The concept of natural selection in biology. Report: Natural selection

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Natural selection and its forms

1. What factors external environment can lead to selection of organisms in nature?
2. Is the relationship between man and nature a selection factor?

Doctrine of natural selection developed by Charles Darwin, who considered selection itself to be the result of the struggle for existence, and its prerequisite was the hereditary variability of organisms.

The genetic essence of natural selection is selective preservation in populations certain genotypes. The hereditary material they contain is passed on to subsequent generations. Thus, natural selection can be defined as selective reproduction genotypes, which best meet the current living conditions of the population. In 9th grade, you have already become familiar with some examples of the action of natural selection that can be observed in experiment or in nature.

Let's consider another experiment showing how, in the course of natural selection, the connection between phenotypes and genotypes in a population is carried out. In nature, there are some types of fruit flies that find their favorite food either on the tops of trees or on the surface soil, but never in the middle. Is it possible to select insects that fly either only downwards or only upwards? Figure 73 shows a diagram of an experiment demonstrating the influence of selection on the genetic composition of populations. Fruit flies were placed in a maze consisting of many chambers, each of which had two exits - up and down. In each of the chambers the animal had to “decide” in which direction to move. The flies, constantly moving upward, eventually ended up in the upper exit from the maze. They were carefully selected for subsequent maintenance. Flies that moved downward ended up in the lower exit from the maze, and they were also selected. Insects, remaining in the chambers of the maze, i.e. those that did not have a specific direction of movement, were collected and removed from the experiment. “Top” and “bottom” flies were kept and bred separately from each other. Gradually, it was possible to create populations, all individuals of which, without exception, had a certain behavioral stereotype (movement up or down). This result was not associated with the appearance of any new genes; everything happened only due to selection, which acted on the variability of phenotypes that already existed in the population (in this case, the variability in the behavior of flies). Thus, the action of natural selection leads to the fact that phenotypes begin to influence the gene pool of populations. What happens if you remove the pressure of natural selection? To answer this question, experimenters allowed flies from the “upper” and “lower” tiers to reproduce together. Soon the initial balance of alleles was restored in the population: some individuals moved up, some down, while others did not show any preferences regarding the direction of movement.

Natural selection changes the composition of the gene pool, “removing” from the population individuals whose characteristics and properties do not provide advantages in the struggle for existence. As a result of selection genetic material“advanced” individuals (i.e., those with properties that increase their chances in the struggle for life) begin to increasingly influence the gene pool of the entire population.

In the course of natural selection, amazing and diverse biological adaptations (adaptations) of organisms to the environmental conditions in which the population lives take place are generated. For example, general adaptations, which include the ability to swim in organisms living in aquatic environment, or the adaptability of the limbs of vertebrates to the terrestrial environment, and particular adaptations:

adaptability to running in horses, antelopes, ostriches, digging in moles, mole rats, or climbing trees (monkeys, woodpeckers, pikas, etc.). Examples of adaptation are camouflage coloring, mimicry (imitation of the peaceful appearance of the external appearance of an animal, well protected from attack by predators), and complex behavioral instincts, and many others. etc. (Fig. 74), It should be remembered that all adaptation is relative. A species that is well adapted to given conditions may be on the verge of extinction if conditions change or a new predator or competitor appears in the environment. It is known, for example, that fish, well protected from predators by thorns and thorns, more often end up in fisherman’s nets, in which they become entangled and held precisely because of the hard outgrowths of the body. It is not for nothing that one of the principles (of evolutionary teaching) in a humorous form sounds like this: “The fittest survive, but they are the fittest only as long as they survive.”

So, opportunities for evolutionary changes in a population are always present. For the time being, they manifest themselves only in the variability of organisms. As soon as selection begins to act, the population responds with adaptive changes.

Previously, you were introduced to the two main forms of natural selection: stabilizing and driving. Let us recall that stabilizing selection is aimed at maintaining existing phenotypes. Its action can be illustrated by Figure 75. This form of selection usually operates where living conditions remain constant for a long time, such as in northern latitudes or on the ocean floor.

The second form of natural selection is driving; In contrast to stabilizing, this form of selection promotes changes in organisms. As a rule, the effects of natural selection become noticeable over long periods of time. Although sometimes driving selection can manifest itself very quickly in response to unexpected and strong changes in external conditions (Fig. 76). A classic example of the action of driving selection is provided by the study of peppered moths, which change color under the influence of soot emissions and sooty tree trunks in industrial areas of England in the 19th century. (Fig. 78).

The third form of natural selection is disruptive, or tearing. Discontinuous selection leads to the emergence within populations of groups of individuals that differ in some characteristics (color, behavior, space, etc.). Disruptive selection promotes the maintenance of two or more phenotypes within populations and eliminates intermediate forms (Fig. 77). There is a kind of rupture in the population according to a certain characteristic. This phenomenon is called polymorphism. Polymorphism is characteristic of many species of animals and plants. For example, sockeye salmon, a salmon fish of the Far East that spends its life in the sea and breeds in small fresh lakes connected to the sea by rivers, has a so-called “residential form”, represented by small dwarf males that never leave the lakes. Among some bird species (skuas, cuckoos, etc.) color morphs are common. The two-spotted ladybug exhibits seasonal polymorphism. Of the two color forms, “red” ladybugs They survive better in winter, and “black” ones survive better in summer. The occurrence of polymorphism is apparently largely determined by the heterogeneity (seasonal or spatial) of the living conditions of the population, which gives rise to selection leading to the emergence of specialized forms (corresponding to heterogeneous conditions) within one population.

The creative role of natural selection.

It must be emphasized that the role of natural selection is not only limited to the elimination of individual non-viable organisms. The driving form of natural selection preserves not individual characteristics of the organism, but their entire complex, all combinations of genes inherent in the organism. Natural selection is often compared to the activity of a sculptor. Just as a sculptor creates a work from a shapeless block of marble that amazes with the harmony of all its parts, so selection creates adaptations and species, removing from the gene pool of a population genotypes that are ineffective from the point of view of survival. This is the creative role of natural selection, since the result of its action is new types of organisms, new forms of life.

Natural selection. Biological adaptations. Forms of natural selection: stabilizing, driving, disruptive. Polymorphism.

1. What is fitness? Why is it relative?
2. What is stabilizing selection? Under what conditions is its effect most noticeable?
3. What is driving selection? Give examples of its action. Under what conditions does this form of selection operate?
4. What is the creative role of natural selection? Give an example that proves that the action of selection is not limited to the elimination of individual traits that reduce the survival of organisms.

Kamensky A. A., Kriksunov E. V., Pasechnik V. V. Biology 10th grade
Submitted by readers from the website

Natural selection is the driving factor of evolution. Mechanism of action of selection. Forms of selection in populations (I.I. Shmalgauzen).

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

The term "Natural selection" was popularized by Charles Darwin when he compared this process with artificial selection, the modern form of which is selective breeding. The idea of ​​comparing artificial and natural selection is that in nature the selection of the most “successful”, “best” organisms also occurs, but in this case the role of “evaluator” of the usefulness of properties is not a person, but the environment. In addition, the material for both natural and artificial selection is 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 called a "self-evident" mechanism because it follows from such simple facts as:

Organisms produce more offspring than can survive;

There is heritable variation in the population of these organisms;

Organisms with different genetic traits have different survival rates and ability to reproduce.

Such conditions create competition between organisms for survival and reproduction and are the minimum necessary conditions for evolution through natural selection. Thus, organisms with hereditary traits that give them a competitive advantage are more likely to pass them on to their offspring than organisms with hereditary traits that do not have such an advantage.

The central concept of the concept of natural selection is the fitness of organisms. Fitness is defined as an organism's ability 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 descendants, but the number of descendants 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 therefore the fitness of that organism will be low.

If any allele increases the fitness of an organism more than other alleles of this gene, then with each generation the share of this allele in the population will increase. That is, selection occurs in favor of this allele. And vice versa, for less beneficial or harmful alleles, their share in populations will decrease, that is, selection will act against these alleles. It is important to note that the influence of certain alleles on the fitness of an organism is not constant - when environmental conditions change, harmful or neutral alleles can become beneficial, and beneficial ones harmful.

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:

Directional selection- changes in the average value of a trait over time, for example an increase in body size;

Disruptive selection- selection for extreme values ​​of a trait and against average values, for example, large and small body sizes;

Stabilizing selection- selection against extreme values ​​of a 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 the individual to potential partners. Traits that have evolved through sexual selection are especially noticeable in the males of some animal species. Characteristics such as large horns and 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 characteristics.

Selection can operate at different 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.

Forms of natural selection

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

Driving selection- a form of natural selection that operates when 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. In this case, other variations of the trait (its deviations in the opposite direction from the average value) are subject to negative selection. As a result, a shift occurs in the population from generation to generation average size sign in a certain direction. In this case, 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).

A classic example of driving selection is the evolution of color in the birch moth. The color of the wings of this butterfly imitates the color of the lichen-covered bark of trees on which it spends the daylight hours. Obviously, such a protective coloration was formed over many generations of previous evolution. However, with the beginning of the industrial revolution in England, this device began to lose its importance. Atmospheric pollution has led to massive death of lichens and darkening of tree trunks. Light butterflies on dark background became easily visible to birds. Beginning in the mid-19th century, mutant dark (melanistic) forms of butterflies began to appear in birch moth populations. Their frequency increased rapidly. By the end of the 19th century, some urban populations of the birch moth consisted almost entirely of dark forms, while rural populations continued to be dominated by light forms. This phenomenon was called industrial melanism. Scientists have found that in polluted areas, birds are more likely to eat light-colored forms, and in clean areas, dark ones. The introduction of air pollution restrictions in the 1950s caused natural selection to reverse course again, and the frequency of dark forms in urban populations began to decline. They are almost as rare these days as they were before the Industrial Revolution.

Driving selection occurs when there is a change environment or adaptation to new conditions when the range expands. It preserves hereditary changes in a certain direction, moving the reaction rate accordingly. For example, during the development of soil as a habitat, various unrelated groups of animals developed limbs that turned into burrowing limbs.

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 expression 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 the greatest contribution to the gene pool of the next generation should be made by individuals with maximum fertility. 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 is. As a result, individuals with average fertility are the most fit.

Selection toward the mean has been found for a variety of traits. In mammals, very low-weight and very high-weight newborns are more likely to die at birth or in the first weeks of life than average-weight newborns. Taking into account the size of the wings of sparrows that died after a storm in the 50s near Leningrad showed that most of them had wings that were too small or too large. And in this case, the average individuals turned out to be the most adapted.

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 allele 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 higher resistance to malaria than homozygotes for the normal allele. Thanks to this, in populations inhabiting malarial areas, heterozygosity for this allele, which is lethal in homozygotes, is created and stably maintained.

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

Thus, stabilizing selection, sweeping aside deviations from the norm, actively shapes 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.

Disruptive 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 a trait. As a result, several new forms may appear from one original one. Darwin described the action of disruptive selection, believing that it underlies divergence, although he could not provide evidence of 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.

The formation of seasonal races in some weeds is explained by the action of disruptive selection. It was shown that the timing of flowering and seed ripening in one of the species of such plants - meadow rattle - is extended almost throughout the summer, with most of the plants flowering and fruiting in mid-summer. However, in hay meadows, those plants that have time to flower and produce seeds before mowing, and those that produce seeds at the end of summer, after mowing, benefit. As a result, two races of rattle are formed - early and late flowering.

Disruptive selection was carried out artificially in experiments with Drosophila. Selection was carried out according to the number of setae; only individuals with small and big amount bristles. As a result, from about the 30th generation, the two lines diverged very much, 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- This is natural selection for reproductive success. The survival of organisms is an important, but not the only component of natural selection. Another important component is attractiveness to individuals of the opposite sex. Darwin called this phenomenon sexual selection. “This form of selection is not determined by the struggle for existence in the relations of organic beings with each other or with external conditions, but by competition between individuals of the same sex, usually males, for the possession of individuals of the other sex.” Traits that reduce the viability of their hosts can emerge and spread if the advantages they provide for reproductive 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 his bright plumage and long tail, somehow managed not to die in the clutches of a predator and survive to puberty, then, therefore, he has good genes.” genes that allowed him to do this. This means that he should be chosen as a father for his children: he will pass on his good genes to them.” By choosing colorful males, females are choosing good genes for their offspring.

According to the “attractive sons” hypothesis, the logic of female choice is somewhat different. If brightly colored males, for whatever reason, are attractive to females, then it is worth choosing a brightly colored father for his future sons, because his sons will inherit the brightly colored genes and will be attractive to females in the next generation. Thus, a positive feedback arises, which leads to the fact that from generation to generation the brightness of the plumage of males becomes more and more intense. The process continues to grow until it reaches the limit of viability.

In the choice of males, females are no more and no less logical than in all their other 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 a watering hole because it feels thirsty. In the same way, females, when choosing bright males, follow their instincts - they like bright tails. All those to whom instinct suggested a different behavior, all of them did not leave offspring. Thus, we were discussing 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 the amazing diversity of shapes, colors and instincts that we observe in the world of living nature .

Positive and negative selection

There are two forms of natural selection: Positive And Cut-off (negative) selection.

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

Eliminating selection eliminates from a population the vast majority of individuals that carry traits that sharply reduce viability under given environmental conditions. Using selection selection, highly deleterious alleles are removed from the population. Also, individuals with chromosomal rearrangements and a set of chromosomes that sharply disrupt the normal functioning of the genetic apparatus can be subjected to cutting selection.

The role of natural selection in evolution

Charles Darwin believed natural selection to be the main driving force of evolution, in modern synthetic theory evolution, it is also the main regulator of the development and adaptation of populations, a mechanism for the emergence of species and supraspecific taxa, although accumulation in late XIX- at the beginning of the 20th century, information on genetics, in particular the discovery of the discrete nature of the inheritance of phenotypic traits, led some researchers to deny the importance of natural selection and, as an alternative, proposed concepts based on assessing the genotype mutation factor as extremely important. The authors of such theories postulated not a gradual, but a very fast (over several generations) spasmodic nature of evolution (mutationism of Hugo de Vries, saltationism of Richard Goldschmidt and other less well-known concepts). The discovery of known correlations among the characters of related species (the law of homological series) by N. I. Vavilov prompted some researchers to formulate the next “anti-Darwinian” hypotheses about evolution, such as nomogenesis, bathmogenesis, autogenesis, ontrogenesis and others. In the 1920s to 1940s in evolutionary biology, those who rejected Darwin's idea of ​​evolution by natural selection (sometimes theories that emphasized natural selection were called "selectionist" theories) saw a revival of interest in this theory due to the revision of classical Darwinism in the light the relatively young science of genetics. The resulting synthetic theory of evolution, often incorrectly called neo-Darwinism, is, among other things, based on a quantitative analysis of the frequency of alleles in populations changing under the influence of natural selection. There are debates where people with a radical approach, as an argument against the synthetic theory of evolution and the role of natural selection, argue that “discoveries of recent decades in various fields of scientific knowledge - from molecular biology with her theory of neutral mutationsMotoo Kimura And paleontology with her theory of punctuated equilibrium Stephen Jay Gould And Niles Eldridge (wherein view understood as a relatively static phase of the evolutionary process) until mathematicians with her theorybifurcations And phase transitions- indicate the insufficiency of the classical synthetic theory of evolution to adequately describe all aspects of biological evolution". The discussion about the role of various factors in evolution began more than 30 years ago and continues to this day, and it is sometimes said that “evolutionary biology (meaning the theory of evolution, of course) has come to the need for its next, third synthesis.”

Material from Wikipedia - the free encyclopedia

Natural selection- the main evolutionary process, as a result of which in a population the number of individuals with maximum fitness (the most favorable traits) increases, 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 adaptation, but is not the only cause of evolution. Maladaptive causes include genetic drift, gene flow, and mutations.

The term "Natural selection" was popularized by Charles Darwin, comparing the process to artificial selection, the modern form of which is selective breeding. The idea of ​​comparing artificial and natural selection is that in nature the selection of the most “successful”, “best” organisms also occurs, but in this case the role of “evaluator” of the usefulness of properties is not a person, but the environment. In addition, the material for both natural and artificial selection is 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 called a "self-evident" mechanism because it follows from such simple facts as:

1. Organisms produce more offspring than can survive;
2. There is heritable variation in the population of these organisms;
3. Organisms with 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 an organism's ability 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 descendants, but the number of descendants 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 therefore the fitness of that 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. Directional selection- changes in the average value of a trait over time, for example an increase in body size;
2. Disruptive selection- selection for extreme values ​​of a trait and against average values, for example, large and small body sizes;
3. Stabilizing selection- selection against extreme values ​​of a 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 the individual to potential partners. Traits that have evolved through sexual selection are especially noticeable in the males of some animal species. Characteristics such as large horns and bright coloring, 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 characteristics.

Selection can operate at different 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, for example group selection, can lead to cooperation (see Evolution#Cooperation).

Forms of natural selection

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

Driving selection

Driving selection- a form of natural selection that operates when 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. In this case, other variations of the trait (its deviations in the opposite direction from the average value) are subject to negative selection. As a result, in a population from generation to generation there is a shift in the average value of the trait in a certain direction. In this case, 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 driving selection is “industrial melanism” in insects. “Industrial melanism” is a sharp increase in the proportion of melanistic (dark-colored) individuals in those populations of insects (for example, butterflies) that live in industrial areas. Due to industrial impact, the tree trunks darkened significantly, and light-colored lichens also died, which is why light-colored butterflies became better visible to birds, and dark-colored ones became less visible. In the 20th century, in some areas, the proportion of dark-colored butterflies in some well-studied moth populations in England reached 95%, while for the first time the dark-colored butterfly ( morpha carbonaria) was captured in 1848.

Driving selection occurs when the environment changes or adapts to new conditions when the range expands. It preserves hereditary changes in a certain direction, moving the reaction rate accordingly. For example, during the development of soil as a habitat, various unrelated groups of animals developed limbs that turned into burrowing limbs.

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 expression 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 the greatest contribution to the gene pool of the next generation should be made by individuals with maximum fertility. 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 is. As a result, individuals with average fertility are the most fit.

Selection toward the mean has been found for a variety of traits. In mammals, very low-weight and very high-weight newborns are more likely to die at birth or in the first weeks of life than average-weight newborns. Taking into account the size of the wings of sparrows that died after a storm in the 50s near Leningrad showed that most of them had wings that were too small or too large. And in this case, the average individuals turned out to be the most adapted.

Disruptive selection

Disruptive 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 a trait. As a result, several new forms may appear from one original one. Darwin described the action 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 the greater rattle in hay meadows. IN normal conditions The flowering and seed ripening periods of this plant cover the entire summer. But in hay meadows, seeds are produced mainly by those plants that manage to bloom and ripen either before the mowing period, or bloom at the end of summer, after mowing. As a result, two races of 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 bristles; only individuals with a small and large number of bristles were retained. As a result, from about the 30th generation, the two lines diverged very much, 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 reproductive success. 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 competition between individuals of one sex, usually males, for the possession of individuals of the other sex.” Traits that reduce the viability of their hosts can emerge and spread if the advantages they provide for reproductive 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 a given male, despite his bright plumage and long tail, managed not to die in the clutches of a predator and survive to sexual maturity, 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 colorful males, females are choosing good genes for their offspring.
• According to the “attractive sons” hypothesis, the logic of female choice is somewhat different. If brightly colored males, for whatever reason, are attractive to females, it is worth choosing a brightly colored father for his future sons, because his sons will inherit the brightly colored 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 increasingly increasing. The process continues to grow 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 a watering hole because it feels thirsty. In the same way, females, when choosing bright males, follow their instincts - they like bright tails. Those for whom instinct suggested 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, which, acting constantly from generation to generation, has formed the amazing variety of forms, colors and instincts that we observe in the world of living nature.

Selection methods: positive and negative selection

There are two forms of artificial selection: Positive And Cut-off (negative) selection.

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

Eliminating selection eliminates from a population the vast majority of individuals that carry traits that sharply reduce viability under given environmental conditions. Using selection selection, highly deleterious alleles are removed from the population. Also, individuals with chromosomal rearrangements and a set of chromosomes that sharply disrupt the normal functioning 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 sterile and, therefore, unable to transmit from generation to generation acquired modifications of structure or instincts. You can set good question- How is it possible to reconcile this case with the theory of natural selection?

- Origin of Species (1859)

Darwin assumed that selection could apply not only to an individual organism, but also to a family. He also said that perhaps, to one degree or another, this could explain people's behavior. He was right, but it was only with the advent of genetics that it became possible to provide a more expanded view of the concept. The first sketch of the “theory of kin selection” 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 an entire family, but also at the gene level.

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11. Okasha, S. Evolution and the Levels of Selection. - Oxford University Press, 2007. - 263 p. - ISBN 0-19-926797-9.
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Literature

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

Links

• - article with good famous examples: color of butterflies, human resistance to malaria, etc.
• - Chapter 4, Natural Selection
• - Modeling for Understanding in Science Education, University of Wisconsin
• from University of Berkeley education website
• Evolution: Education and Outreach

Excerpt describing Natural selection

At four o'clock in the afternoon, Murat's troops entered Moscow. A detachment of Wirtemberg hussars rode ahead, and the Neapolitan king himself rode behind on horseback with a large retinue.
Near the middle of the Arbat, near St. Nicholas the Revealed, Murat stopped, awaiting news from the advance detachment about the situation of the city fortress “le Kremlin”.
A small group of people from the residents remaining in Moscow gathered around Murat. Everyone looked with timid bewilderment at the strange, long-haired boss adorned with feathers and gold.
- Well, is this their king himself? Nothing! – quiet voices were heard.
The translator approached a group of people.
“Take off your hat... take off your hat,” they said in the crowd, turning to each other. The translator turned to one old janitor and asked how far it was from the Kremlin? The janitor, listening in bewilderment to the alien Polish accent and not recognizing the sounds of the translator's dialect as Russian speech, did not understand what was being said to him and hid behind others.
Murat moved towards the translator and ordered 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 translator. A French officer from the advance detachment rode up to Murat and reported that the gates to the fortress were sealed and that there was probably an ambush there.
“Okay,” said Murat and, turning to one of the gentlemen of his retinue, he ordered four light guns to be brought forward and fired at the gate.
The artillery came out at a trot from behind the column following Murat and rode along the Arbat. Having descended to the end of Vzdvizhenka, the artillery stopped and lined up in the square. Several French officers controlled the cannons, positioning them, and looked into the Kremlin through a telescope.
The bell for Vespers was heard in the Kremlin, and this ringing confused the French. They assumed it was a call to arms. Several infantry soldiers ran to the Kutafyevsky Gate. There were logs and planks at the gate. Two rifle shots rang out from under the gate as soon as the officer and his team began to run up to them. The general standing at the cannons shouted command words to the officer, and the officer and 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 of 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 previous expression of gaiety and calm was replaced by a stubborn, concentrated expression of readiness to fight and suffer. For all of them, from the marshal to the last soldier, this place was not Vzdvizhenka, Mokhovaya, Kutafya and Trinity Gate, but this was a new area of ​​a new field, probably a bloody battle. And everyone prepared for this battle. The screams from the gate died down. The guns were deployed. The artillerymen blew off the burnt blazers. The officer commanded “feu!” [fallen!], and two whistling sounds of tins were heard one after another. Grapeshot bullets crackled against 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 across the stone Kremlin died down, a strange sound was heard above the heads of the French. A huge flock of jackdaws rose above the walls and, cawing and rustling with thousands of wings, circled in the air. Along with this sound, a lonely human cry was heard at the gate, and from behind the smoke the figure of a man without a hat, in a caftan, appeared. Holding a gun, he aimed at the French. Feu! - the artillery officer repeated, and at the same time one rifle and two cannon shots were heard. The smoke closed the gate again.
Nothing else moved behind the shields, and the French infantry soldiers and officers went to the gate. There were three wounded and four dead people lying at the gate. Two people in caftans were running away from below, 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 beyond the fence. No one knew who these people were. “Enlevez moi ca,” was the only word said about them, and they were thrown away and cleaned up later so they wouldn’t stink. Thiers alone 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 shot at the French. Some of them were cut down with sabers, and cleared the Kremlin of their presence.]
Murat was informed that the path had been cleared. The French entered the gates and began to camp on Senate Square. The soldiers threw chairs out of the Senate windows 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 settled not as in apartments in the city, but as in a camp located in the city.
Although ragged, hungry, exhausted and reduced to 1/3 of their previous strength, the French soldiers entered Moscow in orderly order. It was an exhausted, exhausted, but still fighting and formidable army. But it was an army only until the minute the soldiers of this army went to their apartments. As soon as the people of the regiments began to disperse to empty and rich houses, the army was destroyed forever and neither residents nor soldiers were formed, but something in between, called marauders. When, five weeks later, the same people left Moscow, they no longer constituted an army. It was a crowd of marauders, each of whom carried or carried with him a bunch of things that seemed valuable and necessary to him. The goal of each of these people when leaving Moscow was not, as before, to conquer, but only to retain what they had acquired. Like that monkey who, having put his hand into the narrow neck of a jug and grabbed a handful of nuts, does not unclench his fist so as not to lose what he has grabbed, and thereby destroys himself, the French, when leaving Moscow, obviously had to die due to the fact that they were dragging with the loot, but it was as impossible for him to throw away this loot as it is impossible for a monkey to unclench a handful of nuts. Ten minutes after each French regiment entered some quarter of Moscow, not a single soldier or officer remained. In the windows of the houses people in greatcoats and boots could be seen walking around the rooms laughing; in the cellars and basements the same people managed the provisions; in the courtyards the same people unlocked or beat down the gates of barns and stables; they lit fires in the kitchens, baked, kneaded and cooked with their hands rolled up, scared, made them laugh and caressed women and children. And there were many of these people everywhere, in shops and in homes; but the army was no longer there.
On the same day, order after order was given by the French commanders to prohibit troops from dispersing throughout the city, to strictly prohibit violence against residents and looting, and to make a general roll call that same evening; but, despite any measures. the people who had previously made up the army dispersed throughout the rich, empty city, abundant in amenities and supplies. Just as a hungry herd walks in a heap across a bare field, but immediately scatters uncontrollably as soon as it attacks rich pastures, so the army scattered uncontrollably throughout the rich city.
There were no inhabitants in Moscow, and the soldiers, like water into sand, were sucked into it and, like an unstoppable star, spread out in all directions from the Kremlin, which they entered first of all. The cavalry soldiers, entering a merchant's house abandoned with all its goods and finding stalls not only for their horses, but also extra ones, still went nearby to occupy another house, which seemed better to them. Many occupied several houses, writing in chalk who occupied it, and arguing and even fighting with other teams. Before they could fit in, the soldiers ran outside to inspect the city and, hearing that everything had been abandoned, rushed to where they could take away valuables for nothing. The commanders went to stop the soldiers and themselves unwittingly became involved in the same actions. In Carriage Row there were shops with carriages, and the generals crowded there, choosing carriages and carriages for themselves. The remaining residents invited their leaders to their place, hoping to thereby protect themselves from robbery. There was an abyss of wealth, and there was no end in sight; everywhere, around the place that the French occupied, there were still unexplored, unoccupied places, in which, as it seemed to the French, there was even more wealth. And Moscow sucked them in further and further. Just as when water pours onto dry land, water and dry land disappear; in the same way, due to the fact that a hungry army entered an abundant, empty city, the army was destroyed, and the abundant city was destroyed; and there was dirt, fires and looting.

The French attributed the fire of Moscow to au patriotisme feroce de Rastopchine [to Rastopchin's wild patriotism]; Russians – to the fanaticism of the French. In essence, there were no reasons for the fire of Moscow in the sense of attributing this fire to the responsibility of one or several persons. Moscow burned down due to the fact that it was placed in such conditions under which every wooden city should burn down, regardless of whether the city had one hundred and thirty bad fire pipes or not. Moscow had to burn due to the fact that the inhabitants left it, and just as inevitably as a heap of shavings should catch fire, on which sparks of fire would rain down for several days. A wooden city, in which there are fires almost every day in the summer under the residents, house owners and under the police, cannot help but burn down when there are no inhabitants in it, but live troops smoking pipes, making fires on Senate Square from Senate chairs and cooking themselves two once a day. In peacetime, as soon as troops settle into quarters in villages in a certain area, 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 an alien army is stationed? Le patriotisme feroce de Rastopchine and the fanaticism of the French are not to blame for anything here. Moscow caught fire from pipes, from kitchens, from fires, from the sloppiness of enemy soldiers and residents - not the owners of the houses. If there were arson (which is very doubtful, because there was no reason for anyone to set fire, and, in any case, it was troublesome and dangerous), then the arson cannot be taken as the cause, since without the arson it would have been the same.
No matter how flattering it was for the French to blame the atrocity of Rostopchin and for the Russians to blame the villain Bonaparte or then to place the heroic torch in the hands of their people, one cannot help but see that there could not have been such a direct cause of the fire, because Moscow had to burn, just as every village and factory had to burn , every house from which the owners will come out and into which strangers will be allowed to run the house and cook their own porridge. Moscow was burned by its inhabitants, it’s true; but not by those residents 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 offer bread, salt and keys to the French, but left it.

The influx of Frenchmen, spreading like a star across Moscow on the day of September 2, reached the block in which Pierre now lived only in the evening.
After the last two days, spent alone and unusually, Pierre was in a state close to madness. His whole being was taken over by one persistent thought. He himself did not know how and when, but this thought now took possession of him so that he did not remember anything from the past, did not understand anything from the present; and everything that he saw and heard happened before him as in a dream.
Pierre left his home only to get rid of the complex tangle of life's demands that gripped him, and which, in his then state, he was able to unravel. He went to Joseph 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 Joseph Alekseevich, a world of eternal, calm and solemn thoughts was associated in his soul, completely opposite to the anxious confusion in which he felt himself being drawn in. He was looking for a quiet refuge and really found it in the office of Joseph Alekseevich. When, in the dead silence of the office, he sat down, leaning on his hands, over the dusty desk of the deceased, in his imagination, calmly and significantly, one after another, the memories of the last days began to appear, especially the Battle of Borodino and that indefinable feeling for him of his insignificance and falsity in comparison with the truth, simplicity and strength of that category of people who were imprinted on his soul under the name they. When Gerasim woke him from his reverie, the thought occurred to Pierre that he would take part in the supposed - as he knew - people's defense 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, during the first solitary and idle day (Pierre tried several times and could not stop his attention on the Masonic manuscripts), he vaguely imagined several times the previously thought about the cabalistic meaning of his name in connection with the name of Bonaparte; but this thought that he, l "Russe Besuhof, was destined to put a limit 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 sole purpose of participating in the people's defense of Moscow), Pierre met the Rostovs and Natasha said to 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, for him to 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 walked with the people beyond the Trekhgornaya Gate. But when he returned home, making sure 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 an 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, in Pierre's opinion, originated from Napoleon alone.
Pierre knew all the details of the attempt by a German student on the life of Bonaparte in Vienna in 1809 and knew that this student had been shot. And the danger to which he exposed his life in fulfilling his intention excited him even more.
Two equally strong feelings irresistibly attracted Pierre to his intention. The first was a feeling of the need for sacrifice and suffering with the awareness of general misfortune, that feeling, as a result of which he went to Mozhaisk on the 25th and arrived in the very heat of battle, now ran away from his home and, instead of the usual luxury and comforts of life, slept without undressing, on on a hard sofa and ate the same food with Gerasim; the other was that vague, 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 Slobodsky Palace, when he suddenly felt that wealth, power, and life, everything that people so diligently arrange and protect - if all this is worth something, then only by the pleasure with which you can give it all up.
It was that feeling as a result of which a hunter recruit drinks his last penny, a drunken man breaks mirrors and glass for no apparent reason and knowing that this will cost him his last money; that feeling as a result of which a person, doing (in a vulgar sense) crazy things, seems to be testing his personal power and strength, declaring the presence of a higher one standing outside human conditions, judgment on life.
From the very day when Pierre first experienced this feeling in the Slobodsky Palace, he was constantly under its influence, but now he only found complete satisfaction with it. In addition, at the moment Pierre was supported in his intention and deprived of the opportunity to renounce him by what he had already done along this path. And his flight from home, and his caftan, and the pistol, and his statement to Rostov that he remained in Moscow - everything would have lost not only its meaning, but all this would have been despicable and ridiculous (to which Pierre was sensitive), if After all this, like others, he left Moscow.

NATURAL SELECTION, the process of selective survival and differential reproduction of organisms, the main driving factor in their evolution. Ideas about the existence of natural selection have been expressed since the beginning of the 19th century by various English naturalists (including A. Wallace). But only Charles Darwin (1842, 1859) assessed it as the main factor in evolution. According to Darwin, natural selection is the result of the struggle for existence; even minor heritable differences between individuals of the same species can provide advantages in this struggle, which is due to the tendency of organisms to reproduce at a high intensity (in geometric progression) and the impossibility of preserving all offspring due to limited natural resources. The death of the overwhelming number of individuals in each generation inevitably leads to natural selection - “survival of the fittest” to given conditions. As a result of the accumulation of beneficial changes over many generations, new adaptations are formed and, ultimately, new species arise. Darwin based his discussions about the action of natural selection primarily on generalizing the experience of domestication of animals and plants by analogy with artificial selection, emphasizing, however, that, unlike human selection, natural selection is determined by the interaction of organisms with environmental conditions and does not have a specific goal.

Systematic research into natural selection, expansion and improvement of methods for studying it began at the end of the 19th century. The use of biometric methods made it possible to establish statistically significant differences between surviving and dead organisms when environmental conditions changed. Thanks to the developments of R. Fisher, J. Haldane, S. Wright and S. S. Chetverikov, who carried out the synthesis of classical Darwinism and genetics, it became possible to begin the experimental study of the genetic foundations of natural selection. The examined natural populations turned out to be literally saturated with mutations, many of which became useful when conditions of existence changed or when combined with other mutations. It was found that the mutation process and free crossing (panmixia) provide genetic heterogeneity of populations and the uniqueness of individuals with different chances of survival; this determines the high intensity and efficiency of natural selection. In addition, it became obvious that natural selection deals not with individual traits, but with entire organisms, and that the genetic essence of natural selection lies in the non-random (differentiated) preservation of certain genotypes in a population, selectively transmitted to subsequent generations. Natural selection is probabilistic in nature, acts on the basis of the mutation process and the existing gene pool, affects the frequency of distribution of genes and their combinations, helps to reduce the negative effects of mutations and the formation of defense mechanisms against their harmful effects, thereby determining the pace and direction of evolution. Under the control of natural selection are not only various traits, but also the factors of evolution themselves, for example, the intensity and nature of mutability, the apparatus of heredity (hence the concept of “evolution of evolution”). In the absence of natural selection, a decrease or loss of fitness of organisms occurs due to the accumulation of undesirable mutations, which manifests itself in an increase in genetic load, including in modern human populations.

There are more than 30 forms of natural selection; none of them exist in pure form, but rather characterizes the tendency of selection in a specific ecological situation. Thus, driving selection contributes to the preservation of a certain deviation from the previous norm and leads to the development of new adaptations through a directed restructuring of the entire gene pool of populations, as well as the genotypes and phenotypes of individuals. It can lead to the dominance of one (or several) pre-existing forms over others. A classic example of its action was the predominance in industrial areas of dark-colored forms of the birch moth butterfly, invisible to birds on tree trunks contaminated with soot (until the mid-19th century, only a light form was found, imitating lichen spots on light birch trunks). The rapid adaptation to poisons of various species of insects and rodents and the emergence of resistance of microorganisms to antibiotics indicate that the pressure of driving selection in natural populations is sufficient to ensure a rapid adaptive response to sudden changes in the environment. As a rule, selection for one trait entails a whole series of transformations. For example, long-term selection for the protein or oil content in corn grains is accompanied by changes in the shape of the grains, the size of the cobs, their location above the soil level, etc.

The result of driving selection in the phylogeny of large taxa is orthoselection, an example of which is the directed evolution of the limb of the horse’s ancestors established by V. O. Kovalevsky (from five-toed to one-toed), which lasted for millions of years and ensured an increase in the speed and economy of running.

Disruptive, or disruptive, selection favors the preservation of extreme deviations and leads to an increase in polymorphism. It manifests itself in cases where none of the intraspecific forms with different genotypes receives an absolute advantage in the struggle for existence due to the diversity of conditions simultaneously occurring in the same territory; in this case, individuals with average or intermediate character traits are eliminated first of all. At the beginning of the 20th century, the Russian botanist N.V. Tsinger showed that the large rattle (Alectoroleophus major), which blooms and bears fruit in unmown meadows throughout the summer, forms two races in mowed meadows: the early spring race, which manages to bear seeds before mowing begins, and late autumn - low plants that are not damaged when mowing, and then quickly bloom and have time to produce seeds before the onset of frost. Another example of polymorphism is the difference in the color of shells in the land snail (Capacea nemoralis), which is food for birds: in dense beech forests, where litter of red-brown litter remains throughout the year, individuals with brown and pink colors are common; in meadows with yellow litter, yellow-colored snails predominate. In mixed deciduous forests, where the nature of the background changes with the onset of a new season, in early spring snails with brown and pink colors dominate, and in summer - with yellow ones. Darwin's finches (Geospizinae) on the Galapagos Islands (classic example of adaptive radiation) - final result long-term disruptive selection, which led to the formation of dozens of closely related species.

If these forms of natural selection lead to changes in both the phenotypic and genetic structure of populations, then stabilizing selection, first described by I. I. Shmalgausen (1938), preserves the average value of traits (norm) in the population and does not allow the genomes of individuals that deviate most from the population to pass into the next generation. this norm. It is aimed at maintaining and increasing stability in a population of an average, previously established phenotype. It is known, for example, that during snow storms, birds survive that, in many respects (length of the wing, beak, body weight, etc.) are close to the average norm, and individuals that deviate from this norm die. The size and shape of flowers in plants pollinated by insects are more stable than in plants pollinated by the wind, which is due to the conjugate evolution of plants and their pollinators, the “culling” of forms that deviate from the norm (for example, a bumblebee cannot penetrate a too narrow corolla of a flower, and the butterfly's proboscis does not touch the stamens that are too short in plants with a long corolla). Thanks to stabilizing selection, with an externally unchanged phenotype, significant genetic changes can occur, ensuring the independence of the development of adaptations from fluctuating environmental conditions. One of the results of the action of stabilizing selection can be considered the “biochemical universality” of life on Earth.

Destabilizing selection (the name was proposed by D.K. Belyaev, 1970) leads to a sharp disruption of ontogenesis regulatory systems, the opening of the mobilization reserve and an increase in phenotypic variability with intensive selection in any particular direction. For example, selection to reduce the aggressiveness of predatory animals in captivity through the restructuring of the neurohumoral system leads to destabilization of the reproduction cycle, shifts in the timing of molting, changes in the position of the tail, ears, coloring, etc.

Genes have been discovered that can be lethal or reduce the viability of organisms in a homozygous state, and in a heterozygous state, on the contrary, increase ecological plasticity and other indicators. In this case, we can talk about the so-called balanced selection, which ensures the maintenance of genetic diversity with a certain ratio of allele frequencies. An example of its action is the increase in resistance in patients with sickle cell anemia (heterozygous for the hemoglobin S gene) to infection with various strains of malarial plasmodium (see Hemoglobins).

An important step in overcoming the desire to explain all the characteristics of organisms by the action of natural selection was the concept of neutral evolution, according to which some of the changes at the level of proteins and nucleic acids occur through the fixation of adaptively neutral or almost neutral mutations. It is possible to select species that appear in peripheral populations “suddenly” from a geochronological point of view. Even earlier, it was proven that catastrophic selection, in which a small number of individuals and even a single organism survive during periods of sudden environmental changes, can become the basis for the formation of a new species due to chromosomal rearrangement and a change in the ecological niche. Thus, the formation of the xerophytic, endemic species Clarkia lingulata in the Sierra Nevada Mountains in California is explained by severe drought, which caused massive plant death, which became catastrophic in peripheral populations.

Natural selection that affects the secondary sexual characteristics of individuals is called sexual (for example, the bright nuptial coloration of males in many species of fish and birds, inviting calls, specific odors, highly developed tools for tournament combat in mammals). These traits are useful because they increase the possibility of their carriers participating in the reproduction of offspring. In sexual selection, males are most active, which is beneficial for the species as a whole, because females remain safer during the breeding season.

There is also group selection, which promotes the preservation of traits useful to a family, flock, or colony. Its special case in colonial insects is the selection of relatives, in which sterile castes (workers, soldiers, etc.) provide (often at the cost of own life) the survival of fertile individuals (queens) and larvae and thereby the preservation of the entire colony. The altruistic behavior of parents, pretending to be wounded in order to lure the predator away from their children, threatens the death of the imitator, but in general increases the chances of survival of its offspring.

Although ideas about the leading role of natural selection in evolution have been confirmed in many experiments, they are still subject to criticism based on the idea that organisms cannot be formed as a result of a random combination of mutations. This ignores the fact that each act of natural selection is carried out on the basis of the previous results of its own action, which, in turn, predetermine the forms, intensity and directions of natural selection, and therefore the paths and patterns of evolution.

Lit.: Shmalgauzen I.I. Factors of evolution. 2nd ed. M., 1968; Mayr E. Zoological species and evolution. M., 1968; Sheppard F. M. Natural selection and heredity. M., 1970; Lewontin R. Genetic basis of evolution. M., 1978; Wilson D. S. The natural selection of populations and communities. Menlo Park, 1980; Gall Ya. M. Research on natural selection // Development of evolutionary theory in the USSR. L., 1983; Gause G.F. Ecology and some problems of the origin of species // Ecology and evolutionary theory. L., 1984; Ratner V. A. Brief outline of the theory of molecular evolution. Novosibirsk, 1992; Dawkins R. The Selfish General M., 1993; Sober E. The nature of selection: evolutionary theory in philosophical focus. Chi., 1993; Darwin Ch. Origin of Species... 2nd ed. St. Petersburg, 2001; Coyne J., Orr N. A. Speciation. Sunderland, 2004; Gavrilets S. Fitness landscapes and the origin of species. Princeton, 2004; Yablokov A.V., Yusufov A.G. Evolutionary teaching. 5th ed. M., 2004; Severtsov A. S. Theory of evolution. M., 2005; Kolchinsky E. I. E. Mayr and modern evolutionary synthesis. M., 2006.

The idea of ​​comparing artificial and natural selection is that in nature the selection of the most “successful”, “best” organisms also occurs, but in this case the role of “evaluator” of the usefulness of properties is not a person, but the habitat. In addition, the material for both natural and artificial selection is 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 called a "self-evident" mechanism because it follows from such simple facts as:

1. Organisms produce more offspring than can survive;
2. There is heritable variation in the population of these organisms;
3. Organisms with 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 descendants, but the number of descendants 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 therefore the fitness of that 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. Directional selection- changes in the average value of a trait over time, for example an increase in body size;
2. Disruptive selection- selection for extreme values ​​of a trait and against average values, for example, large and small body sizes;
3. Stabilizing selection- selection against extreme values ​​of a 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 the individual to potential partners. Traits that have evolved through sexual selection are especially noticeable in the males of some animal species. Characteristics such as large horns and bright coloring, 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 characteristics.

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, for example, group selection, can lead to cooperation (see Evolution#Cooperation).

Forms of natural selection

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

Driving selection

Driving selection- a form of natural selection that operates when 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. In this case, other variations of the trait (its deviations in the opposite direction from the average value) are subject to negative selection. As a result, in a population from generation to generation there is a shift in the average value of the trait in a certain direction. In this case, 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 driving selection is “industrial melanism” in insects. “Industrial melanism” is a sharp increase in the proportion of melanistic (dark-colored) individuals in those populations of insects (for example, butterflies) that live in industrial areas. Due to industrial impact, the tree trunks darkened significantly, and light-colored lichens also died, which is why light-colored butterflies became better visible to birds, and dark-colored ones became less visible. In the 20th century, in a number of areas, the proportion of dark-colored butterflies in some well-studied moth populations in England reached 95%, while for the first time a dark-colored butterfly ( morpha carbonaria) was captured in 1848.

Driving selection occurs when the environment changes or adapts to new conditions when the range expands. It preserves hereditary changes in a certain direction, moving the reaction rate accordingly. For example, during the development of soil as a habitat, various unrelated groups of animals developed limbs that turned into burrowing limbs.

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 expression 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 the greatest contribution to the gene pool of the next generation should be made by individuals with maximum fertility. 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 is. As a result, individuals with average fertility are the most fit.

Selection toward the mean has been found for a variety of traits. In mammals, very low-weight and very high-weight newborns are more likely to die at birth or in the first weeks of life than average-weight newborns. Taking into account the size of the wings of sparrows that died after a storm in the 50s near Leningrad showed that most of them had wings that were too small or too large. And in this case, the average individuals turned out to be the most adapted.

Disruptive selection

Disruptive 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 a trait. As a result, several new forms may appear from one original one. Darwin described the action 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 the greater rattle in hay meadows. Under normal conditions, the flowering and seed ripening periods of this plant cover the entire summer. But in hay meadows, seeds are produced mainly by those plants that manage to bloom and ripen either before the mowing period, or bloom at the end of summer, after mowing. As a result, two races of 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 bristles; only individuals with a small and large number of bristles were retained. As a result, from about the 30th generation, the two lines diverged very much, 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 reproductive success. 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 competition between individuals of one sex, usually males, for the possession of individuals of the other sex.” Traits that reduce the viability of their hosts can emerge and spread if the advantages they provide for reproductive 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 a given male, despite his bright plumage and long tail, managed not to die in the clutches of a predator and survive to sexual maturity, 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 colorful males, females are choosing good genes for their offspring.
• According to the “attractive sons” hypothesis, the logic of female choice is somewhat different. If brightly colored males, for whatever reason, are attractive to females, it is worth choosing a brightly colored father for his future sons, because his sons will inherit the brightly colored 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 becomes more and more intense. The process continues to grow 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 a watering hole because it feels thirsty. In the same way, females, when choosing bright males, follow their instincts - they like bright tails. Those for whom instinct suggested 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, which, acting constantly from generation to generation, has formed the amazing variety of forms, colors and instincts that we observe in the world of living nature.

Selection methods: positive and negative selection

There are two forms of artificial selection: Positive And Cut-off (negative) selection.

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

Eliminating selection eliminates from a population the vast majority of individuals that carry traits that sharply reduce viability under given environmental conditions. Using selection selection, highly deleterious alleles are removed from the population. Also, individuals with chromosomal rearrangements and a set of chromosomes that sharply disrupt the normal functioning 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 sterile and, therefore, unable to transmit from generation to generation acquired modifications of structure or instincts. A good question to ask is how reconcilable is this case with the theory of natural selection?

- Origin of Species (1859)

Darwin assumed that selection could apply not only to an individual organism, but also to a family. He also said that perhaps, to one degree or another, this could explain people's behavior. He was right, but it was only with the advent of genetics that it became possible to provide a more expanded view of the concept. The first sketch of the “theory of kin selection” 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 an entire family, but also at the level of the 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. -