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

Living in natural conditions, there is individual variability, which can manifest itself in three forms - useful, neutral and harmful. Usually, organisms with harmful variability die at various stages of individual development. The neutral variability of organisms does not affect their viability. Individuals with beneficial variability survive by superiority in intraspecific, interspecific, or adverse conditions. environment.

driving selection

When environmental conditions change, those individuals of the species survive in which hereditary variability has manifested itself and, in connection with this, signs and properties have developed that correspond to new conditions, and those individuals that did not have such variability die. During his voyage, Darwin discovered that on oceanic islands where strong winds prevail, there are few long-winged insects and many insects with rudimentary wings and wingless insects. As Darwin explains, insects with normal wings could not withstand the strong winds on these islands and died. And insects with rudimentary wings and wingless did not rise at all into the air and hid in the cracks, finding shelter there. This process, which was accompanied by hereditary variability and natural selection and continued for many thousands of years, led to a decrease in the number of long-winged insects on these islands and the appearance of individuals with rudimentary wings and wingless insects. Natural selection, which ensures the emergence and development of new features and properties of organisms, is called motive selection.

Disruptive selection

Disruptive selection- this is a form of natural selection, leading to the formation of a number of polymorphic forms that differ from each other within the same population.

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. 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.

So, populations of lake frogs living in ponds with heterogeneous illumination, with alternating areas overgrown with duckweed, reed, cattail, with "windows" 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 leads to an 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 manifested in the display of their bright 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 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. Eventually, the tail reaches a length where its detriment to the viability of the male is balanced by its attractiveness in the eyes of the 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 efficiency of selection. 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. In the same way, favorable new gene combinations that manage to form in some local fraction of a given population are broken apart and eliminated as a result of crossing with individuals of neighboring fractions.

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 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 variability in crossing 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) Directional selection (moving)

Rice. one.

Directed 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.

Fig 2. The 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.

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 the leopard, is colored with small spots resembling sun glare, and in the 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. four.

Stabilizing selection in relatively constant conditions environment, natural selection is directed against individuals whose characteristics 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.

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 (Hb S) alley and leads to their death in 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:

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

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

the North American opossum is 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 (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 of brown color, specimens of earth snails often have brown and pink shells, in areas with rough 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 plant Gilia achilleaefolia 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

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

Green N. Biology - Moscow, 2003

Kamlyuk L.V. Biology in questions and answers - Minsk, 1994

Lemeza N.A. Biology manual - Minsk, 1998

From Wikipedia, the free encyclopedia

Natural selection- the main evolutionary process, as a result of which the number of individuals with maximum fitness (the most favorable traits) increases in the population, while the number of individuals with unfavorable traits decreases. In the light of the modern synthetic theory of evolution, natural selection is seen as main reason development of adaptations , speciation and origin of supraspecific taxa . Natural selection is the only known cause of adaptations, but not the only cause of evolution. Non-adaptive causes include genetic drift, gene flow, and mutations.

The term "natural selection" was popularized by Charles Darwin, comparing this process with artificial selection, modern form which is the selection . The idea of ​​comparing artificial and natural selection is that in nature the selection of the most “successful”, “best” organisms also takes place, but in this case it is not a person who acts as an “appraiser” of the usefulness of properties, but the environment. In addition, the material for both natural and artificial selection are small hereditary changes that accumulate from generation to generation.

Mechanism of natural selection

In the process of natural selection, mutations are fixed that increase the fitness of organisms. Natural selection is often referred to as a "self-evident" mechanism because it follows from simple facts such as:

1. Organisms produce more offspring than can survive;
2. In the population of these organisms, there is hereditary variability;
3. Organisms that have different genetic traits have different survival rates and ability to reproduce.

The central concept of the concept of natural selection is the fitness of organisms. Fitness is defined as the ability of an organism to survive and reproduce, which determines the size of its genetic contribution to the next generation. However, the main thing in determining fitness is not the total number of offspring, but the number of offspring with a given genotype (relative fitness). For example, if the offspring of a successful and rapidly reproducing organism are weak and do not reproduce well, then the genetic contribution and, accordingly, the fitness of this organism will be low.

Natural selection for traits that can vary over some range of values ​​(such as the size of an organism) can be divided into three types:

1. Directed Selection- changes in the average value of the trait over time, for example, an increase in body size;
2. Disruptive selection- selection for the extreme values ​​of the trait and against the average values, for example, large and small body sizes;
3. Stabilizing selection- selection against the extreme values ​​of the trait, which leads to a decrease in the variance of the trait.

A special case of natural selection is sexual selection, the substrate of which is any trait that increases the success of mating by increasing the attractiveness of an individual for potential partners. Traits that have evolved through sexual selection are particularly evident in the males of certain animal species. Traits such as large horns, bright colors, on the one hand, can attract predators and reduce the survival rate of males, and on the other hand, this is balanced by the reproductive success of males with similar pronounced traits.

Selection can operate at various levels of organization such as genes, cells, individual organisms, groups of organisms, and species. Moreover, selection can act simultaneously at different levels. Selection at levels above the individual, such as group selection, can lead to cooperation (see Evolution#Cooperation).

Forms of natural selection

There are different classifications of forms of selection. A classification based on the nature of the influence of selection forms on the variability of a trait in a population is widely used.

driving selection

driving selection- a form of natural selection that operates under directed changing environmental conditions. Described by Darwin and Wallace. In this case, individuals with traits that deviate in a certain direction from the average value receive advantages. At the same time, other variations of the trait (its deviations in the opposite direction from the average value) are subjected to negative selection. As a result, in the population from generation to generation, there is a shift in the average value of the trait in a certain direction. At the same time, the pressure of driving selection must correspond to the adaptive capabilities of the population and the rate of mutational changes (otherwise, environmental pressure can lead to extinction).

An example of the action of motive selection is "industrial melanism" in insects. "Industrial melanism" is a sharp increase in the proportion of melanistic (having a dark color) individuals in those populations of insects (for example, butterflies) that live in industrial areas. Due to industrial impact, tree trunks darkened significantly, and light lichens also died, which made light butterflies more visible to birds, and dark ones worse. In the 20th century, in a number of areas, the proportion of dark-colored butterflies in some well-studied populations of the birch moth in England reached 95%, while for the first time a dark butterfly ( Morfa carbonaria) was captured in 1848.

Driving selection is carried out when the environment changes or adapts to new conditions with the expansion of the range. It preserves hereditary changes in a certain direction, moving the reaction rate accordingly. For example, during the development of the soil as a habitat for various unrelated groups of animals, the limbs turned into burrowing ones.

Stabilizing selection

Stabilizing selection- a form of natural selection, in which its action is directed against individuals with extreme deviations from the average norm, in favor of individuals with an average severity of the trait. The concept of stabilizing selection was introduced into science and analyzed by I. I. Shmalgauzen.

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 sparrows that died after a storm in the 50s near Leningrad 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.

Disruptive selection

Disruptive (tearing) selection- a form of natural selection, in which conditions favor two or more extreme variants (directions) of variability, but do not favor the intermediate, average state of the trait. As a result, several new forms may appear from one initial one. Darwin described the operation of disruptive selection, believing that it underlies divergence, although he could not provide evidence for its existence in nature. Disruptive selection contributes to the emergence and maintenance of population polymorphism, and in some cases can cause speciation.

One of the possible situations in nature in which disruptive selection comes into play is when a polymorphic population occupies a heterogeneous habitat. Wherein different forms adapt to different ecological niches or sub-niches.

An example of disruptive selection is the formation of two races in a large rattle in hay meadows. Under normal conditions, the flowering and seed ripening periods of this plant cover the whole summer. But in hay meadows, seeds are produced mainly by those plants that have time to bloom and ripen either before the mowing period, or bloom at the end of summer, after mowing. As a result, two races of the rattle are formed - early and late flowering.

Disruptive selection was carried out artificially in experiments with Drosophila. The selection was carried out according to the number of setae, leaving only individuals with a small and large number of setae. As a result, from about the 30th generation, the two lines diverged very strongly, despite the fact that the flies continued to interbreed with each other, exchanging genes. In a number of other experiments (with plants), intensive crossing prevented effective action disruptive selection.

sexual selection

sexual selection This is natural selection for success in reproduction. 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. Darwin called this phenomenon sexual selection. "This form of selection is determined not by the struggle for existence in the relations of organic beings among themselves or with external conditions, but by the rivalry between individuals of one sex, usually males, for the possession of individuals of the other sex." 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.

Two hypotheses about the mechanisms of sexual selection are common.

• According to the “good genes” hypothesis, the female “argues” as follows: “If this male, despite the bright plumage and long tail, managed not to die in the clutches of a predator and survive to puberty, then he has good genes that allowed him to do this. Therefore, he should be chosen as the father of his children: he will pass on his good genes to them. By choosing bright males, females choose good genes for their offspring.
• According to the “attractive sons” hypothesis, the logic of female selection is somewhat different. If bright males, for whatever reason, are attractive to females, 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 increases more and more. The process goes on increasing until it reaches the limit of viability.

When choosing males, females do not think about the reasons for their behavior. 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. In the same way, females, choosing bright males, follow their instincts - they like bright tails. Those who instinctively prompted a different behavior did not leave offspring. The logic of the struggle for existence and natural selection is the logic of a blind and automatic process that, acting constantly from generation to generation, has formed that amazing variety of forms, colors and instincts that we observe in the world of wildlife.

Selection methods: positive and negative selection

There are two forms of artificial selection: Positive and Clipping (negative) selection.

Positive selection increases the number of individuals in the population that have useful traits that increase the viability of the species as a whole.

Cut-off selection culls out from the population the vast majority of individuals that carry traits that sharply reduce viability under given environmental conditions. With the help of cut-off selection, strongly harmful alleles are removed from the population. Also, individuals with chromosomal rearrangements and a set of chromosomes that sharply disrupt normal work genetic apparatus.

The role of natural selection in evolution

In the example of the worker ant, we have an insect extremely different from its parents, yet absolutely barren and therefore unable to transmit from generation to generation acquired modifications of structure or instincts. One can ask a good question - to what extent is it possible to reconcile this case with the theory of natural selection?

- Origin of Species (1859)

Darwin assumed that selection could be applied not only to the individual organism, but also to the family. He also said that, perhaps, to one degree or another, this can also explain the behavior of people. He turned out to be right, but it was not until the advent of genetics that it became possible to provide a more expanded view of this concept. The first outline of the "kind selection theory" was made by the English biologist William Hamilton in 1963, who was the first to propose considering natural selection not only at the level of an individual or a whole family, but also at the level of a gene.

see also

Write a review on the article "Natural Selection"

Notes

1. , With. 43-47.
2. , p. 251-252.
3. OrrHA// Nat Rev Genet. - 2009. - Vol. 10(8). - P. 531-539.
4. Haldane J// Nature. - 1959. - Vol. 183. - P. 710-713.
5. Lande R, Arnold SJ The measurement of selection on correlated characters // Evolution. - 1983. - Vol. 37.-P. 1210–26. - DOI:10.2307/2408842.
6. .
7. , Chapter 14.
8. Andersson M, Simmons L// Trends Ecol Evol. - 2001. - Vol. 21(6). - P. 296-302.
9. Kokko H, Brooks R, McNamara J, Houston A// Proc Biol Sci. - 2002. - Vol. 269. - P. 1331-1340.
10. Hunt J, Brooks R, Jennions MD, Smith MJ, Bentsen CL, Bussière LF// Nature. - 2004. - Vol. 432. - P. 1024-1027.
11. Okasha, S. Evolution and the Levels of Selection. - Oxford University Press, 2007. - 263 p. - ISBN 0-19-926797-9.
12. Mayr E// Philos. Trans. R. Soc. Lond., B, Biol. sci. - 1998. - T. 353. - pp. 307–14.
13. Maynard Smith J// Novartis Found. Symp. - 1998. - T. 213. - pp. 211–217.
14. Gould SJ, Lloyd EA//Proc. Natl. Acad. sci. U.S.A. - 1999. - T. 96, No. 21. - S. 11904–11909.

Literature

• Lua error: attempt to index local "entity" (a nil value).

Links

• - an article with well-known examples: the color of butterflies, people's resistance to malaria, etc.
• - Chapter 4, Natural Selection
• - Modeling for Understanding in Science Education, University of Wisconsin
• University of Berkeley education website
• Evolution: Education and Outreach

An excerpt characterizing Natural selection

At four o'clock in the afternoon, Murat's troops entered Moscow. In front rode a detachment of Wirtemberg hussars, behind on horseback, with a large retinue, the Neapolitan king himself rode.
Near the middle of the Arbat, near Nikola Yavlenny, Murat stopped, waiting for news from the advance detachment about the situation in the city fortress "le Kremlin".
Around Murat, a small group of people from the residents who remained in Moscow gathered. Everyone looked with timid bewilderment at the strange, long-haired chief adorned with feathers and gold.
- Well, is it himself, or what, their king? Nothing! quiet voices were heard.
The interpreter drove up to a bunch of people.
“Take off your hat… take off your hat,” they started talking in the crowd, addressing each other. The interpreter turned to an old janitor and asked how far it was to the Kremlin? The janitor, listening with bewilderment to the Polish accent alien to him and not recognizing the sounds of the interpreter as Russian, did not understand what was said to him and hid behind the others.
Murat moved up to the interpreter and ordered him to ask where the Russian troops were. One of the Russian people understood what was being asked of him, and several voices suddenly began to answer the interpreter. A French officer from the advance detachment rode up to Murat and reported that the gates to the fortress were closed up and that there was probably an ambush there.
- Good, - said Murat and, turning to one of the gentlemen of his retinue, he ordered four light guns to be advanced and fired at the gates.
Artillery trotted out from behind the column following Murat and drove along the Arbat. Having descended to the end of Vzdvizhenka, the artillery stopped and lined up on the square. Several French officers disposed of the cannons, placing them, and looked at the Kremlin through a telescope.
In the Kremlin, the bell was heard for Vespers, and this ringing embarrassed the French. They assumed it was a call to arms. Several infantry soldiers ran to the Kutafiev Gate. Logs and plank shields lay in the gates. Two rifle shots rang out from under the gate as soon as the officer with the team began to run up to them. The general, who was standing by the guns, shouted command words to the officer, and the officer with the soldiers ran back.
Three more shots were heard from the gate.
One shot hit a French soldier in the leg, and a strange cry from a few voices was heard from behind the shields. On the faces of the French general, officers and soldiers at the same time, as if on command, the former expression of cheerfulness and calmness was replaced by a stubborn, concentrated expression of readiness for struggle and suffering. For all of them, from the marshal to the last soldier, this place was not Vzdvizhenka, Mokhovaya, Kutafya and Trinity Gates, but it was a new area of ​​a new field, probably a bloody battle. And everyone is ready for this battle. The screams from the gates ceased. The guns were advanced. The gunners blew off their burnt overcoats. The officer commanded "feu!" [fall!], and two whistling sounds of tin cans were heard one after another. Card-shot bullets crackled on the stone of the gate, logs and shields; and two clouds of smoke wavered in the square.
A few moments after the rolling of shots on the stone Kremlin had died down, a strange sound was heard over the heads of the French. A huge flock of jackdaws rose above the walls and, croaking and rustling with thousands of wings, circled in the air. Together with this sound, a lonely human cry was heard at the gate, and from behind the smoke appeared the figure of a man without a hat, in a caftan. Holding a gun, he aimed at the French. Feu! - repeated the artillery officer, and at the same time one rifle and two gun shots were heard. The smoke closed the gate again.
Nothing else moved behind the shields, and the French infantry soldiers with officers went to the gate. There were three wounded and four dead people in the gate. Two men in caftans ran downstairs, along the walls, towards Znamenka.
- Enlevez moi ca, [Take it away,] - said the officer, pointing to the logs and corpses; and the French, having finished off the wounded, threw the corpses down behind the fence. Who these people were, no one knew. “Enlevez moi ca” is only said about them, and they were thrown away and cleaned up afterwards so that they would not stink. One Thiers dedicated several eloquent lines to their memory: “Ces miserables avaient envahi la citadelle sacree, s "etaient empares des fusils de l" arsenal, et tiraient (ces miserables) sur les Francais. On en sabra quelques "uns et on purgea le Kremlin de leur presence. [These unfortunates filled the sacred fortress, took possession of the guns of the arsenal and fired at the French. Some of them were chopped down with sabers, and the Kremlin was cleared of their presence.]
Murat was informed that the path had been cleared. The French entered the gate and began to camp on the Senate Square. Soldiers threw chairs out of the windows of the senate into the square and laid out fires.
Other detachments passed through the Kremlin and were stationed along Maroseyka, Lubyanka, and Pokrovka. Still others were located along Vzdvizhenka, Znamenka, Nikolskaya, Tverskaya. Everywhere, not finding owners, the French were placed not like in the city in apartments, but like in a camp located in the city.
Although ragged, hungry, exhausted and reduced to 1/3 of their former strength, the French soldiers entered Moscow in orderly order. It was an exhausted, exhausted, but still fighting and formidable army. But this was an army only until the moment when the soldiers of this army dispersed to their quarters. As soon as the people of the regiments began to disperse to empty and rich houses, the army was forever destroyed and not residents and not soldiers were formed, but something in between, called marauders. When, after five weeks, the same people left Moscow, they no longer constituted an army. It was a crowd of marauders, each of whom was carrying or carrying with him a bunch of things that he thought were valuable and needed. The goal of each of these people when leaving Moscow was not, as before, to win, but only to keep what they had acquired. Like that monkey who, having put his hand into the narrow throat of a jug and seized a handful of nuts, does not open his fist so as not to lose what he has seized, and this destroys himself, the French, when leaving Moscow, obviously had to die due to the fact that they were dragging with loot, but it was as impossible for him to give up this loot as it is impossible for a monkey to unclench a handful of nuts. Ten minutes after the entry of each French regiment into some quarter of Moscow, not a single soldier and officer remained. In the windows of the houses one could see people in overcoats and boots, laughingly pacing around the rooms; in the cellars, in the cellars, the same people were in charge with provisions; in the yards, the same people unlocked or beat off the gates of sheds and stables; fires were laid out in the kitchens, with rolled up hands they baked, kneaded and boiled, frightened, made laugh and caressed women and children. And there were many of these people everywhere, both in shops and in houses; but the troops were gone.
On the same day, order after order was given by the French commanders to forbid the troops to disperse around the city, to strictly prohibit the violence of the inhabitants and looting, to make a general roll call that very evening; but no matter what measures. the people who had previously made up the army spread out over the rich, abundant in amenities and supplies, empty city. Just as a hungry herd marches in a heap across a bare field, but immediately disperses irresistibly as soon as it attacks rich pastures, so the army dispersed irresistibly throughout a rich city.
There were no inhabitants in Moscow, and the soldiers, like water into the sand, soaked into it and spread like an unstoppable star in all directions from the Kremlin, into which they entered first of all. The cavalry soldiers, entering the merchant's house left with all the good and finding stalls not only for their horses, but also superfluous, nevertheless went side by side to occupy another house, which seemed better to them. Many occupied several houses, writing with chalk what he was doing, and arguing and even fighting with other teams. Not having time to fit yet, the soldiers ran out into the street to inspect the city and, according to the rumor that everything was abandoned, rushed to where they could pick up valuable things for free. The commanders went to stop the soldiers and themselves were involuntarily involved in the same actions. There were shops with carriages in Karetny Ryad, and the generals crowded there, choosing carriages and carriages for themselves. The remaining residents invited the chiefs to their place, hoping that they would be protected from robbery. There was an abyss of wealth, and there was no end in sight; everywhere, around the place that the French had occupied, there were still unexplored, unoccupied places in which, as it seemed to the French, there were still more riches. And Moscow sucked them further and further into itself. Exactly as due to the fact that water is poured onto dry land, water and dry land disappear; in the same way, because a hungry army entered a plentiful, empty city, the army was destroyed, and a plentiful city was destroyed; and there was dirt, fires and looting.

The French attributed the fire of Moscow to au patriotisme feroce de Rastopchine [Rastopchin's wild patriotism]; Russians - to the fanaticism of the French. In essence, there were no such reasons and could not be. Moscow burned down due to the fact that it was placed in such conditions under which any wooden city must burn down, regardless of whether or not there are one hundred and thirty bad fire pipes in the city. Moscow had to burn down due to the fact that the inhabitants left it, and just as inevitably as a pile of shavings should catch fire, on which sparks of fire would fall for several days. A wooden city, in which there are fires almost every day in the summer with residents, owners of houses and with the police, cannot help but burn when there are no inhabitants in it, but troops live, smoking pipes, laying fires on Senate Square from Senate chairs and cooking themselves two times a day. In peacetime it is necessary for troops to settle down in apartments in villages in a certain area, and the number of fires in this area immediately increases. To what extent should the probability of fires increase in an empty wooden city in which a foreign army is stationed? Le patriotisme feroce de Rastopchine and the savagery of the French are not to blame for anything here. Moscow caught fire from pipes, from kitchens, from bonfires, from the slovenliness of enemy soldiers, residents - not the owners of houses. If there were arson (which is very doubtful, because there was no reason for anyone to set fire, and, in any case, troublesome and dangerous), then arson cannot be taken as a reason, since without arson it would be the same.
No matter how flattering it was for the French to blame the atrocities of Rastopchin and for the Russians to blame the villain Bonaparte or then to put the heroic torch into the hands of their people, one cannot help but see that there could not be such a direct cause of the fire, because Moscow had to burn down, as every village, factory should burn down , any house from which the owners will come out and into which they will be allowed to host and cook their own porridge of strangers. Moscow is burned down by the inhabitants, it is true; but not by those inhabitants who remained in it, but by those who left it. Moscow, occupied by the enemy, did not remain intact, like Berlin, Vienna and other cities, only due to the fact that its inhabitants did not bring bread of salt and keys to the French, but left it.

On the day of September 2, the French invasion, spreading like a star across Moscow, reached the quarter in which Pierre now lived, only in the evening.
Pierre was in a state close to insanity after the last two, solitary and unusually spent days. His whole being was seized by one obsessive thought. He himself did not know how and when, but this thought now took possession of him so that he remembered nothing of the past, did not understand anything of the present; and everything he saw and heard happened before him as in a dream.
Pierre left his home only in order to get rid of the complex confusion of the demands of life that had seized him, and which he, in his then state, but was able to unravel. He went to Iosif Alekseevich's apartment under the pretext of sorting through the books and papers of the deceased, only because he was looking for peace from life's anxiety - and with the memory of Iosif Alekseevich, a world of eternal, calm and solemn thoughts was associated in his soul, completely opposite to the disturbing confusion in which he felt drawn in. He was looking for a quiet refuge and indeed found it in the office of Joseph Alekseevich. When, in the dead silence of the office, he sat down, leaning on his hands, over a dusty desk the deceased, in his imagination calmly and significantly, one after another, memories of the last days began to appear, especially the battle of Borodino and that indefinable feeling for him of his insignificance and deceit in comparison with the truth, simplicity and strength of that category of people who were imprinted in him in soul called they. When Gerasim woke him from his reverie, Pierre had the idea that he would take part in the alleged - as he knew - people's defense of Moscow. And for this purpose, he immediately asked Gerasim to get him a caftan and a pistol and announced to him his intention, hiding his name, to stay in the house of Joseph Alekseevich. Then, in the course of the first solitary and idle day spent (Pierre tried several times and could not stop his attention on Masonic manuscripts), several times he vaguely imagined the thought that had previously come about the cabalistic meaning of his name in connection with the name of Bonaparte; but this thought that he, l "Russe Besuhof, is destined to put an end to the power of the beast, came to him only as one of the dreams that run through his imagination for no reason and without a trace.
When, having bought a caftan (with the aim of only participating in the people's defense of Moscow), Pierre met the Rostovs and Natasha told him: “Are you staying? Oh, how good it is! - the thought flashed through his head that it would really be good, even if they took Moscow, he would stay in it and fulfill what was predetermined for him.
The next day, with one thought not to feel sorry for himself and not to lag behind them in anything, he went with the people beyond the Trekhgornaya outpost. But when he returned home, convinced that Moscow would not be defended, he suddenly felt that what had previously seemed to him only a possibility had now become a necessity and inevitability. He had to, hiding his name, stay in Moscow, meet Napoleon and kill him in order to either die or stop the misfortune of all of Europe, which, according to Pierre, came from Napoleon alone.
Pierre knew all the details of the attempt on the life of a German student by Bonaparte in Vienna in 1809 and knew that this student was shot. And the danger to which he exposed his life in the fulfillment of his intention excited him even more.
Two equally strong feelings irresistibly attracted Pierre to his intention. The first was the feeling of the need for sacrifice and suffering in the consciousness of general misfortune, that feeling, as a result of which he went to Mozhaisk on the 25th and drove into the heat of battle, now ran away from his house and, instead of the usual luxury and comforts of life, slept without undressing on hard couch and ate the same meal with Gerasim; the other was that indefinite, exclusively Russian feeling of contempt for everything conventional, artificial, human, for everything that is considered by most people to be the highest good of the world. For the first time, Pierre experienced this strange and charming feeling in the Sloboda Palace, when he suddenly felt that wealth, and power, and life, everything that people arrange and cherish with such diligence - if all this is worth something, then only for the pleasure with which all this can be thrown.
It was the feeling that makes a hunter-recruit drink his last penny, a drunken man breaks mirrors and glass for no apparent reason and knowing that it will cost him his last money; that feeling, as a result of which a person, committing (in the vulgar sense) crazy deeds, as if tries his personal power and strength, declaring the presence of a higher, standing outside human conditions, judgment over life.
From the very day that Pierre first experienced this feeling in the Sloboda Palace, he was incessantly under his influence, but now he only found him complete satisfaction. In addition, at the present moment, Pierre was supported in his intention and deprived of the opportunity to renounce him by what he had already done along the way. And his flight from home, and his caftan, and the pistol, and his statement to Rostov that he was staying in Moscow - everything would not only lose its meaning, but all this would be contemptible and ridiculous (to which Pierre was sensitive), if after all this, like the others, he left Moscow.

The idea of ​​comparing artificial and natural selection is that in nature the selection of the most “successful”, “best” organisms also takes place, but in this case it is not a person who acts as an “appraiser” of the usefulness of properties, but the environment. In addition, the material for both natural and artificial selection are small hereditary changes that accumulate from generation to generation.

Mechanism of natural selection

In the process of natural selection, mutations are fixed that increase the adaptability of organisms to their environment. Natural selection is often referred to as a "self-evident" mechanism because it follows from simple facts such as:

1. Organisms produce more offspring than can survive;
2. In the population of these organisms, there is hereditary variability;
3. Organisms that have different genetic traits have different survival rates and ability to reproduce.

The central concept of the concept of natural selection is the fitness of organisms. Fitness is defined as the ability of an organism to survive and reproduce in its existing environment. This determines the size of his genetic contribution to the next generation. However, the main thing in determining fitness is not the total number of offspring, but the number of offspring with a given genotype (relative fitness). For example, if the offspring of a successful and rapidly reproducing organism are weak and do not reproduce well, then the genetic contribution and, accordingly, the fitness of this organism will be low.

Natural selection for traits that can vary over some range of values ​​(such as the size of an organism) can be divided into three types:

1. Directed Selection- changes in the average value of the trait over time, for example, an increase in body size;
2. Disruptive selection- selection for the extreme values ​​of the trait and against the average values, for example, large and small body sizes;
3. Stabilizing selection- selection against the extreme values ​​of the trait, which leads to a decrease in the variance of the trait.

A special case of natural selection is sexual selection, the substrate of which is any trait that increases the success of mating by increasing the attractiveness of an individual for potential partners. Traits that have evolved through sexual selection are particularly evident in the males of certain animal species. Traits such as large horns, bright colors, on the one hand, can attract predators and reduce the survival rate of males, and on the other hand, this is balanced by the reproductive success of males with similar pronounced traits.

Selection can operate at various levels of organization such as genes, cells, individual organisms, groups of organisms, and species. Moreover, selection can act simultaneously at different levels. Selection at levels above the individual, such as group selection, can lead to cooperation (see Evolution#Cooperation).

Forms of natural selection

There are different classifications of forms of selection. A classification based on the nature of the influence of selection forms on the variability of a trait in a population is widely used.

driving selection

driving selection- a form of natural selection that operates under directed changing environmental conditions. Described by Darwin and Wallace. In this case, individuals with traits that deviate in a certain direction from the average value receive advantages. At the same time, other variations of the trait (its deviations in the opposite direction from the average value) are subjected to negative selection. As a result, in the population from generation to generation, there is a shift in the average value of the trait in a certain direction. At the same time, the pressure of driving selection must correspond to the adaptive capabilities of the population and the rate of mutational changes (otherwise, environmental pressure can lead to extinction).

An example of the action of motive selection is "industrial melanism" in insects. "Industrial melanism" is a sharp increase in the proportion of melanistic (having a dark color) individuals in those populations of insects (for example, butterflies) that live in industrial areas. Due to industrial impact, tree trunks darkened significantly, and light lichens also died, which made light butterflies more visible to birds, and dark ones worse. In the 20th century, in a number of regions, the proportion of dark-colored butterflies in some well-studied populations of the birch-moth in England reached 95%, while for the first time the dark-colored butterfly ( Morfa carbonaria) was captured in 1848.

Driving selection is carried out when the environment changes or adapts to new conditions with the expansion of the range. It preserves hereditary changes in a certain direction, moving the norm of the reaction accordingly. For example, during the development of the soil as a habitat for various unrelated groups of animals, the limbs turned into burrowing ones.

Stabilizing selection

Stabilizing selection- a form of natural selection, in which its action is directed against individuals with extreme deviations from the average norm, in favor of individuals with an average severity of the trait. The concept of stabilizing selection was introduced into science and analyzed by I. I. Shmalgauzen.

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 sparrows that died after a storm in the 50s near Leningrad 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.

Disruptive selection

Disruptive (tearing) selection- a form of natural selection, in which conditions favor two or more extreme variants (directions) of variability, but do not favor the intermediate, average state of the trait. As a result, several new forms may appear from one initial one. Darwin described the operation of disruptive selection, believing that it underlies divergence, although he could not provide evidence for its existence in nature. Disruptive selection contributes to the emergence and maintenance of population polymorphism, and in some cases can cause speciation.

One of the possible situations in nature in which disruptive selection comes into play is when a polymorphic population occupies a heterogeneous habitat. At the same time, different forms adapt to different ecological niches or subniches.

An example of disruptive selection is the formation of two races in a large rattle in hay meadows. Under normal conditions, the flowering and seed ripening periods of this plant cover the whole summer. But in hay meadows, seeds are produced mainly by those plants that have time to bloom and ripen either before the mowing period, or bloom at the end of summer, after mowing. As a result, two races of the rattle are formed - early and late flowering.

Disruptive selection was carried out artificially in experiments with Drosophila. The selection was carried out according to the number of setae, leaving only individuals with a small and large number of setae. As a result, from about the 30th generation, the two lines diverged very strongly, despite the fact that the flies continued to interbreed with each other, exchanging genes. In a number of other experiments (with plants), intensive crossing prevented the effective action of disruptive selection.

sexual selection

sexual selection This is natural selection for success in reproduction. 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. Darwin called this phenomenon sexual selection. "This form of selection is determined not by the struggle for existence in the relations of organic beings among themselves or with external conditions, but by the rivalry between individuals of one sex, usually males, for the possession of individuals of the other sex." 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.

Two hypotheses about the mechanisms of sexual selection are common.

• According to the “good genes” hypothesis, the female “reasons” as follows: “If this male, despite the bright plumage and long tail, managed not to die in the clutches of a predator and survive to puberty, then he has good genes that allowed him to do this . Therefore, he should be chosen as the father of his children: he will pass on his good genes to them. By choosing bright males, females choose good genes for their offspring.
• According to the “attractive sons” hypothesis, the logic of female selection is somewhat different. If bright males, for whatever reason, are attractive to females, 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 increases more and more. The process goes on increasing until it reaches the limit of viability.

When choosing males, females do not think about the reasons for their behavior. 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. In the same way, females, choosing bright males, follow their instincts - they like bright tails. Those who instinctively prompted a different behavior did not leave offspring. The logic of the struggle for existence and natural selection is the logic of a blind and automatic process that, acting constantly from generation to generation, has formed that amazing variety of forms, colors and instincts that we observe in the world of wildlife.

Selection methods: positive and negative selection

There are two forms of artificial selection: Positive and Clipping (negative) selection.

Positive selection increases the number of individuals in the population that have useful traits that increase the viability of the species as a whole.

Cut-off selection culls out from the population the vast majority of individuals that carry traits that sharply reduce viability under given environmental conditions. With the help of cut-off selection, strongly harmful alleles are removed from the population. Also, individuals with chromosomal rearrangements and a set of chromosomes that sharply disrupt the normal operation of the genetic apparatus can be subjected to cutting selection.

The role of natural selection in evolution

In the example of the worker ant, we have an insect extremely different from its parents, yet absolutely barren and therefore unable to transmit from generation to generation acquired modifications of structure or instincts. One can ask a good question - to what extent is it possible to reconcile this case with the theory of natural selection?

- Origin of Species (1859)

Darwin assumed that selection could be applied not only to the individual organism, but also to the family. He also said that, perhaps, to one degree or another, this can also explain the behavior of people. He turned out to be right, but it was not until the advent of genetics that it became possible to provide a more expanded view of this concept. The first outline of the "kind selection theory" was made by the English biologist William Hamilton in 1963, who was the first to propose considering natural selection not only at the level of an individual or a whole family, but also at the level of a gene.

see also

Notes

1. , With. 43-47.
2. , p. 251-252.
3. Orr H.A. Fitness and its role in evolutionary genetics // Nature Reviews Genetics. - 2009. - Vol. 10, no. 8. - P. 531-539. - DOI:10.1038/nrg2603. - PMID 19546856 .
4. Haldane J.B.S. The theory of natural selection today // Nature. - 1959. - Vol. 183, no. 4663. - P. 710-713. - PMID 13644170 .
5. Lande R., Arnold S. J. The measurement of selection on correlated characters // Evolution. - 1983. - Vol. 37, no. 6. - P. 1210-1226. -