Albert Gurulev. Albert Gurulev - Rosstan (collection)

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 main factor 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 reasoning about the action of natural selection primarily on a generalization of 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 conditions environment and doesn't have specific purpose.

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 change. 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 to act 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). Quick addiction to poisons various types insects and rodents, the emergence of resistance of microorganisms to antibiotics indicates 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 whole line 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, snails with brown and pink colors dominate in early spring, and yellow ones in summer. Darwin's finches (Geospizinae) on the Galapagos Islands ( classic example 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 random combination 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 essay theories 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.

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, How:

1. Organisms produce more offspring than can survive;
2. There is heritable variability 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 total number descendants, and 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 dispersion of the trait and a decrease in diversity.

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 simultaneously act on different levels. Selection at levels above the individual, for example group selection, can lead to cooperation (see Evolution#Cooperation).

Forms of natural selection

Exist different classifications forms of selection. 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 conditions external environment. 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. Wherein different shapes adapt to various 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 individuals 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 main hypotheses about the mechanisms of sexual selection have been proposed. According to the “good genes” hypothesis, the female “reasons” as follows: “If this male, despite his bright plumage and a long tail, somehow managed not to die in the clutches of a predator and survive to puberty, then, therefore, he has good 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

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.

see also

Notes

1. , With. 43-47
2. , p. 251-252
3. Orr H.A. Fitness and its role in evolutionary genetics // Nat Rev Genet. - 2009. - Vol. 10(8). - P. 531-539.
4. Haldane J The theory of natural selection today // 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. , Chapter 14
7. Andersson M, Simmons L Sexual selection and mate choice // Trends Ecol Evol. - 2001. - Vol. 21(6). - P. 296-302.
8. Kokko H, Brooks R, McNamara J, Houston A The sexual selection continuum // Proc Biol Sci. - 2002. - Vol. 269. - P. 1331-1340.
9. Hunt J, Brooks R, Jennions MD, Smith MJ, Bentsen CL, Bussière LF High-quality male field crickets invest heavily in sexual display but die young // Nature. - 2004. - Vol. 432. - P. 1024-1027.
10. Okasha, S. Evolution and the Levels of Selection. - Oxford University Press, 2007. - 263 p. - ISBN 0-19-926797-9
11. Mayr E The objects of selection // Philos. Trans. R. Soc. Lond., B, Biol. Sci. - 1998. - T. 353. - P. 307–14.
12. Maynard Smith J The units of selection // Novartis Found. Symp. - 1998. - T. 213. - P. 211–217.
13. Gould SJ, Lloyd EA Individuality and adaptation across levels of selection: how shall we name and generalize the unit of Darwinism? // Proc. Natl. Acad. Sci. U.S.A.. - 1999. - T. 96. - No. 21. - P. 11904–11909.
14. Ethology. Ru: Moral animal, part 2
15. Kin selection. The evolution of cooperation and altruism.

Links

• “Forms of Natural Selection” - an article with well-known examples: the color of butterflies, human resistance to malaria, etc.
• On the Origin of Species by Charles Darwin – Chapter 4, Natural Selection

Natural selection is a process originally defined by Charles Darwin as leading to the survival and preferential reproduction of individuals more adapted to given environmental conditions and possessing useful hereditary traits. In accordance with Darwin's theory and the modern synthetic theory of evolution, the main material for natural selection is random hereditary changes - recombination of genotypes, mutations and their combinations.

In the absence of the sexual process, natural selection leads to an increase in the proportion of a given genotype in the next generation. However, natural selection is “blind” in the sense that it “evaluates” phenotypes rather than genotypes, and the preferential transmission of the genes of an individual with useful traits to the next generation occurs regardless of whether these traits are heritable.

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 under directed changes in 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, the proportion of dark-colored butterflies in some well-studied moth populations in England reached 95% in some areas, while the first dark-colored butterfly (morfa 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- 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. Schmalhausen.

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- 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.

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- 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 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 main hypotheses about the mechanisms of sexual selection have been proposed. 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, there is a positive Feedback, which leads to the fact that from generation to generation the brightness of the plumage of males increases more and more. 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 .

Anthropology and concepts of biology Kurchanov Nikolay Anatolievich

Natural selection

Natural selection

Natural selection is the most important factor in evolution. Darwinism (namely, STE is built on the basis of Darwinism), as noted above, is called the theory of natural selection.

A brief and successful definition of selection can be formulated by I. Lerner: “Selection is the differential reproduction of genotypes”(Lerner J., 1958). This definition shows that reproduction does not mean more intense, but more efficient reproduction. Natural selection was very well characterized by one of the founders of modern cytogenetics, S. Darlington (1903–1981), as a process of transfer “...from the chemical level of mutation to the biological level of adaptation”(Darlington S., 1958).

The role of natural selection is one of the key controversial issues in evolutionary biology throughout its history.

By the middle of the 20th century, thanks to the fundamental theoretical developments of I. I. Shmalhausen and J. Simpson, the idea of ​​three forms of selection was formed in evolutionary biology.

Stabilizing selection- this is the preferential survival of organisms that have characteristics that do not have noticeable deviations from the norm characteristic of a given population. The most obvious result of the action of stabilizing selection is the stabilization of the already existing norm of reaction for a given trait.

Driving selection– contributes to a shift in the average value of the characteristic. With a directed change in the environment, individuals with individual characteristics corresponding to this change. This selection helps to consolidate new form to replace the old one, which came into conflict with the changed environmental conditions.

Disruptive selection– selection directed against individuals with an average value of traits and leading to a break in the population into several groups for a given trait.

This division was well confirmed in subsequent experimental studies.

The variability of a trait in a population is described by a normal distribution curve. A normal genotype leads to the development of an individual whose characteristics are close to the average norm ( fashion) variation curve of a given trait. The more the genotype of an individual is changed, the less common such individuals are. If the genotype is changed so much that ontogeny cannot lead to the development of a sexually mature individual, then such an individual is outside the variation curve (lethal mutations).

In addition to the three forms of selection noted, there are many other classifications. In population genetics, attention is focused on changes in the frequency of alleles in a population and the following selection options are highlighted (Ayala F., Kaiger J., 1988):

– selection against a recessive allele;

– selection against the dominant allele;

– selection against the neutral allele;

– selection in favor of heterozygotes;

– selection against heterozygotes;

– frequency-dependent selection.

The last option is quite interesting. It is characterized by an increase in the probability of crossing depending on the frequency of the genotype, and often selection occurs in favor of a rare allele.

An important role in nature is played by selection in favor of heterozygotes, leading to stable polymorphism of populations. In evolutionary ecology special meaning is given by selection for reproductive strategies, which we will consider below. A very specific type of selection is sexual selection.

There are many other classifications of natural selection, and there is not always a consensus among evolutionists.

Chapter IV. Natural selection, or survival of the most