyed by predators. Such "experiencing stations" (Naumov, 1939) play the role of natural reserves, within which rodents are relatively inaccessible to predators. The number of predators begins to fall, and the number of rodents begins to increase in accordance with their specific breeding potential.

In general, the dependencies here resemble the relationships shown in the figure. An increase in the number of prey causes an increase in the number of predators, and a subsequent decrease in the number of prey also reduces the number of predators. For individual species, however, very complex numerical relationships are observed, which we will analyze here in the most concise terms.

The outcome of the eliminating activity of a predator depends on the characteristics of the prey, specific features of the predator, and environmental conditions. In difficult conditions of the biocenosis, the problem is solved with great difficulty. Gauze in a number of works took the path of dividing the problem. Choosing, as an object, ciliates, Gauze artificially created a limited "microcosm", consisting, for example, of two species - predator and prey. Two ciliates were taken - Paramaecium caudatum (prey) and Didinium nasutum (predator). Didinium is a fast swimmer (faster than Paramecia) and sucks out its prey. Therefore, in a homogeneous "microcosm", that is, in culture medium without "shelters", the predator, in the end, completely destroys the paramecium and dies itself. Completely different results were obtained in a heterogeneous "microcosm" (its role was played by a test tube containing 0.5 cm 3 of the nutrient mixture, in which the paramecia were partially hidden). In this case, the outcome was different. Sometimes the predator died, and the prey multiplied. However, if new numbers of ciliates were periodically introduced into the microcosm, then periodic “waves of life” arose, during which an increase in the number of prey caused a subsequent increase in the number of a predator, and a quantitative depression first caused a decrease in the population of a predator.

Thus, environmental conditions significantly affect the result of the described relationships.

Now let's move on to the properties of the predator. If a predator has powerful means of attack (like Didinium), then its effect on the prey population is sharper, and in a certain territory the predator, under certain conditions, can completely exterminate the prey, or create incentives for the prey to move (if it has the appropriate morpho- physiological organizational capabilities) to another habitat. If, however, the prey is well protected, able to resist, runs fast, or breeds intensively, and the predator possesses relatively weak means of attack, then the phenomenon is limited to the periodic fluctuations indicated above. In a natural setting, different relationships can be observed and, therefore, on average, the role of a predator has a noticeable evolutionary significance. The dependence of predators of euryphages and stenophages on fluctuations in prey is, of course, different.

Of great importance feeding regime. Years or periods of nutritional deficiencies sharply reduce the resistance of individuals of a given species to all the elimination factors listed above. Starvation entails a decrease in activity, a decrease in defensive instincts, a weakening of resistance against infections, a decrease in fertility, etc. For example, squirrel, in years of food abundance, gives 2-3 litters of 4-5 baby squirrels in each, its barrenness does not exceed 5 -ten%. In the years of famine, barrenness reaches 20-25%, the number of litters is on average 1-5, the number of young 2-3 baby squirrels. During the years of strong breeding of lemmings, the latter, under the influence of a lack of food, rush in large masses to new habitats. Many animals die when trying to overcome water barriers, and mainly from attacks by predators. Lemmings are followed by snowy owls, foxes, arctic foxes and hungry reindeer. After such wanderings, the number of animals decreases sharply.

Thus, each species constantly experiences the elimination pressure of biotic and abiotic environmental factors. All of the factors listed above act together as a system of factors. Some of them in a given year are close to the optimum for a given species, while others, on the contrary, have an eliminating effect. Combinations of specific factors (such as temperature and humidity) also have a huge impact on the body. As a rule, it is combinations of various environmental factors that act.

The survival potential under these conditions is determined by two reasons. First, it depends on the state of the leading factors for this type. If, for example, for a given species, temperature and humidity are of the greatest importance, and the state of these factors is optimal, then the known unfavorability of the remaining factors will affect the abundance of the species to a lesser extent.

However, the degree of resistance of the species to eliminating environmental factors is of decisive importance. The resistance of a species is determined by its ecological valency, which is understood as the scope of its ability to adapt to changing environmental conditions. The valency can be wide, and such species are called euryadaptive, or relatively narrow (stenadaptive species). However, no matter how wide the valence is, it is never, as a rule, equivalent in relation to all eliminating factors. A species, for example, may have a wide ecological valence in relation to temperature fluctuations (eurythermal species), but be highly specialized in terms of feeding regime (stenophages), or be stenothermic, but at the same time euryphage, etc. In addition, and euryadaptation has its limits. For example, pasyuk is a typical example of a euryadaptive form, however, as we have seen, its ecological valence has its own definite limits.

In any case, the degree of euryadaptation, in relation to this environmental factor and to all factors of the station and biocenosis as a whole, is the basis for characterizing the survival potential of a species, and the survival potential, on average, is directly proportional to the ecological valency of the species.

Let us give some explanatory examples. In years with a reduced feeding regime, the survival potential of euryphages is higher than that of stenophages. Some predators, with a lack of one type of food, switch to another, which allows them to avoid difficult conditions. The omnivorousness of a number of species of insects allows them to stay alive with a lack of certain plants. Stenophagi die under these conditions. Therefore, for example, the fight against harmful insects or nematodes - euryphages, as a rule, is more difficult than with stenophages.

So, the biotic potential of a species, its vitality, is a certain resultant of two quantities - the potential for reproduction and the potential for survival, which in turn is determined by the degree of ecological valence of the species. Under the influence of the combination of the above elimination factors, the number of adults of a given generation is always less than the number of newborns. This fact has been relatively well studied through quantitative analysis of the dynamics of the number of offspring born in a given year and its further fate. As a rule (as Darwin pointed out), there is a high mortality among young individuals, which leads to a rapid decrease in the number of offspring. Analyzing the composition of the species population by age and calculating the percentage of each age group to the total number of individuals (this can be done, in particular, in relation to game animals and birds), it can be established that the decrease in the number always obeys a certain curve. For example, the figure shows the decrease in the number of offspring of a squirrel. As can be seen, in the first year of life, mortality is high, then its rate falls and the mortality of adult forms becomes less intense.

Similar curves may already be drawn for a very large number of species. The same figure shows the dynamics of the number of spruce ages. It is easy to see the similarity of these curves, despite the profound differences between biological objects (squirrel and spruce). It is obvious that we are dealing here with a common cause. This latter is the struggle for existence, to which all biological objects are equally subject. The curves show that the struggle for existence has a completely obvious eliminating meaning: some of the individuals die. Thus, the struggle for existence is a natural eliminating factor that determines the extermination of the less adapted and the residual survival of the more adapted.

Elimination types

It is important to find out what is the evolutionary significance of the elimination action of the struggle for existence. If some individuals die and others survive, then the question arises as to what determines this difference.

The answer to this question will become clear if we take into account the nature of elimination, its types, which we will now consider.

a) Individual non-selective (random) elimination concerns individual individuals. Such, for example, is the death of a swallow on the tenacious thorns of a burdock. This death is accidental and is rarely observed (a similar case is described for one bat). However, there are quite a lot of such cases in the life of plants and animals, and they can have a certain significance, for example, during the nesting period, when the accidental death of a lactating female entails the death of all her offspring. Theoretically, one can imagine that any mutant, and hence its offspring, can die in this way.

b) Group indiscriminate (random) elimination it no longer concerns individual individuals, but a group of individuals and is determined by the more widespread action of some random destructive factor, for example, a limited forest fire, a local winter flood, a mountain landslide, a sudden local frost (especially after rain), washing away parts of animals or plants with streams showers, etc., etc. In such cases, both the “adapted” and the “unadapted” perish. In this case, death can affect groups of individuals of a certain genotypic composition. For example, if a mutant has arisen that has not had time to multiply in large quantities, slowly spreads and has a small area (center) of distribution, then random group elimination can cover the entire individual composition of the mutant's offspring. Regardless of the relative usefulness or harmfulness of a given mutation, all its carriers can be destroyed. Thus, random group elimination in such cases can affect the genetic composition of the species, although it still does not have a leading evolutionary significance.

in) catastrophic indiscriminate elimination arises with an even wider distribution of destructive factors, for example, unusual frost, floods, forest fires that have engulfed large areas, exceptional drought, lava flows and other disasters that have spread over vast areas. And in this case, both "adapted" and "unadapted" perish. Nevertheless, this form of elimination can be of great evolutionary importance, influencing the genetic composition of the species even more effectively and exerting a powerful influence on entire biocenoses.

Naumov (1939) observed that as a result of showers in the steppe part of southern Ukraine, rodent burrows were flooded, which led to a sharp decrease in the number of voles. At the same time, the local population of the Kurgan mouse did not noticeably change. This is explained by the greater mobility of mice compared to voles. During the spring snowmelt, rodent burrows are closed with ice plugs, and voles die of starvation, while mice survive, as they store food in underground chambers. The selected example shows the effect of biological inequality of two kinds in relation to the same external factor. Obviously, such relationships can result in the evolution of biocenoses (succession) and a change in the species composition of individual genera, families, etc.

An example of catastrophic elimination is the mass death of desman during winter floods or the death of gray partridges in the harsh winter of 1839/40, etc. The main sign of catastrophic elimination is the mass destruction of individuals of the species, regardless of their survival potential.

G) Total (general) elimination. This form of elimination should also be singled out, under the conditions of which the entire population of the species perishes, i.e., all the individuals included in its composition. This form of elimination is also indiscriminate. It is possible in those cases when the range of the species is small, or when the latter is completely covered by the influence of any unfavorable factors. Probably, total elimination was, for example, the cause of the death of a mammoth in Siberia. It is easy to imagine that total elimination can lead to the death of the entire population of some endemic, occupying, for example, one mountain peak or a small island, completely covered by some kind of natural disaster, etc.

From what has been said about total elimination, it is clear that an absolute distinction between the enumerated forms of indiscriminate elimination is impossible. Much is determined, as we see, by the number of species, the number of individuals included in its composition. Elimination, which has a group value for some species, will be total for others. Much is also determined by the properties of those living forms that have been exposed to these eliminating factors. For example, a limited wildfire would be detrimental to plants, while animals could get away from it. However, the animal population is not equal in this regard. Very small soil forms living in the forest floor will die in large numbers. The same will happen with many insects, for example, forest ants, many beetles, etc. Many amphibians will die, for example, toads, grass frogs, viviparous lizards, etc. - in general, all those forms whose retreat rate is lower than the rate of fire spread . Mammals and birds will in most cases be able to get away. However, here too much is determined by the stage of individual development. There will not be much difference between a beetle egg and a bird egg, a butterfly caterpillar and a chick. In all cases, of course, the forms that are in the early stages of individual development suffer the most.

e) Selective elimination has the greatest evolutionary significance, since in this case the main effect of the struggle for existence is ensured, i.e., the death of the least adapted and the survival of the most adapted. Selective elimination is based on the genetic heterogeneity of individuals or their groups, and, consequently, on the nature of modifications and the resulting biological inequality of various forms. It is in this case that the natural perfection and progressive evolution of the species occurs.

Intraspecific and interspecific struggle for existence

Selective elimination is the most characteristic moment of the struggle for existence, its actual expression. Through the selective elimination of unsatisfactory forms, the residual preservation of the fittest individuals or groups of individuals is achieved.

The question arises, within which particular groups of individuals does selective elimination have the greatest evolutionary significance? Darwin pointed out that this question is connected with the question of the intensity of the struggle for existence. He attached the greatest importance to the intraspecific struggle for existence. The sharpest competition between forms occurs within the same species, since the needs of individuals of the same species are closest to each other, and, consequently, the competition between them is much more pronounced.

We already know that individuals of the same species are biologically unequal, that is, they have different chances of resistance to destructive environmental factors. This biological inequality is obviously expressed in the fact that different individuals have some differences in biotic potential.

Further, we know that there is indirect and direct competition between individuals, and that (according to Darwin) it is the more intense, the closer the competing individuals are to each other in terms of their needs. From this it is obvious that each individual of a species has, so to speak, a double life “load”: a) it resists, to the extent of its biotic potential, eliminating environmental factors and b) competes mainly for food and space with other individuals of the species. It is equally obvious that the struggle against eliminating factors is the more intense, the more intense the competition with other individuals of the species. After all, this competition is, as it were, an "additional burden" that aggravates the struggle for existence. From what has been said, it becomes clear that, in total, the struggle for existence is especially intense between individuals with close vital interests, i.e., individuals characterized by the same ecological niche.

A niche is understood as a complex of material environmental conditions within which individuals a) turn out to be the most adapted, b) extract food resources, and c) have the opportunity to reproduce most intensively. More precisely, a niche is a complex of material environmental conditions in which the biotic potential of a species is most fully expressed.

For example, for the red bug, its niche is the soil. The bug feeds on the corpses of insects, sucking out their juices with the help of its proboscis. The soil serves as a source of moisture for it. The author often observed that the red bug plunges its proboscis into the ground and sucks water. Vegetation cover serves as a refuge for him. Reproduction also takes place on earth. Females make small burrows in the soil where eggs are laid. Attachment to the soil, as a niche, also caused changes in the organization of the red bug. The rear (flying) pair of wings has been turned into rudiments. Consequently, attachment to the soil led to the loss of the ability to fly. Another good example- muskrat niche. All its vital needs, and above all the necessary abundant food, are satisfied in the floodplain basins and backwaters of the rivers. It is remarkable that reproduction is also associated with the water element. The author repeatedly observed the "games" of desmans in the water, and under the conditions of a specially arranged vivarium, attempts to coitus made in the water (Paramonov, 1932). Thus, the water mass of floodplain lakes and backwaters, rich in vegetation and other food resources, becomes a desman niche, to which it is adapted in all the leading features of its morphophysiological organization. That is why the burrows of desmans, as a rule, have the only way out - into the water.

Since individuals of the same species, as a rule, are characterized by the same or qualitatively similar niches, it is the intraspecific struggle for existence that is most intense. Thus, Darwin quite correctly singled out intraspecific struggle as an independent category of competitive relationships between organisms. Let us consider some examples of intraspecific struggle for existence, established both by field observations and experimental studies. Let us recall the relationship between the white and blue fox described above (an indirect intraspecific struggle for existence). In the conditions of the mainland tundra, white fox prevails, and in the conditions of the Commander Islands - blue. Another example is the relationship between the typical and melanistic forms of the moth moth. The typical light-winged form (Amphidasis betularia) at first dominated, but in the 1960s the dark-winged form (A. b. doubledayria) began to multiply vigorously in England (in the vicinity of Manchester). The latter supplanted the typical (light-winged) first in England, and then (in the 80s) the same process became widespread in Western Europe. Dementiev (1940) refers to the following examples. The blue goose (Anser coerulescens) has been supplanted by the white mutant in most of its range. On the island of St. Vincent (Antilles group of islands), a melanistic mutant of the sunbird Coereba saccharina arose. In 1878 the mutant became numerically predominant, in 1903 the typical form was found in only one copy, etc.

Experimental data also confirm the existence of an intraspecific struggle for existence. An example is the excellent study by Sukachev (1923) on the mortality of various intraspecific genetic forms of the common dandelion (Taraxacum officinale). In the plots, dandelion was sown as part of three hereditary forms, conditionally designated as A, B and C. The crops were mixed and clean, in conditions of a rare and dense planting. The percentage of mortality in different conditions was investigated, as can be seen from the table.

Consider the data of these tables.

The table shows that the various intraspecific forms are differentiated in terms of survival potential. Moreover, it is stated here that under different conditions, the survival potential also changes. So, in a rare pure culture, mortality increases in row C-A-B, in a thick pure - B-A-C, in a rare mixed and in a thick mixed C-A-B.

The table shows that forms A, B and C have different breeding potential. Consequently, it is quite obvious that within a species there is differentiation in terms of the degree of reproduction potential. For example, under conditions of mixed cultures, form C has the highest potential for reproduction, form A has the lowest.

Finally, the data from both tables show that dense crops have higher mortality, while rare crops have lower mortality. Fertility changes in the same way. Sukachev's data indicate that the biotic potential of intraspecific forms is not the same and that, consequently, the population of the species does indeed consist of biologically unequal groups. The presented material also shows that within the species there is a struggle for existence, resulting in selective elimination, in the course of which the forms that have the smallest, under given conditions, biotic potential, i.e., the least adapted to them, are destroyed. Finally, Sukachev's data emphasize that the survival of the fittest (having the highest biotic potential) occurs not through their selection, but through the extermination of the least fit.

Interspecies struggle for existence can also be quite intense. Some examples of it have been given above. In a number of cases, namely, if the interests of the species are mutually close, the intensity of the interspecific struggle is no less great than the intraspecific one. For example, very intense competition is observed between two types of crayfish - eastern narrow-toed (Astacus leptodactylus) and broad-toed (A. astacus), with the first replacing the second.

Even between species of different systematic groups, competition is very high. For example, Zakarian (1930) observed that the plant petrosimonia (P. brachiata) tends to outcompete other species growing in the same experimental plots. So, in one observation, in the month of March, petrosimonia and two more species grew on the same site - Salsoda crassa and Sueda splendens. It was counted: 64 individuals of petrosionia, 126 - S. crassa and 21 - S. splendens. By autumn, only the petrosimony remained. Thus, in the conditions of one and the same station, intense competition occurs between species. Only when species are profoundly different in their needs does competition between them weaken. Then the law (Darwin) about the greatest amount of life with the greatest variety comes into effect.

It should be kept in mind that "interspecific struggle" is not always necessarily less intense than "intraspecific". The intensity of competition is determined by many factors, and above all by the degree of proximity of the occupied niches. If two species occupy the same niche, then the competition between them will be in the nature of "intraspecific struggle". Gause (1935) investigated a similar case. Two ciliates Paramaecium aurelia and Glaucoma scintillans were introduced into the "microcosm". If P. aurelia is reared separately, then the number of individuals grows to some saturating level. The same thing happens in an isolated glaucoma culture. If both ciliates feed in the microcosm, glaucoma, which has a high rate of reproduction, manages to capture all the food resources by the time when the paramecium is just beginning to grow in numbers, and as a result, the latter is completely replaced. Similar results occur in a culture containing two species of paramecia, and P. aurelia completely displaces another species that uses food resources less productively - P. caudatum. However, a complication arises here, consisting in the fact that the advantages of one species over another, as already indicated above (for relationships between rats), depend on environmental conditions. In Gause's experiments, it turned out that if the microcosm contains the products of vital activity of microorganisms living in it, then P. aurelia wins; if the microcosm is flushed with pure saline, P. caudatum may displace P. aurelia.

Now let's move on to species with different niches. Two paramecia were placed in the microcosm - P. aurelia and P. bursaria. The second species has a dark color, depending on the symbiotic algae living in its plasma. The algae release (in the light) oxygen, and this makes P. bursaria less dependent on environmental oxygen. It can freely exist at the bottom of the test tube, where the settling yeast cells accumulate. Infusoria feed on them. R. aurelia is more oxygen-loving (oxyphilic) and stays in the upper parts of the test tube. Both forms consume both yeast and bacteria, but the former are used more efficiently by P. bursaria and the latter by P. aurelia. Thus, their niches do not match. The figure shows that under these conditions permanent coexistence of both species is possible. Thus, as we see, the experimental data confirm Darwin's position on the decrease in the intensity of competition with a divergence of interests (divergence of characters), and thus the usefulness of divergence.

Classical examples of the struggle for existence are the relationships that arise between different types of trees in a forest. In the forest, competition between trees is easily observed, during which some individuals find themselves in a privileged position, while others are at different levels of oppression.

In forestry, there are: 1) exclusively dominant trunks (I), 2) dominant ones with a less well-developed crown (II), 3) dominant ones, the crowns of which are in the initial stages of degeneration (III), 4) oppressed trunks (IV), 5) moribund and dying trunks (V). Different types of trees in different conditions of existence clearly crowd out each other. So, in Denmark, the displacement of birch by beech was traced. Pure birch forests have survived only in desert and sandy areas, but where the soil is in any way suitable for beech, it drowns out the birch. In this state, she can live for a long time, but ultimately dies, since her beech is long-lived, and its crown is more powerful. In addition, beech gives undergrowth under the crowns of birch, while the latter cannot grow under the canopy of beech.

Natural selection

From the struggle for existence follows, as a consequence of it, natural selection. Darwin was unable to rely on direct observations directly confirming the operation of natural selection. To illustrate it, he used, as he himself pointed out, "imaginary" examples. True, these examples breathe life itself. However, they were not strong evidence for natural selection. Subsequently, the situation changed, and little by little works began to appear in which the facts of natural selection were substantiated.

The facts testifying in favor of the theory of natural selection can be divided into two groups: indirect evidence of natural selection and direct evidence.

Indirect evidence of natural selection. This includes groups of facts that receive their most satisfactory or even the only explanation on the basis of the theory of natural selection. From a large number of such facts, we will focus on the following: protective coloration and the form and phenomena of mimicry, features of adaptive traits of entomophilous, ornithophilous and teriophilous flowers, adaptive traits of island insects, adaptive behavior, how! proof of selection.

1. Protective coloration and shape. By patronizing coloration and form, or cryptic coloration and form, is understood the similarity of organisms (in color or form) with the objects of their normal life environment.

The phenomena of cryptic similarity are widespread in nature. Consider some examples of cryptic coloring and form.

The Russian zoologist V. A. Wagner (1901) described a spider (Drassus polihovi), which rests on the branches of trees and is remarkably similar to buds. Its abdomen is covered with folds similar to the integumentary scales of the kidneys. The spider makes short and quick movements, immediately assuming a resting position and imitating a kidney. Thus, cryptic similarity is associated with cryptic behavior (rest posture) - a fact unusually characteristic of the phenomena described, which are widespread among animals, including vertebrates. Thus, many arboreal birds have plumage dyed and ornamented to match the color and surface of the bark. Such birds (for example, many owls, eagle owls, owls, cuckoos, nightjars, pikas, etc.) are completely invisible in a resting position. This applies especially to females. Their cryptic resemblance to the bark is of great importance for the reason that it is usually the female who sits on the eggs, or guards the chicks; Therefore, in those cases where the males of forest species (for example, black grouse and capercaillie) differ well from each other in color, their females are colored very similarly (uniformly). For the same reason, for example, in the common pheasant (Phasianus colchicus), colored geographical varieties are characteristic only of males, while the females of all geographical subspecies of this bird are colored very similarly, patronizingly. Similar phenomena are observed in other animals.

Patterns of cryptic coloring. The main feature of cryptic phenomena is that those parts of the body that are exposed to the eye of a predator are cryptically colored. So, for example, in butterflies that fold their wings in a roof-like manner (as a result of which the upper sides of the front wings are facing the observer), the cryptic coloration is always present precisely on this upper side. The remaining parts of the wing, covered (in the resting posture) and therefore invisible, can and often do have bright colors. For example, the red-winged ribbonworm (Catoeala nupta and other species) has bright red stripes on its hindwings. During the fast flight of this butterfly, they flash before the eyes. However, it is enough for it to sit on the bark, as immediately the cryptically colored (to match the color of the bark) forewings lean like a roof on the bright hindwings, and the butterfly disappears from the eyes, if only the broken curve of its flight has been lost sight of. This phenomenon is even more effective in Kallima, in which cryptic similarity reaches a high degree of specialization.

Like all diurnal butterflies, their wings do not fold behind their backs in a roof-like manner (like those of night bats), but parallel to each other. Therefore, in the resting pose, the upper sides of the wings are hidden, while the lower ones face the observer. In this case, the hidden upper sides have a bright color visible during flight (for example, yellow stripes on a bluish background), and the outer lower sides have a critical color. Wallace, who observed the callim on about. Sumatra, indicates that it is enough for a butterfly to sit on a tree branch, and it is lost, which is facilitated not only by the cryptic color of the wings, but also by their cryptic pattern and shape, which are unusually similar to a leaf blade with a petiole.

So, cryptic coloration, firstly, is present in those individuals to whom it is especially useful (for example, in females), and secondly, it develops in those parts of the body that are exposed to the predator's eye (where it is needed, as a masking means). Thirdly, cryptic phenomena are always associated with the resting posture, i.e., with critical behavior that enhances the cryptic effect of masking (Oudemans, 1903).

However, these remarkable phenomena do not end there. Stick insects (Phasmidae), first studied by Bats (1862), are known to be strikingly knot-like. The posture of rest (critical behavior) further enhances this similarity. If you touch the stick insect, then for some time it sways like a blade of grass swayed by the wind (protective movements). If you take a stick insect in your hands, it falls into a state of thanatosis (reflex temporary and easily ending immobility). At the same time, the stick insect folds its legs along the body and becomes completely indistinguishable from a dry blade of grass. The phenomenon of thanatosis is characteristic of many insects.

2. Mimicry. This is the name of the similarity of some animals (imitators, or imitators) with others, which have the meaning of "models", and "imitators" derive one or another benefit from the similarity with the "model". Mimicry is widespread among insects, in particular in our Russian nature. Some flies of the family Syrphidae imitate wasps and bumblebees, while many insects belonging to various orders, as well as some spiders, are biologically related to ants and form a group of so-called myrmecophiles, strikingly similar to ants. Some butterflies imitate others, inedible ones, with which they fly together.

In Africa, the butterfly Papilio dardanus is found, which has a very large range, from Abyssinia to the Cape Colony inclusive and from the eastern coast to Senegal and the Gold Coast. In addition, R. dardanus is found in Madagascar. The form that lives on this island has in general features typical for the genus in the pattern and contour of the wing, reminiscent of our Russian swallowtails.

A completely different picture is observed on the African continent. Here, with the exception of Abyssinia, where typical females of P. dardanus are found, a wide polymorphism of the species under consideration is observed. This polymorphism is associated in this case with mimicry.

In South Africa, namely in the Cape Colony, the females of P. dardanus are completely changed. Their wings are devoid of balancers and are deceptively reminiscent of the wings of another local butterfly Amauris echeria (also without balancers):

This is the "model" that the native R. dardanus mimics. Moreover, A. echeria also lives in Natal, and forms here a special local form, which is connected by a series of transitions with the Cape forms of the same species. And so, the females of P. dardanus imitating this species give a parallel series of transitional forms (from Cape to Natal), imitating the transitional forms of the "model".

However, the described phenomenon is not limited to this. In the Cape Colony, besides A. echeria, two more butterflies A. niavius ​​and Danais chrysippus fly. Accordingly, the local females of P. dardanus produce two more imitation forms. One of them imitates D. chrysippus, and the other A. niavius.

Thus, P. dardanus has several forms of females that mimic several "models", namely the Cape and Natal forms of A. echeria. A. niavius, Danais chrysippus.

A natural question arises: what is the biological meaning of these imitations? The "models" were found to be inedible butterflies. Insectivorous animals, in any case, avoid them. In this case, the birds are unconditionally oriented by sight, and a certain color (and shape) of the wings of butterflies is conditioned-reflex associated with sensations unpleasant for birds (apparently, taste). Consequently, the "imitators" (in this case, the females of P. dardanus), while remaining actually edible, but at the same time having a resemblance to an inedible "model", are to a certain extent protected from attack by birds, "taking" them for the latter.

3. Explanation of cryptic phenomena and mimicry based on the theory of natural selection. The phenomena of cryptic form and behavior, as well as the phenomena of mimicry described above, are so widespread in various groups of organisms that it is impossible not to see a certain pattern in them that requires a causal explanation. The latter is entirely achieved on the basis of the theory of natural selection. However, other explanations have been proposed. Some researchers admit that, for example, cryptic coloration, pattern and shape are the result of the influence of physico-chemical factors, exercise or the consequence of special mental factors, etc.

Let's consider these assumptions. Is it possible, for example, to assume that the "ancestor" callim "exercised" in resemblance to a leaf, or the females of P. dardanus - in resemblance to the corresponding "models"? The absurdity of such an "explanation" is self-evident. Equally absurd is the assumption that the question is about the influence of climate, temperature, humidity, food, etc.

How did these factors make the stick insect look like a knot and the callima look like a leaf? Why did these factors cryptically affect the underside of the wings of the Callima and the upper side of the wings of the Red Ribbon? Obviously, the attempt to reduce patronizing color and form, or mimicry, to a purely physiological action external factors - barren. You should think about the fact that in the callima and the ribbon maiden, protective coloration is available only on those sides of the wings that are turned (in a resting position) to the external environment. The same sides of the wings, which are hidden in the resting position, not only do not have a protective coloration in these species, but, on the contrary, have a bright pattern that is sharply striking. In many crepuscular and nocturnal moths, a small part of the hindwings remains visible in the resting posture. And so, it is this part of the hind wings that has a cryptic coloration, while the rest of them, hidden from the gaze of an insectivorous bird, does not have this cryptic coloration.

Obviously, in such cases it is just as absurd to speak of exercise, the influence of food, light, temperature, moisture, etc., etc., as in the previous examples.

If, therefore, the phenomena of cryptic similarity and mimicry are inexplicable from the above points of view, then, on the contrary, they receive a satisfactory explanation in the light of the theory of selection.

Indeed, from the factors described above, it is quite clear that cryptic similarity and mimicry are beneficial to their owners. All those hereditary changes that caused the emergence of cryptic similarity were kept by virtue of their usefulness. Consequently, from generation to generation, selection for cryptic qualities was made in a natural way.

Mimicry is explained in a similar way. For example, it was established that from the testicles of the same form of P. dardanus, females of the three types indicated above can appear. Consequently, various forms of P. dardanus females may appear in a known locality, but in fact those that best mimic the local model will survive. The rest, even if they appeared, are much less likely to survive, since there is no corresponding inedible model in the given area, and therefore, birds will destroy such "groundless" imitators.

This general explanation requires, however, some deciphering. If, for example, we try to analyze the cryptic resemblance of a callima to a leaf, it will immediately become clear that it is composed of a very large number of elements. The resemblance of callima to a leaf blade is detailed, not general. Such is the general leaf-like shape of the folded wings, the balancers, which, when folded, correspond to the leaf stalk, the median line of the cryptic wing pattern, which imitates the median vein of the leaf; elements of lateral venation; wing spots imitating fungal leaf spots, general coloration of the underside of the wings imitating the color of dry coffee leaf and, finally, the behavior of the callima, which uses its cryptic resemblance to a leaf with the help of an appropriate resting posture.

All these elements of cryptic coloration, form and behavior could not have arisen suddenly. The same is true for the described cases of mimicry. Such a sudden formation of all these elements would be a miracle. However, miracles do not happen, and it is quite clear that the cryptic elements of the Kallima were formed historically. From the point of view of the theory of selection, cryptic similarity and mimicry arose as an accidental and, moreover, approximate similarity. However, once having arisen, it then, as useful, was preserved. Having been retained in generations, the initial cryptic similarity was expressed in different individuals to varying degrees and in a different number of elements. In some individuals, the cryptic effect of a given feature (for example, the color of the wing) or the effect of resemblance to an inedible form was more perfect than in others. However, it is natural that in the presence of vigilant insectivorous birds in a given area, individuals with the highest effect and the largest number of cryptic or mimicry traits had an advantage.

Thus, in the long series of generations, the most cryptically perfect forms survived. It is natural, therefore, that cryptic similarity and mimicry must be improved. Each cryptic feature was amplified, and the number of such cryptic features was accumulated. This is how historically, for example, the complex of cryptic features of the callima described above was formed. It does not follow from what has been said, of course, that the organism as a whole is a simple result of the summation of features. The cryptic effects certainly accumulated, but this accumulation is always associated with a general overmature of the organism as a result of combination by crossing. This issue is discussed below.

However, if cryptic similarity and mimicry were to be improved in the course of history, then we should expect that different kinds, for example, species of butterflies, must also be in geological modernity at various stages of this adaptive perfection. What is theoretically expected is actually observed in nature. In fact, the critical coloration and shape in different species is expressed with varying degrees of perfection. In some cases, the insect does not have special cryptic features. However, its coloring corresponds to the general color of the area, for example, the forest. For example, many moths, spreading their wings, sit on the white bark of a birch and, having dark wings, stand out sharply against the light background of the bark. Nevertheless, they remain invisible, as they look like one of the possible black spots of the bark of this tree. Such cases are very common. The author of these lines observed one dusky butterfly from the Notodontitae family - Lophopteryx camelina. With folded wings, the butterfly resembles a yellow sliver of bark. The butterfly flew off the tree and "stuck" in the needles of a pine tree, not far from the ground, remaining completely immobile. Well visible on the green foyer, it is still not striking due to its resemblance to a yellow sliver. Dropped into a net, it remained in a state of thanatosis, and its resemblance to a splinter of bark continued to mislead. Similar phenomena of approximate resemblance to one of the possible objects in a given situation can be called non-special critical coloring.

From such cases one can find many passages to a more special likeness.

Our Polygonium c-album, for example, sitting on the forest floor, becomes like a piece of a dried leaf. Butterfly Diphtera alpium, sitting on the bark, imitates the pattern and color of lichen, etc.

In these cases, the question is about a more special cryptic coloration.

Having picked up the rows of species from non-special to cryptic coloration, we will get a picture of the development of this phenomenon. However, even more convincing is the fact that the improvement of critical traits can be stated within the same species. Thus, Schwanvin (1940) showed that within one and the same species of butterfly Zaretes isidora, several forms can be identified in which cryptic features (similarity to a dry leaf) reach varying degrees of perfection. The figure shows a more primitive form of Zaretes isidora forma itis. As you can see, a longitudinal strip (Up) stretches along the hind wing, imitating the median vein of a dry leaf. However, this imitation is not yet perfect. The continuation of the "median vein" of the leaf within the front wing is still unclear, and in addition, the cryptic effect is reduced by the presence of other bands (E 3 , Ua, E 3 p), which break the resemblance to the median vein of the leaf. In another form, Zaretes isidora f. strigosa - the resemblance to a leaf is much greater. The median “vein” (Up) is clearer, E 3 has partially disintegrated, Ua is in a state of complete destruction, just like E 3 r. On the forewing, the median vein has developed significantly, and a series of dark bands at the base of the forewing is covered by a process of degradation. Due to this, the effect of imitation of the median vein of the leaf intensified. If we now compare these butterflies with callima, we will see that her cryptic effect is even more perfect. So in Zaretes, the continuation of the line imitating the median vein of the leaf is somewhat displaced on the front wing. This is not observed in callima. Thus, on the examples of both forms and the callima, it is revealed that the resemblance to the leaf is obviously achieved by successive displacement and destruction of all those parts of the pattern that violate the cryptic effect. This example shows that the resemblance to the sheet did not appear suddenly, but developed and improved. Moreover, both forms - Zaretes forma itis and f. strigosa are examples of varying degrees of effect achieved. These phenomena fully correspond to the theory of selection and are, therefore, an indirect proof of it.

However, even more significant is the fact that the median vein of the wing of the Callima arose in part from other elements of the design than in Zaretes. Therefore, the same effect has a different origin. The imitation of the leaf blade has been achieved in various ways. It is quite obvious that the factor that determined these results was not climate or exercise, but the eye of a predator. Birds exterminated forms less similar to a leaf, while forms more similar to it survived.

As for the mental factors that supposedly caused the described phenomena, the best evidence refuting this absurd idea is the cases of mimicry in plants, when, for example, an insect serves as a model, and a flower is an imitator.

The picture shows a flower of the orchid Ophrys muscifera, which is unusually similar to a bumblebee. This resemblance is based on the following:

1) The flower is pollinated by insects. 2) The flower has no smell, and the insect pollinating it does not seek nectar and does not receive it. 3) Visitors to the flower are only males. 4) The flower, to a certain extent, resembles the female of the same insect species. 5) The male, sitting on a flower, behaves in the same way as when copulating with a female, 6) If you remove parts of the flower that cause it to resemble a female, then the flower does not attract males (Kozo-Polyansky, 1939). All these features allow us to state that the cryptic features of the flower are a remarkable adaptation to pollination. In this case, it is quite clear that neither the theory of "exercise" nor the influence of climatic and mental factors explain anything. The described case is understandable only from the point of view of selection theory and is one of the most elegant indirect proofs of it (Kozo-Polyansky, 1939).

The study of the main regularities of mimicry leads to the same conclusion. Here are the most important of these regularities (Carpenter and Ford, 1936).

a) Mimicry affects only visible or so-called visual signs.

b) The systematic features of the model and the imitator can be, and usually are, completely different (i.e., they belong to completely different systematic groups). But in appearance (visually) the simulator is unusually similar to the model.

c) Simulator and model, as a rule, occupy the same area of ​​distribution.

d) Simulators and model aircraft fly together.

e) The imitator deviates from the usual appearance of the systematic group to which it belongs.

These patterns cannot be explained by an exercise in model similarity. The absurdity of this "explanation" is self-evident, especially in relation to plant imitators. No less absurd is this explanation in relation to insects, which just provide the greatest number of examples of mimicry. In general, there can be no question of an animal, and even more so a plant, imitating its appearance under the model through exercise. It could be assumed that the model and the imitator, living together, are influenced by the same factors, and therefore are similar.

It turns out, however, that the food of the model and the imitator, as well as the environment in which they develop, are quite often profoundly different. Therefore, the physiological explanation of mimicry does not give anything. Only the theory of selection satisfactorily explains mimicry. Like cryptic coloration, mimicry arose and developed in view of its usefulness. The acquisition of mimic traits increases the survival potential and, consequently, the biotic potential of the species. Therefore, the selection went in the direction of the development of imitating traits through the destruction of less successful imitators. Later we will see that this conclusion was confirmed experimentally.

4. Aposematic colors and shapes. It can be seen from the foregoing that mimicry is based on the similarity of the imitator with the model. This similarity is based on the fact that, for example, the model is inedible, and therefore, the similarity with it deceives the enemy, who "mistakes" an edible insect for an inedible one. Thus, in their origin, mimic species are obviously related to model species. Inedibility is due to an unpleasant smell, poisonous or burning properties of secretions, stinging organs, etc. These properties are usually associated with bright and noticeable colors, sharp patterns, for example, alternating dark and bright yellow stripes, as we see in wasps, or a bright red or yellow background, on which there are black spots (like ladybugs), etc. The inedible caterpillars of many butterflies have a very bright and variegated color. With these bright colors and drawings, the insect, as it were, "declares" its inedibility, for example, birds learn by personal experience to distinguish between such insects and, as a rule, do not touch them. From this it becomes clear that the resemblance to such inedible insects has a useful value and takes on the role of a visual adaptation, which develops in edible insects. This is where the phenomenon of mimicry comes from. We will see later that this explanation of mimicry has been experimentally confirmed. Warning colors and drawings are called aposematic, and the corresponding mimicry drawings are called pseudo-aposematic.

5. Let us finally dwell on the phenomena recognition coloration associated sometimes with the corresponding behavior. An example is the recognizable coloration of the moorhen (Zhitkov and Buturlin, 1916). The color of the feather cover of this bird is cryptic. Only the undertail is painted clean White color. Moorhen adheres to dense swamp thickets. The brood of the bird consists of about 12 chicks. It is difficult to keep this group of chicks together in dense thickets. Birds can easily fight off their mother, lose sight of her and become the prey of even small predators. And now the moorhen, making her way in the thickets, raises her tail high, exposing the white undertail, which serves as a “guiding sign” for the chicks, guided by which they unmistakably follow their mother.

Thus, the white undertail of the moorhen is a device that increases the survival rate of offspring.

The described case is interesting, however, from the other side. A white undertail is present in many birds and may not have the meaning described above. Similar remarks were made by anti-Darwinists, who pointed out that a feature arises without regard to its usefulness.

However, this remark is only evidence of a misunderstanding of the theory of selection. A sign becomes an adaptation only in conditions of certain relations with the surrounding life situation. In other conditions, it may be indifferent. Thus, the analyzed example is additional evidence of the fact that adaptation is not something absolute, but only a manifestation of the relationship of a given trait to specific environmental conditions.

6. Features of adaptive traits of entomo-, ornitho- and teriophilous flowers. We have already described the adaptations of entomophilous flowers for pollination by insects. The emergence of these adaptations under the influence of selection is self-evident, since it is impossible to explain the adaptation of entomophilous flowers to insects by any other theories.

No less striking examples of the action of selection are the adaptive traits of ornitho- and teriophilous flowers.

Ornithophilous flowers are adapted for pollination by birds. Birds navigate by sight. The flowers should be brightly colored, while the smell does not matter. Therefore, ornithophilous flowers are usually odorless. However, they have a bright color that attracts birds. For example, flowers pollinated by hummingbirds are bright red, blue, or green, corresponding to the pure colors of the solar spectrum. If within the same group of plants there are ornithophilous forms, then they have the colors of the spectrum, while others do not have such a color. Thus, it is quite obvious that the ornithophilous coloration of flowers is an adaptation to bird visits. However, the most remarkable thing is that ornithophilous flowers are adapted to birds not only in color, but also in their structure. So, they have an increase in the strength of flowers due to the development of mechanical tissues (in xerophytes) or an increase in turgor (in plants of humid tropical regions). Ornithophilous flowers secrete copious liquid or slimy nectar.

In the flower of the ornithophilous plant Holmskioldia sanguinea, the calyx is sympetalous. It occurred by the fusion of five leafy organs and has the shape of a fiery red funnel. The corolla of the flower, of the same color, has the shape of a hunting horn. The stamens are curved and protrude somewhat outwards, like the pistil. The flower is odorless; the largest release of nectar is in the early morning, during the hours of flight of the Cirnirys pectoralis sunbird. Birds plunge their curved beak into the corolla, sitting on a flower, or stopping in front of it in the air, like a hummingbird, that is, fluttering their wings. The beak exactly matches the curve of the corolla. The impression is that the beak seems to be cast in the form of a corolla, and the latter is like a bird's mask. When the beak is submerged, the anthers touch the forehead feathers and pollinate it. When visiting another flower, the pollen easily falls on the stigma and cross-pollination occurs (Porsch, 1924).

Finally, let us dwell on flowers that can be called theriophilic, that is, mammals adapted for pollination, in particular, bats. Theriophilic flowers have a number of peculiar features. Bats can easily damage a flower. In this regard, the theriophilic flowers, adapted for pollination by bats, are distinguished by an extraordinary strength of tissues, and their individual parts (as in the case of ornithophilous flowers)) are spliced ​​with each other. Since bats fly at twilight, the teriophilous flowers emit a scent_ only at this time. In the twilight hours, they also observed the release of nectar (Porsch). For their part, some bats that use colors are adapted to the latter. Thus, the long-tongued vampire (Glossophaga soricina) has an elongated muzzle, and the tongue is elongated and equipped with a brush that collects nectar.

Thus, the structure and color of the flower, the nature of the smell or its absence, as well as the time of release of nectar, are adapted with amazing accuracy to visitors (butterflies, bumblebees, birds, mammals), corresponding to their organization, flight time, and behavioral characteristics.

It scarcely needs proof that, without the theory of selection, all the adaptations described would have to be attributed to a completely incomprehensible and mysterious "capacity" to acquire an expedient structure adapted in every detail to flower visitors. On the contrary, the theory of selection provides a completely natural explanation for the phenomena described. The act of cross pollination is a vital quality, without which the reproduction of offspring is difficult. Therefore, the better a plant is adapted to its pollinator, the more likely it is to reproduce.

Thus, adaptations were inevitably honed to a high degree of perfection, where they were biologically necessary.

It is remarkable that this perfection and precision of adaptation is especially high when the flower is visited by only one particular nectar consumer. If this is not the case, then adaptations to them, as a rule, are of a more general, universal nature.

7. Let us now dwell on island wingless insects, as an example of indirect evidence of natural selection. Referring to Wollaston, Darwin pointed to the fact that Fr. Madera of 550 species of beetles, 200 species are incapable of flight. This phenomenon is accompanied by the following signs. A number of facts indicate that quite often flying beetles are blown into the sea by the wind and die. On the other hand, Wollaston noticed that Madeira beetles hide as long as the wind is blowing and there is no sun. Further, it was stated that wingless insects are especially characteristic of islands that are strongly blown by winds. From these facts, Darwin concluded that the winglessness of the insects of such islands was developed by selection. The flying forms are blown away by the wind and perish, while the wingless are preserved. Consequently, through the constant elimination of winged forms, the flightless fauna of windswept oceanic islands is formed.

These assumptions were completely confirmed. It was found that the percentage of wingless forms on windswept islands is always much higher than on the continents. So, on the Croset Islands, for 17 genera of insects - 14 are wingless. In the Kerguelen Islands, out of a total of eight endemic fly species, only one has wings.

One could, of course, say that selection has nothing to do with it. For example, wingless mutants are observed in Drosophila. Therefore, winglessness is the result of mutations, and selection only "picks up" the mutation if it is beneficial, as is the case on windy islands. However, it is the winglessness of island insects that well reveals the creative role of selection. Let's consider a corresponding example.

One of the Kerguelen wingless flies, besides winglessness, has another feature: it always keeps on the underside of the leaves of plants that are resistant to the wind. At the same time, the paws of this fly are equipped with tenacious claws. In another Kerguelen fly - Amalopteryx maritima - along with the rudimentation of the wings, the thighs of the hind legs have strong developed muscles, which is the reason for the ability of the fly to jump. Further, these insects are characterized by interesting behavior. As soon as the sun is covered with clouds (a harbinger of the wind), flightless insects immediately hide, going into the ground, hiding in the thick of herbaceous vegetation, moving to the underside of the leaves, etc. Consequently, winglessness or rudimentation of wings is associated with a number of other features of organization and behavior . It is easy to see the irreducibility of such "island" qualities in one mutation. The question is about the accumulation, the action of selection of a whole complex of "island" features.

One of the most remarkable indirect evidence of natural selection is the peculiarities of the Kerguelen flowering plants. There are no plants pollinated by insects on these islands. This fact will become clear if we remember that flight is associated with death. Therefore, on the wind-blown Kerguelen Islands, there are only wind-pollinated plants. It is obvious that insect pollinated plants could not stay on the islands due to the lack of appropriate insects. In this regard, adaptations for pollination by insects, in particular, bright colors, have also disappeared in Kerguelen flowering plants. For example, in carnations (Lyallia, Colobanthus), the petals are devoid of bright color, while in local buttercups (Ranunculus crassipes, R. trullifolius), the petals are reduced to the degree of narrow stripes. For the reasons mentioned above, the flora of the Kerguelen Islands is striking in the poverty of colors and, according to one of the naturalists who observed it, acquired a "melancholy shade." These phenomena reveal the action of natural selection with extraordinary clarity.

8. Adaptive Behavior as Indirect Evidence for Selection. The behavior of animals in many cases clearly indicates that it has developed under the influence of selection. Kaftanovsky (1938) indicates that guillemots lay their eggs on eaves densely populated by other murres. Violent fights take place between the birds because of each place. Newly arrived guillemots are met by other birds with sensitive blows of a strong beak. Nevertheless, the guillemot stubbornly adheres to these densely populated cornices, despite the fact that there are free ones nearby. The reasons for this behavior are explained quite simply. Kaftanovsky points out that diffuse, i.e., sparsely populated colonies are attacked by predatory gulls, while densely populated colonies are not attacked by the latter or are easily driven away by a collective attack.

It is clear how the instinct of coloniality developed among guillemots. Individuals lacking such instincts are subject to continuous elimination, and individuals that seek to lay their eggs in the midst of a densely populated bird colony are in the most favorable position.

Particularly illustrative examples of adaptive behavior associated with purely instinctive actions, for example, in insects. This includes, for example, the activity of many hymenoptera, including some paralyzing wasps described by Fabre and other researchers. Some wasps attack, for example, spiders, strike their nerve centers with their sting and lay their testicle on the body of the spider. The hatched larva feeds on live but paralyzed prey. A wasp that paralyzes a spider unmistakably stings its nerve centers, and on the other hand, a spider that is aggressive towards other insects is helpless against the type of wasp that is its specific enemy. Such a pair of specific species - a wasp and a spider, a paralyzing predator and its prey, therefore, are, as it were, adapted to each other. The wasp attacks only a certain type of spider, and the spider is defenseless against a certain type of wasp. It is quite obvious that the formation of such a fixed connection between two specific species is explicable only on the basis of the theory of selection. The question is about the historical connections between the forms that are most suitable to each other in the described relations.

Let's move on to direct evidence of the existence of natural selection in nature.

Direct evidence for natural selection

A significant amount of direct evidence of natural selection has been obtained through relevant field observations. Of the relatively large number of facts, we present only a few.

1. During a storm in New England, 136 sparrows died. Bumpes (Bumpes, 1899) studied the length of their wings, tail and beak, and it turned out that death was selective. The largest percentage of the dead were sparrows, which differed either in longer wings than in normal forms, or, on the contrary, in shorter wings. Thus, it turned out that in this case there was a selection for the average norm, while the deviating forms perished. Here we have the action of selection based on the inequality of individuals in relation to the eliminating factor - the storm.

2. Weldon (Weldon, 1898) established the fact of the reverse order - the survival of one intraspecific form under normal conditions, and another under changed conditions. Weldon studied the variability of one crab, in which there is a certain relationship between the width of the forehead and the length of the body, which is expressed in the fact that when the length of the body changes, the width of the forehead also changes. It was found that between 1803 and 1898 the average width of the forehead in crabs of a certain length gradually decreased. Weldon found that this change is associated with adaptive changes dependent on the emergence of new conditions of existence. In Plymouth, where observations were made, a pier was built, which weakened the effect of the tides. As a result, the seabed of the Plymouth coast began to be intensively clogged with soil particles brought by rivers and organic sediments. Wastewater. These changes affected the fauna of the bottom, and Weldon connected with them the changes in the width of the forehead of crabs. To verify this, the following experiment was set up. Crabs with narrower and wider foreheads were placed in aquariums. The water contained an admixture of clay, which, with the help of a stirrer, remained in an agitated state. A total of 248 crabs were placed in the aquariums. Soon part of the crabs (154) died, and it turned out that they all belonged to the “broad-eye” group, while the remaining 94, the survivors, belonged to the “narrow-eye” group. It was found that in the latter, the filtration of water in the gill cavity is more perfect than in the "broad-browed", which was the reason for the death of the latter. Thus, in the conditions of a clean bottom, "narrow-shaped" forms did not have an advantage and the quantitative ratios were not in their favor. When the conditions changed, the selection for "narrow-mindedness" began.

The described example also throws light on the elimination of sparrows (1). Some authors consider the results of Bempes's observations as evidence that selection does not create anything new, but only preserves the average rate (Berg, 1921). The results of Weldon's observations refute this. Obviously, under conditions typical for the area, the average norm survives. Under other conditions, the average norm may be eliminated and deviating forms will survive. It is clear that in the course of geological time, when conditions change, as a rule, it is precisely the latter that will occur. In the new conditions, new features will also come to the fore.

The dependence of evolution on environmental conditions is very clearly seen from the following example.

3. Harrison (Harrison, 1920) observed the numerical ratios of individuals of the butterfly Oporabia autumnata living in two different forest areas in the Cleveland region (Yorkshire, England). According to Garrison, around 1800, a mixed forest of pine, birch and alder was split in two. After a storm in the southern half of the forest, part of the pines died, and they were replaced by birches. On the contrary, birches and alders have become rare in the northern part. Thus, the forest turned out to be divided into two stations: pines dominated in one, and birches dominated in the other.

It was in this forest that the said butterfly lived. In 1907, it was noticed that its population had differentiated into two forms - dark-winged and light-winged. The first dominated in the pine forest (96%), and the second - in the birch forest (85%). Dusk birds (nightjars) and bats ate these insects, and Garrison found wings of destroyed butterflies on the forest floor. It turned out that in the dark pine forest the wings lying on the ground belonged predominantly to the light form, although the numerical ratio of the dark variety to the light one in the pine forest was 24:1. Consequently, in the dark forest, birds and bats grabbed the light variety, as more noticeable. In this example, it is clearly seen that the correspondence between the color of a butterfly and the color of its station is constantly maintained by the action of natural selection.

Let us now turn to the experimental evidence of natural selection. The latter mainly concern the protective effect of cryptic, sematic and aposematic coloration and mimicry.

4. Poulton (1899) experimented with 600 urticaria pupae. The pupae were located on various colored backgrounds, corresponding and not corresponding to their coloration. It turned out that if the color of the pupae corresponded to the color of the background, in total, 57% of them were destroyed by birds, while on an unsuitable background, against which the pupae were clearly visible, 90% were destroyed. Similar experiments were undertaken by Cesnola (di-Cesnola, 1904), who showed that praying mantises planted on a background that did not match their coloration were completely destroyed by birds. The methodology of these researchers was, however, elementary. Chesnola experimented with a small number of praying mantises.

The data of Belyaev and Geller are much more convincing.

5. Belyaev (1927), like Chesnola, experimented with praying mantises. A site measuring 120 m 2 was cleared of tall plants and acquired a faded brown color. 60 praying mantises were placed on the site, tied to pegs driven into the ground at a distance of 1 m from one another. The praying mantises were brown, straw-yellow and green, and the brown praying mantises were hardly visible against the pale brown background of the site. The fighters were chasers-heathens, who kept on the fence of the site and ate praying mantises. Thus, the selection process is clearly shown in the experiment.

Similar data on a large material are shown by Heller (Heller, 1928). Insects were planted on experimental sites in a checkerboard pattern. The fighters were chickens.

A clear selection took place, since insects that did not match the color of the soil were destroyed by 95.2%, and in the case of homochromia, on the contrary, 55.8% survived.

The experiments of Belyaev and Geller are also interesting in another respect: they show that homochromy does not fully guarantee survival, but only increases the biotic potential of this form. Finally, one more conclusion needs to be emphasized. Praying mantises belonged to the same species, and their color differences are intraspecific variations. The experiments of Belyaev and Geller thus showed that selection takes place within the population of a species.

6. Carrik (1936) experimented with caterpillars, observing the protective value of cryptic coloration. He established that the wren, for example, did not notice moth caterpillars that had a cryptic coloration. However, it was enough for the caterpillar to move, as the wren immediately attacked it. Similar observations have been made by other authors, and they prove that cryptic coloration is closely related to cryptic behavior (rest posture) and guarding movements.

7. The above examples show the true meaning of cryptic coloration. Let us now turn to the meaning of mimicry. Mostler (Mostler, 1935) tried to establish the extent to which aposematic and pseudo-aposematic coloration has an effect. Mostler experimented with wasps, bumblebees, and bees, as well as with flies that mimic the former. It has been shown on a large body of material that birds, as a rule, do not eat Hymenoptera, except for specially adapted birds, which is connected, apparently, with taste reflexes. This reflex is developed in order personal experience. When hymenoptera-mimicking flies were offered to young birds, they initially ate them. However, when they had previously been offered hymenoptera, and they had developed a negative reflex to these insects, they stopped taking imitation flies as well. Experience brilliantly showed the importance of aposematic and pseudo-aposematic coloration.

The next experience is especially important. Mühlmann (Miihlmann, 1934), experimenting with birds, used flour worms as food. The worms were painted with harmless paint, and the birds ate them willingly. After that, the experience was modified. Birds were offered the same colored worms, but some of them were dyed with a mixture of paint, with unpleasant tasting substances. Birds stopped taking such worms, but they did not take simply colored ones, that is, edible ones. Relationships arose that resemble those that exist between the imitator and the model. Painted with an unpleasant mixture played the role of a model, just painted - an imitator. It has therefore been shown that the similarity of the simulator to the model has a protective value. Then the experience was modified as follows. Mulman set out to find out to what extent birds are able to distinguish patterns. The paint was applied to certain segments of the body of the worms, they were given a certain pattern, and in this form the worms were included in the experiment described above. It turned out that the birds distinguished the drawings and did not take definitely colored worms if the latter were unpleasant in taste. This result throws light on the process of improving the cryptic drawing. If birds distinguish a pattern, then the more perfect, for example, the critical resemblance of a butterfly's wing to a leaf, the greater its chances of survival. In the light of Muhlmann's experiments, this conclusion acquires a high degree of certainty.

sexual selection

The theory of sexual selection has caused the greatest number of objections, even from many Darwinists. It turned out that in some cases its use can be disputed and that, for example, the bright coloration of males can be explained differently. Thus, Wallace assumed that color and pattern do not affect the choice of females and that the strength of the male, which is manifested in a brighter color, is of greatest importance. Thus, Wallace essentially denied sexual selection. They tried to reject the theory of sexual selection on the grounds that it was built on anthropomorphism, that is, on the mechanical transfer of human emotions to animals. Such a mechanical extrapolation of human ideas of beauty to animals is indeed erroneous. Of course, we do not know what the turkey "thinks" about the turkey flaunting in front of her, but we cannot, on the basis of simple observations, either deny or defend the theory of sexual selection. Zhitkov (1910), on the basis of a number of field observations, indicates, for example, that mating of black grouse and fights of turukhtans, very often, occur without the participation of females and that, consequently, there is no choice of males. Zhitkov also pointed out that at grouse leks the most active males fight in the central parts of the lek. The rest, weaker and younger, stay on the outskirts of it, closer to the females, which is why "with a greater degree of probability it can be assumed that the attention of the female often falls to their lot."

Such facts seem to speak against the theory of sexual selection. It has also been suggested that the bright coloration of males is not attractive, but intimidating. Fausek (1906) developed this theory in particular detail. Undoubtedly, the theory of frightening (threatening) coloring cannot be denied.

It should be said, however, that these considerations do not essentially refute the theory of sexual selection. This primarily applies to the above-mentioned observations of Zhitkov, according to which black grouse lek even in the absence of females, and fighting scythes (male black grouse) do not pay any attention to females even if they are present. The first observation only shows that adaptations to the mating season are as relative as any adaptation. The behavior of scythes on the current becomes an adaptation in the presence of certain relationships, namely in the presence of females. In other respects, the same phenomena do not matter as adaptations to the mating season. This observation of Zhitkov proves nothing else. As regards his second observation, we are now well aware of direct influence of courtship on sexual arousal of males and females . It can be thought that it is the males who are on display after display, in a state of increased sexual arousal, more actively approach the females and that it is they who have the greatest success, while the males who do not participate in display and in fights, due to the lack of sexual arousal, remain on the sidelines. Thus, in the case of black grouse, we may be dealing with a form of sexual selection in which the active side is the male. This form of sexual selection is no doubt a special case of natural selection. The strength of the male, his armament, his adaptations for active defense and attack are of great vital importance in the struggle for existence. For example, large fangs can be important both in the fight for the female and in defense against enemies. Thus, in such cases, one can speak of a coincidence of sexual and natural selection, and mating with a more energetic and stronger male (if his signs and properties are hereditarily determined), of course, increases the standard of living of the population that arose from such males. We certainly observe this form of sexual selection in highly organized mammals (canines, deer, seals) and in birds. If in this case the phenomena described by Zhitkov arise, then one must not forget the relativity of any adaptations and expect that greater strength and better armament in all cases ensure the mating of these males, and not other, weaker ones. Secondly, when discussing the reality of the form of sexual selection under consideration, one more factor must be taken into account, namely, the height of the organization. It is impossible, for example, to "refute" the theory of sexual selection, using examples from the relations between the sexes in lowly organized forms. Strictly speaking, sexual selection, in contrast to natural selection, occurs through the selection of appropriate individuals, and therefore is associated with a high development of the nervous system and sensory organs. Therefore, it can be argued that the importance of sexual selection increases as organization increases. From this point of view, an interesting attempt by J. S. Huxley (1940) to approach the relationship between the sexes in a historical aspect. He distinguishes the following three main groups of these relations. A - forms without crossing, in which gametes are connected regardless of any contact between individuals, for example, by releasing eggs and sperm into the water, as we see in coelenterates, annelids and most bony fish. Naturally, sexual selection is out of the question here. B - forms with mating, however, only for coition, without subsequent long-term cohabitation of the sexes. In this case, we see the development of special devices that attract both sexes to each other. This includes two categories of phenomena: a) The development of the ability to mate with one individual. For example: detection of the opposite sex with the help of the organs of smell, sight, hearing, excitation of sexual reflexes by touching or grasping (in some crabs, in tailless amphibians), sexual games that stimulate mating (newts, some Diptera, etc.), wrestling and intimidation (stag beetles, lizards, sticklebacks, praying mantises, etc.). b) The development of the ability to mate with more than one individual with the help of: a) fighting, b) displaying, c) fighting and displaying (as is observed in turukhtans, black grouse, birds of paradise). C - long-term cohabitation of the sexes, not only for the time of coitus, but also in the course of further relationships. Mating occurs: a) with one individual or b) with several individuals, and the mating is associated with a struggle, or a struggle in combination with attracting attention, etc. This includes the links between the sexes within the classes of birds and mammals.

Huxley's scheme is based on the progressive development of the reproductive organ system, and this is its drawback. It would be more correct to build this scheme on the progressive development of the nervous system. In fact, it is hardly correct to place in one rubric the sexual games of newts and Drosophila, the relationship between male stag beetles and lizards. If we classify the relationship between the sexes according to the level of development of the nervous system, we can state that sexual selection, in its typical forms, manifests itself in higher animals (vertebrates, especially birds and mammals) capable of conditioned reflex activity.

It is only necessary not to forget the relativity of the significance of sexual selection. For example, the strongest male will not always be the most successful. On grouse leks, coitus is not always provided only for males participating in lekking. But on average, the strongest, most active males still have more chances than the rest. Criticism of the first type of selection theory, when mating depends on competition between males, is based on a misinterpretation of the theory of adaptations. Critics impose on Darwinism the idea of ​​the absolute meaning of contraptions, and then, citing cases where such contraptions are not valid, claim that they do not matter at all. In fact, all adaptation, as we know, is relative, and therefore sexual selection does not always follow the scheme proposed by Darwin.

Central to the theory of sexual selection is the problem of the bright colors of the males of many birds (and other animals, but especially birds). After all, it is the bright, unmasking coloration of males, which contradicts the theory of natural selection, that requires explanation. Darwin and put forward an ingenious theory, according to which females choose the most beautiful males. This theory can only be refuted or confirmed experimentally. There is little data on this. Let us present, however, the following results of experimental observations (Cinat-Thomson, 1926) on sexual selection in the budgerigar (Melopsittacus undulatus). The males of this bird have lush feathers forming a collar, which has a number of large dark spots (1-5) or 1-3 smaller ones. The more spots, the better the collar is developed. According to the number of spots, the males were designated No. 1, No. 2, No. 3, and G. D., respectively. It turned out that females prefer males with a large number of spots. Males No. 2 and No. 4 were planted in the cage. All females chose males No. 4. Then the following experiments were made. Males had additional dark feathers glued onto their collars. Males No. 4, No. 3, No. 2 and No. 1 were subjected to experiments. Control experiments showed that females choose males No. 3 and No. 4. These males were left with their natural outfit. Then “decorated males” No. 2 + 1 and No. I + II were put into the enclosure (Roman numerals indicate the number of glued feathers). Although their success was less than expected, it still turned out to be double their previous success (when these males did not have glued feathers). In another experiment, male No. 4 (which was successful) was cut off the lush collar and removed the dark feathers on it. He was put into the aviary and suffered a complete failure. With all the possible inaccuracies of the methods (the data would be more accurate using variational statistics), nevertheless, these experiments show that females distinguish and choose males according to their dress.

Thus, the existence of sexual selection has been established experimentally. It should be emphasized that in the experiments of Cinat-Thomson, females choose males, which confirms the central position of the theory of sexual selection as a factor that determines the bright coloration of males.

The question of sexual selection has recently received interesting coverage in the works of a number of authors, including Mashkovtsev, who, on the basis of literary data and his own observations (Mashkovtsev, 1940), came to the conclusion that the presence of a male has a stimulating effect on the development of the ovary and the number of eggs in females. Of great importance is also the general situation of the mating season, the presence of a nest, the appearance of spring greenery, thawed patches, etc. If, for example, females sit without males and without a nest, then the ovaries develop only to a small extent. On the contrary, if you put up a nest and let the males in, then rapid ovulation (development of eggs) and intensive development of the ovaries begin. Thus, external environmental factors, as well as the nest and the male (its smell and appearance), affect the female, stimulating ovogenesis. If we compare these data with at least the experiments of Cinat Thomson, it becomes clear that the sense organs (primarily the organs of vision) in birds are of great importance in the occurrence of sexual arousal in females. The signs of the male (as well as the presence of a nest and the corresponding ecological situation), through the sense organs, apparently excite the activity of the pituitary gland of the female, which secretes the gonadotropic hormone (stimulator of ovarian function). We see that external stimuli, and especially the presence of a male, is a powerful factor that enhances the female's sexual production. The presented data undoubtedly confirm the main provisions of Darwin's theory of sexual selection. In this case, it becomes highly probable that sexual selection, being a special form of natural selection, plays an enormous role as a factor that increases the fertility of the female. An increase in the multiplication factor (under certain favorable general conditions) leads to an increase in the general biotic potential of the species. These relationships remove the negative value of the unmasking coloration of males, and it becomes a factor in the progressive development and life success of the species.

Sexual selection and sexual dimorphism. It can be seen from the foregoing that sexual selection is associated with morphophysiological differences between male and female. It is known that male and female differ in their secondary sexual characteristics and that these latter arise under the influence of male and female sex hormones produced in the gonads. Experiments with transplantation of the gonads from a male to a female and from the latter to a male convincingly demonstrate the dependence of secondary sexual characteristics on the hormonal activity of the gonads. These relations make it possible, as it were, to reduce sexual dimorphism to purely hormonal influences and to see in them the reasons for the differences between male and female. With such a formulation of the question, the theory of sexual selection becomes, as it were, superfluous. Of course, at the lower stages of phylogenetic development, the problem of sexual dimorphism can be solved on the basis of the theory of the sex hormonal effect. We can also consider that sexual dimorphism in these cases is determined by genetic factors. For example, in roundworms, sexual dimorphism is very pronounced, and males are well distinguished from females by their secondary sexual characteristics, while it is difficult to talk about sexual selection within this group of organisms. Neither competition between males nor the choice of a male by a female takes place here, although the relationship between the sexes in nematodes should be assigned to the second rubric of J. S. Huxley. The male and female enter into coitus, and it is preceded by the male wrapping around the body of the female. The male wraps his tail around her, gropes for the genital opening and introduces his spicules, then pouring out the seed from the ejaculatory canal. These phenomena are not related to sexual selection. Numerous observations of the author on the behavior of males show that coitus occurs as a result of chance encounters.

In higher animals - in invertebrates (insects), and even more so in vertebrates - sexual selection is undeniable. Consequently, the question arises, what is the cause of sexual dimorphism here - sexual selection or the shaping influence of hormonal factors? This question must be answered. Historically, sexual dimorphism arose in its hormonal connections. Therefore, it is present in the lower groups, in which there is no sexual selection. However, in higher forms, especially in birds and mammals, historically hormonal factors give way to sexual selection, and sexual dimorphism acquires the significance of a special form of variability that serves as material for the emergence of sexual selection. The bright coloration, strength and armament of the male are a direct consequence of the influence of sex hormones. However, it was precisely under the influence of sexual selection that the offspring of those males in which their distinctive features were developed most fully and expressively were predominantly bred. Thus, through the sexual selection of external characters, the hormonal effect of the sex gland and, consequently, the selection for sexual dimorphism intensified.

Snezhinsky Polytechnic College

Report on biology on the topic:

"Natural selection"

Completed by: 1st year student

F-18D groups

Yakunina Elena

Checked by: Budalova I.B.

Snezhinsk 2009

Natural selection

a) Destabilizing selection

b) Sexual selection

c) Group selection

d) Directed selection (moving)

e) Stabilizing selection

f) Disruptive (dismembering) selection

Conclusion

Bibliography

Natural selection

Natural selection- the result of the struggle for existence; it is based on preferential survival and leaving offspring with the most adapted individuals of each species and the death of less adapted organisms.

The mutation process, population fluctuations, isolation create genetic heterogeneity within a species. But their action is not directed. Evolution, on the other hand, is a directed process associated with the development of adaptations, with a progressive complication of the structure and functions of animals and plants. There is only one directed evolutionary factor - natural selection.

Either certain individuals or entire groups can be subject to selection. As a result of group selection, traits and properties are often accumulated that are unfavorable for an individual, but useful for the population and the whole species (a stinging bee dies, but attacking the enemy, it saves the family). In any case, selection preserves the organisms most adapted to a given environment and operates within populations. Thus, it is populations that are the field of action of selection.

Natural selection should be understood as selective (differential) reproduction of genotypes (or gene complexes). In the process of natural selection, it is not so much the survival or death of individuals that is important, but their differential reproduction. Success in reproduction of different individuals can serve as an objective genetic-evolutionary criterion of natural selection. The biological significance of an individual that has given offspring is determined by the contribution of its genotype to the gene pool of the population. Selection from generation to generation according to phenotypes leads to the selection of genotypes, since not traits, but gene complexes are transmitted to descendants. For evolution, not only genotypes are important, but also phenotypes and phenotypic variability.

During expression, a gene can influence many traits. Therefore, the scope of selection can include not only properties that increase the likelihood of leaving offspring, but also traits that are not directly related to reproduction. They are selected indirectly as a result of correlations.

a) Destabilizing selection

Destabilizing selection- this is the destruction of correlations in the body with intensive selection in each specific direction. An example is the case when selection aimed at reducing aggressiveness leads to destabilization of the breeding cycle.

Stabilizing selection narrows the reaction rate. However, in nature there are cases when the ecological niche of a species may become wider over time. In this case, the selective advantage is obtained by individuals and populations with a wider reaction rate, while maintaining the same average value of the trait. This form of natural selection was first described by the American evolutionist George G. Simpson under the name centrifugal selection. As a result, a process occurs that is the reverse of stabilizing selection: mutations with a wider reaction rate gain an advantage.

Thus, populations of marsh frogs living in ponds with heterogeneous illumination, with alternating areas overgrown with duckweed, reed, cattail, with “windows” of open water, are characterized by a wide range of color variability (the result of a destabilizing form of natural selection). On the contrary, in water bodies with uniform illumination and coloration (ponds completely overgrown with duckweed, or open ponds), the range of variability in frog coloration is narrow (the result of the action of a stabilizing form of natural selection).

Thus, a destabilizing form of selection in goes to the expansion of the reaction rate.

b) sexual selection

sexual selection- natural selection within the same sex, aimed at developing traits that give mainly the opportunity to leave the largest number of descendants.

In males of many species, pronounced secondary sexual characteristics are found that at first glance seem maladaptive: the tail of a peacock, the bright feathers of birds of paradise and parrots, the scarlet combs of roosters, the enchanting colors of tropical fish, the songs of birds and frogs, etc. Many of these features make life difficult for their carriers, making them easily visible to predators. It would seem that these signs do not give any advantages to their carriers in the struggle for existence, and yet they are very widespread in nature. What role did natural selection play in their origin and spread?

We already know that the survival of organisms is an important but not the only component of natural selection. Another important component is attractiveness to members of the opposite sex. Charles Darwin called this phenomenon sexual selection. He first mentioned this form of selection in The Origin of Species and later analyzed it in detail in The Descent of Man and Sexual Selection. He believed that "this form of selection is determined not by the struggle for existence in the relationship of organic beings among themselves or with external conditions, but by the rivalry between individuals of the same sex, usually males, for the possession of individuals of the other sex."

Sexual selection is natural selection for success in reproduction. Traits that reduce the viability of their carriers can emerge and spread if the advantages they provide in breeding success are significantly greater than their disadvantages for survival. A male that lives a short time but is liked by females and therefore produces many offspring has a much higher cumulative fitness than one that lives long but leaves few offspring. In many animal species, the vast majority of males do not participate in reproduction at all. In each generation, fierce competition for females arises between males. This competition can be direct, and manifest itself in the form of a struggle for territories or tournament fights. It can also occur in an indirect form and be determined by the choice of females. In cases where females choose males, male competition is shown in displaying their flamboyant appearance or complex courtship behavior. Females choose those males that they like the most. As a rule, these are the brightest males. But why do females like bright males?

Rice. 7. The bright colors of birds arise in evolution due to sexual selection.

The fitness of the female depends on how objectively she is able to assess the potential fitness of the future father of her children. She must choose a male whose sons will be highly adaptable and attractive to females.

Two main hypotheses about the mechanisms of sexual selection have been proposed.

According to the “attractive sons” hypothesis, the logic of female selection is somewhat different. If bright males, for whatever reason, are attractive to females, then it is worth choosing a bright father for your future sons, because his sons will inherit the bright color genes and will be attractive to females in the next generation. Thus, a positive feedback occurs, which leads to the fact that from generation to generation the brightness of the plumage of males is more and more enhanced. The process goes on increasing until it reaches the limit of viability. Imagine a situation where females choose males with a longer tail. Long-tailed males produce more offspring than males with short and medium tails. From generation to generation, the length of the tail increases, because females choose males not with a certain tail size, but with a larger than average size. In the end, the tail reaches such a length that its harm to the viability of the male is balanced by its attractiveness in the eyes of females.

In explaining these hypotheses, we tried to understand the logic of the action of female birds. It may seem that we expect too much from them, that such complex fitness calculations are hardly accessible to them. In fact, in choosing males, females are no more and no less logical than in all other behaviors. When an animal feels thirsty, it does not reason that it should drink water in order to restore the water-salt balance in the body - it goes to the watering hole because it feels thirsty. When a worker bee stings a predator attacking a hive, she does not calculate how much by this self-sacrifice she increases the cumulative fitness of her sisters - she follows instinct. In the same way, females, choosing bright males, follow their instincts - they like bright tails. All those who instinctively prompted a different behavior, all of them left no offspring. Thus, we discussed not the logic of females, but the logic of the struggle for existence and natural selection - a blind and automatic process that, acting constantly from generation to generation, has formed all that amazing variety of forms, colors and instincts that we observe in the world of wildlife. .

c) Group selection

Group selection is often also called group selection, it is the differential reproduction of different local populations. Wright compares population systems of two types - a large continuous population and a number of small semi-isolated colonies - in relation to the theoretical selection efficiency. It is assumed that the total size of both population systems is the same and the organisms interbreed freely.

In a large contiguous population, selection is relatively inefficient in terms of increasing the frequency of favorable but rare recessive mutations. In addition, any tendency to increase the frequency of any favorable allele in one part of a given large population is counteracted by crossing with neighboring subpopulations in which that allele is rare. Similarly, favorable new gene combinations that have managed to form in some local fraction of a given population are broken up and eliminated as a result of crossing with individuals of neighboring shares.

All these difficulties are eliminated to a large extent in a population system that resembles in its structure a series of separate islands. Here, selection, or selection in conjunction with genetic drift, can quickly and effectively increase the frequency of some rare favorable allele in one or more small colonies. New favorable combinations of genes can also easily gain a foothold in one or more small colonies. Isolation protects the gene pools of these colonies from "flooding" as a result of migration from other colonies that do not have such favorable genes, and from crossing with them. Up to this point, only individual selection or - for some colonies - individual selection combined with genetic drift has been included in the model.

Let us now assume that the environment in which this population system is located has changed, as a result of which the adaptability of the former genotypes has decreased. In a new environment, new favorable genes or combinations of genes that are fixed in some colonies have a high potential adaptive value for the population system as a whole. All conditions are now in place for group selection to take effect. The less fit colonies gradually shrink and die out, while the more fit colonies expand and replace them throughout the area occupied by a given population system. Such a subdivided population system acquires a new set of adaptive traits as a result of individual selection within certain colonies, followed by differential reproduction of different colonies. The combination of group and individual selection can lead to results that cannot be achieved through individual selection alone.

It has been established that group selection is a second-order process that complements the main process of individual selection. As a second order process, group selection must be slow, probably much slower than individual selection. Updating populations takes more time than updating individuals.

The concept of group selection has been widely accepted in some circles, but has been rejected by other scientists. They argue that the various possible patterns of individual selection are capable of producing all the effects attributed to group selection. Wade conducted a series of breeding experiments with the flour beetle (Tribolium castaneum) in order to ascertain the effectiveness of group selection, and found that the beetles responded to this type of selection. In addition, when a trait is simultaneously affected by individual and group selection and, moreover, in the same direction, the rate of change of this trait is higher than in the case of individual selection alone (Even moderate immigration (6 and 12%) does not prevent differentiation populations caused by group selection.

One of the features of the organic world, which is difficult to explain on the basis of individual selection, but can be considered as the result of group selection, is sexual reproduction. Although models have been created in which sexual reproduction is favored by individual selection, they appear to be unrealistic. Sexual reproduction is the process that creates recombination variation in interbreeding populations. It is not the parental genotypes that break up in the process of recombination that benefit from sexual reproduction, but the population of future generations, in which the margin of variability increases. This implies participation as one of the factors of the selective process at the population level.

G)

Rice. 1. Driving form of natural selection

Directional selection (moving) was described by Ch. Darwin, and the modern doctrine of driving selection was developed by J. Simpson.

The essence of this form of selection is that it causes a progressive or unidirectional change in the genetic composition of populations, which manifests itself in a shift in the average values ​​of the selected traits in the direction of their strengthening or weakening. It occurs when a population is in the process of adapting to a new environment, or when there is a gradual change in the environment, followed by a gradual change in the population.

With a long-term change in the external environment, a part of the individuals of the species with some deviations from the average norm may gain an advantage in life and reproduction. This will lead to a change in the genetic structure, the emergence of evolutionarily new adaptations and a restructuring of the organization of the species. The variation curve shifts in the direction of adaptation to new conditions of existence.

Figure 2. Dependence of the frequency of dark forms of the birch moth on the degree of atmospheric pollution

Light-colored forms were invisible on birch trunks covered with lichens. With the intensive development of industry, sulfur dioxide produced by burning coal caused the death of lichens in industrial areas, and as a result, dark bark of trees was discovered. On a dark background, light-colored moths were pecked by robins and thrushes, while melanic forms survived and successfully reproduced, which are less noticeable against a dark background. Over the past 100 years, more than 80 species of butterflies have developed dark forms. This phenomenon is now known under the name of industrial (industrial) melanism. Driving selection leads to the emergence of a new species.

Rice. 3. Industrial melanism. Dark forms of butterflies are invisible on dark trunks, and light ones on light ones.

Insects, lizards and a number of other inhabitants of the grass are green or brown in color, the inhabitants of the desert are the color of sand. The fur of animals living in the forests, such as a leopard, is colored with small spots resembling sun glare, while in a tiger it imitates the color and shadow from the stems of reeds or reeds. This coloring is called patronizing.

In predators, it was fixed due to the fact that its owners could sneak up on prey unnoticed, and in organisms that are prey, due to the fact that the prey remained less noticeable to predators. How did she appear? Numerous mutations gave and give a wide variety of forms that differ in color. In a number of cases, the coloring of the animal turned out to be close to the background of the environment, i.e. hid the animal, played the role of a patron. Those animals in which the protective coloration was weakly expressed were left without food or became victims themselves, and their relatives with the best protective coloration emerged victorious in the interspecific struggle for existence.

Directed selection underlies artificial selection, in which selective breeding of individuals with desirable phenotypic traits increases the frequency of those traits in a population. In a series of experiments, Falconer chose the heaviest individuals from a population of six-week-old mice and let them mate with each other. He did the same with the lightest mice. Such selective crossing on the basis of body weight led to the creation of two populations, in one of which the mass increased, and in the other it decreased.

After the selection was stopped, neither group returned to its original weight (approximately 22 grams). This shows that artificial selection for phenotypic traits has led to some genotypic selection and partial loss of some alleles by both populations.

e) Stabilizing selection

Rice. 4. Stabilizing form of natural selection

Stabilizing selection in relatively constant environmental conditions, natural selection is directed against individuals whose characters deviate from the average norm in one direction or another.

Stabilizing selection preserves the state of the population, which ensures its maximum fitness under constant conditions of existence. In each generation, individuals that deviate from the average optimal value in terms of adaptive characteristics are removed.

Many examples of the action of stabilizing selection in nature have been described. For example, at first glance it seems that individuals with maximum fecundity should make the greatest contribution to the gene pool of the next generation.

However, observations of natural populations of birds and mammals show that this is not the case. The more chicks or cubs in the nest, the more difficult it is to feed them, the smaller and weaker each of them. As a result, individuals with average fecundity turn out to be the most adapted.

Selection in favor of averages has been found for a variety of traits. In mammals, very low and very high birth weight newborns are more likely to die at birth or in the first weeks of life than middle weight newborns. Accounting for the size of the wings of birds that died after the storm showed that most of them had too small or too large wings. And in this case, the average individuals turned out to be the most adapted.

What is the reason for the constant appearance of poorly adapted forms in constant conditions of existence? Why is natural selection unable to once and for all clear a population of unwanted evasive forms? The reason is not only and not so much in the constant emergence of more and more new mutations. The reason is that heterozygous genotypes are often the fittest. When crossing, they constantly give splitting and homozygous descendants with reduced fitness appear in their offspring. This phenomenon is called balanced polymorphism.

Fig.5. Map of the distribution of sickle cell anemia in malarial areas. Colors indicate malarial areas. The shaded area shows high frequency sickle cell anemia

The most widely known example of such a polymorphism is sickle cell anemia. This severe blood disease occurs in people homozygous for the mutant hemoglobin allele (Hb S) and leads to their death at an early age. In most human populations, the frequency of this alley is very low and approximately equal to the frequency of its occurrence due to mutations. However, it is quite common in areas of the world where malaria is common. It turned out that heterozygotes for Hb S have a higher resistance to malaria than homozygotes for the normal alley. Due to this, in populations inhabiting malarial areas, heterozygosity is created and stably maintained for this lethal alley in the homozygote.

Stabilizing selection is a mechanism for the accumulation of variability in natural populations. The outstanding scientist I. I. Shmalgauzen was the first to pay attention to this feature of stabilizing selection. He showed that even under stable conditions of existence, neither natural selection nor evolution ceases. Even remaining phenotypically unchanged, the population does not cease to evolve. Its genetic makeup is constantly changing. Stabilizing selection creates such genetic systems that provide the formation of similar optimal phenotypes on the basis of a wide variety of genotypes. Such genetic mechanisms as dominance, epistasis, complementary action of genes, incomplete penetrance, and other means of hiding genetic variability owe their existence to stabilizing selection.

The stabilizing form of natural selection protects the existing genotype from the destructive influence of the mutation process, which explains, for example, the existence of such ancient forms as the tuatara and ginkgo.

Thanks to stabilizing selection, "living fossils" that live in relatively constant environmental conditions have survived to this day:

1. tuatara, bearing the features of reptiles of the Mesozoic era;

2. coelacanth, a descendant of lobe-finned fish, widespread in the Paleozoic era;

3. North American opossum - a marsupial known from the Cretaceous period;

The stabilizing form of selection acts as long as the conditions that led to the formation of a particular trait or property persist.

It is important to note here that the constancy of conditions does not mean their immutability. During the year, environmental conditions change regularly. Stabilizing selection adapts populations to these seasonal changes. Breeding cycles are timed to them, so that the young are born in that season of the year when food resources are maximum. All deviations from this optimal cycle, reproducible from year to year, are eliminated by stabilizing selection. Descendants born too early die from starvation, too late - they do not have time to prepare for winter. How do animals and plants know when winter is coming? On the onset of frost? No, it's not a very reliable pointer. Short-term temperature fluctuations can be very deceptive. If in some year it gets warmer earlier than usual, this does not mean at all that spring has come. Those who react too quickly to this unreliable signal risk being left without offspring. It is better to wait for a more reliable sign of spring - an increase in daylight hours. In most animal species, it is this signal that triggers the mechanisms of seasonal changes in vital functions: cycles of reproduction, molting, migration, etc. I.I. Schmalhausen convincingly showed that these universal adaptations arise as a result of stabilizing selection.

Thus, stabilizing selection, sweeping aside deviations from the norm, actively forms genetic mechanisms that ensure the stable development of organisms and the formation of optimal phenotypes based on various genotypes. It ensures the stable functioning of organisms in a wide range of fluctuations in external conditions familiar to the species.

f) Disruptive (dismembering) selection

Rice. 6. Disruptive form of natural selection

Disruptive (dismembering) selection favors the preservation of extreme types and the elimination of intermediate ones. As a result, it leads to the preservation and strengthening of polymorphism. Disruptive selection operates in a variety of environmental conditions found in the same area, and maintains several phenotypically different forms at the expense of individuals with an average norm. If environmental conditions have changed so much that the bulk of the species loses fitness, then individuals with extreme deviations from the average norm acquire an advantage. Such forms multiply rapidly and on the basis of one group several new ones are formed.

A model of disruptive selection can be the situation of the emergence of dwarf races of predatory fish in a water body with little food. Often, juveniles of the year do not have enough food in the form of fish fry. In this case, the advantage is gained by the fastest growing ones, which very quickly reach a size that allows them to eat their fellows. On the other hand, squints with the maximum delay in growth rate will be in an advantageous position, since their small size allows them to remain planktivorous for a long time. A similar situation through stabilizing selection can lead to the emergence of two races of predatory fish.

An interesting example is given by Darwin regarding insects - inhabitants of small oceanic islands. They fly well or are completely devoid of wings. Apparently, the insects were blown out to sea by sudden gusts of wind; only those that could either resist the wind or not fly at all survived. Selection in this direction has led to the fact that out of 550 species of beetles on the island of Madeira, 200 are flightless.

Another example: in forests where soils are brown, earth snail specimens often have brown and pink shells, in areas with coarse and yellow grass, yellow color prevails, etc.

Populations adapted to ecologically dissimilar habitats may occupy contiguous geographic areas; for example, in coastal areas of California, the Giliaachilleaefolia plant is represented by two races. One race - "sunny" - grows on open grassy southern slopes, while the "shady" race is found in shady oak forests and sequoia groves. These races differ in the size of the petals - a trait determined genetically.

The main result of this selection is the formation of population polymorphism, i.e. the presence of several groups that differ in some way or in the isolation of populations that differ in their properties, which may be the cause of divergence.

Conclusion

Like other elementary evolutionary factors, natural selection causes changes in the ratio of alleles in the gene pools of populations. Natural selection plays a creative role in evolution. By excluding genotypes with low adaptive value from reproduction, while preserving favorable gene combinations of different merits, he transforms the picture of genotypic variability, which is formed initially under the influence of random factors, in a biologically expedient direction.

Bibliography

1) Vlasova Z.A. Biology. Student Handbook - Moscow, 1997

2) Green N. Biology - Moscow, 2003

3) Kamluk L.V. Biology in questions and answers - Minsk, 1994

4) Lemeza N.A. Biology manual - Minsk, 1998