Natural selection 3 types. How to pass her 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, 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 .

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:

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.

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:

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 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.

Stabilizing selection

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.

Sexual selection

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.

Notes

, With. 43-47.
, p. 251-252.
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.
Haldane J.B.S. The theory of natural selection today // Nature. - 1959. - Vol. 183, no. 4663. - P. 710-713. - PMID 13644170.
Lande R., Arnold S. J. The measurement of selection on correlated characters // Evolution. - 1983. - Vol. 37, no. 6. - P. 1210-1226. -

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

The mutation process, fluctuations in population numbers, and isolation create genetic heterogeneity within a species. But their action is undirected. Evolution is a directed process associated with the development of adaptations, with the progressive complication of the structure and functions of animals and plants. There is only one directed evolutionary factor - natural selection.

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

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

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

a) Destabilizing selection

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

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

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

Thus, a destabilizing form of selection leads to an expansion of the reaction norm.

b) Sexual selection

Sexual selection- natural selection within one sex, aimed at developing characteristics that primarily give the opportunity to leave the largest number of descendants.

Males of many species display clearly expressed secondary sexual characteristics that at first glance seem non-adaptive: the tail of a peacock, the bright feathers of birds of paradise and parrots, the scarlet crests of roosters, the enchanting colors of tropical fish, the songs of birds and frogs, etc. Many of these features complicate the life of their carriers and make them easily noticeable to predators. It would seem that these characteristics do not provide any advantages to their carriers in the struggle for existence, and yet they are very widespread in nature. What role did natural selection play in their emergence and spread?

We already know that the survival of organisms is an important, but not the only component of natural selection. Another important component is attractiveness to individuals of the opposite sex. Charles Darwin called this phenomenon sexual selection. He first mentioned this form of selection in On the Origin of Species and then analyzed it in detail in The Descent of Man and Sexual Selection. He believed that “this form of selection is determined not by the struggle for existence in the 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.”

Sexual selection is natural selection for reproductive success. 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. A male who lives short but is liked by females and therefore produces many offspring has much higher overall fitness than one who lives long but produces few offspring. In many animal species, the vast majority of males do not participate in reproduction at all. In each generation, fierce competition arises between males for females. This competition can be direct, and manifest itself in the form of struggle for territory or tournament battles. It can also occur in an indirect form and be determined by the choice of females. In cases where females choose males, male competition manifests itself through displays of flamboyant appearance or complex courtship behavior. Females choose the males they like best. As a rule, these are the brightest males. But why do females like bright males?

Rice. 7.

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

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

According to the “attractive sons” hypothesis, the logic of female 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. Let's imagine a situation where females choose males with a longer tail. Long-tailed males produce more offspring than males with short and medium tails. From generation to generation, the length of the tail increases because females choose males not with a certain tail size, but with a larger than average size. Eventually, the tail reaches a length where its detriment to the male's vitality is balanced by its attractiveness in the eyes of females.

In explaining these hypotheses, we tried to understand the logic of the actions of female birds. It may seem that we expect too much from them, that such complex calculations of fitness are hardly possible for them. In fact, females are no more or less logical in their choice of males 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. When a worker bee stings a predator attacking a hive, she does not calculate how much with this self-sacrifice she increases the overall fitness of her sisters - she follows instinct. 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 .

c) Group selection

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

In a large continuous population, selection is relatively ineffective in increasing the frequency of favorable but rare recessive mutations. Moreover, any tendency toward an increase in the frequency of any favorable allele in one part of a given large population is counteracted by interbreeding with neighboring subpopulations in which that allele is rare. In the same way, favorable new gene combinations that managed to form in some local lobe of a given population are broken down into parts and eliminated as a result of crossing with individuals from neighboring lobes.

All these difficulties are largely eliminated in a population system whose structure resembles a series of individual islands. Here selection, or selection together with genetic drift, can quickly and efficiently increase the frequency of some rare favorable allele in one or more small colonies. New favorable gene combinations can also easily become established in one or more small colonies. Isolation protects the gene pools of these colonies from being “flooded” as a result of migration from other colonies that do not have such favorable genes, and from crossing with them. Up to this point, the model has included only individual selection or, for some colonies, individual selection combined with genetic drift.

Let us now assume that the environment in which this population system is located has changed, as a result of which the adaptability of the previous genotypes has decreased. In a new environment, new favorable genes or combinations of genes that become established in some colonies have high potential adaptive value for the population system as a whole. Now all the conditions are in place for group selection to come into play. Less adapted colonies gradually decline and die out, and colonies that are more adapted expand and replace them throughout the area occupied by a given population system. Such a subdivided population system acquires a new set of adaptive characteristics as a result of individual selection within some colonies, followed by differential reproduction of different colonies. The combination of group and individual selection can produce results that cannot be achieved by individual selection alone.

It has been established that group selection is a second-order process that complements the main process of individual selection. As a second-order process, group selection must proceed slowly, probably much more slowly than individual selection. Renewing populations takes longer than updating individuals.

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

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

G) Directional selection (driving)

Rice. 1.

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

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

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

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

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

Rice. 3.

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

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

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

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

d) Stabilizing selection

Rice. 4.

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

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

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- and very-high-weight newborns are more likely to die at birth or in the first weeks of life than average-weight newborns. A study of the size of the wings of birds that died after the storm 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.

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

Fig.5.

The most widely known example of such a polymorphism is sickle cell anemia. This severe blood disease occurs in people homozygous for the mutant hemoglobin alley (Hb S) and leads to their death at an early age. In most human populations, the frequency of this alley is very low and approximately equal to the frequency of its occurrence due to mutations. However, it is quite common in areas of the world where malaria is common. It turned out that heterozygotes for Hb S have higher resistance to malaria than homozygotes for the normal alley. Thanks to this, in populations inhabiting malarial areas, heterozygosity for this lethal homozygous alley 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. Such genetic mechanisms as dominance, epistasis, complementary action of genes, incomplete penetrance and other means of hiding genetic variability owe their existence to stabilizing selection.

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

Thanks to stabilizing selection, “living fossils” living in relatively constant environmental conditions have survived to this day:

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

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

the North American opossum is a marsupial known since the Cretaceous period;

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

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

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

f) Disruptive (dismembering) selection

Rice. 6.

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

A model of disruptive selection could be the situation of the emergence of dwarf predatory fish in a body of food with little food. Often, underyearling squirrels do not have enough food in the form of fish fry. In this case, the advantage goes to the fastest growing ones, which very quickly reach a size that allows them to eat their fellows. On the other hand, the bee-eater with the maximum delay in growth rate will be in an advantageous position, since their small size allows them to remain planktivores for a long time. Such a situation, through stabilizing selection, can lead to the emergence of two predatory fish.

An interesting example is given by Darwin regarding insects - inhabitants of small oceanic islands. They fly beautifully or have no wings at all. Apparently, the insects were carried out to sea by sudden gusts of wind; Only those that could either withstand the wind or did not fly at all survived. Selection in this direction has led to the fact that on the island of Madeira, out of 550 species of beetles, 200 are flightless.

Another example: in forests where the soil is brown, individuals of the earth snail often have brown and pink colored shells, in areas with coarse and yellow grass yellow color predominates, etc.

Populations adapted to ecologically dissimilar habitats may occupy adjacent geographic areas; for example, in the coastal regions of California, the plant Gilia achilleaefolia is represented by two races. One race, the “sun” race, grows on open, grassy, south-facing slopes, while the “shade” race is found in shady oak and redwood groves. These races differ in the size of the petals - a genetically determined feature.

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

Conclusion

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

Bibliography

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

Green N. Biology - Moscow, 2003

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

Lemeza N.A. A manual on biology - Minsk, 1998

The doctrine of natural selection was created by Charles Darwin and A. Wallace, who considered it as the main creative force directing the evolutionary process and determining its specific forms.

Natural selection is the process by which predominantly individuals with hereditary characteristics useful for given conditions survive and leave offspring.

Assessing natural selection from the standpoint of genetics, we can conclude that it essentially selects positive mutations and genetic combinations that arise during sexual reproduction, improving survival in populations, and rejects all negative mutations and combinations that worsen the survival of organisms. The latter simply die. Natural selection can also act at the level of reproduction of organisms, when weakened individuals either do not produce full-fledged offspring or do not leave offspring at all (for example, males who lost mating fights with stronger rivals; plants in conditions of light or nutrition deficiency, etc.) .

In this case, not just some specific positive or negative qualities of organisms are selected or discarded, but entire genotypes that carry these characteristics (including many other characteristics that influence the further course and speed of evolutionary processes).

Forms of natural selection

Currently, there are three main forms of natural selection, which are given in school textbooks on general biology.

Stabilizing natural selection

This form of natural selection is characteristic of stable conditions of existence that do not change for a long time. Therefore, in populations there is an accumulation of adaptations and selection of genotypes (and the phenotypes they form) that are appropriate specifically for existing conditions. When populations reach a certain set of adaptations that are optimal and sufficient for survival in given conditions, stabilizing selection begins to act, cutting off extreme variants of variability and favoring the preservation of some average conservative characteristics. All mutations and sexual recombinations that lead to deviations from this norm are eliminated by stabilizing selection.

For example, the length of the limbs of hares should provide them with sufficiently fast and stable movement, allowing them to escape from a pursuing predator. If the limbs are too short, the hares will not be able to escape from predators and will become easy prey before they have time to give birth. This is how carriers of short-legged genes are removed from hare populations. If the limbs are too long, the hares' running will become unstable, they will topple over, and predators will easily be able to catch up with them. This will lead to the removal of carriers of long-legged genes from hare populations. Only individuals with an optimal length of limbs and their optimal ratio to the size of the body will be able to survive and give birth to offspring. This is a manifestation of stabilizing selection. Under its pressure, genotypes that differ from some average and reasonable norm under given conditions are eliminated. The formation of protective (camouflaging) coloration also occurs in many animal species.

The same applies to the shape and size of flowers, which should ensure sustainable pollination by insects. If the flowers have a too narrow corolla or short stamens and pistils, then insects will not be able to reach them with their paws and proboscis and the flowers will be unpollinated and will not produce seeds. Thus, the formation of optimal sizes and shapes of flowers and inflorescences occurs.

Over very long periods of stabilizing selection, some species of organisms may arise whose phenotypes remain virtually unchanged for many millions of years, although their genotypes, of course, have undergone changes during this time. Examples include the lobe-finned fish coelacanth, sharks, scorpions and some other organisms.

Driving selection

This form of selection is typical for changing environmental conditions, when directed selection occurs in the direction of a changing factor. This is how mutations accumulate and the phenotype changes, associated with this factor and leading to a deviation from the average norm. An example is industrial melaninogenesis, which manifested itself in birch moth butterflies and some other species of lepidoptera, when, under the influence of industrial soot, birch tree trunks darkened and white butterflies (the result of stabilizing selection) became noticeable against this background, which caused them to be quickly eaten by birds. The benefit went to dark mutants, which successfully reproduced in new conditions and became the dominant form in birch moth populations.

A shift in the average value of a trait towards the active factor can explain the appearance of heat-loving and cold-loving, moisture-loving and drought-resistant, salt-loving species and forms in different representatives of the living world.

As a consequence of the action of driving selection, there have been numerous cases of adaptation of fungi, bacteria and other pathogens of human, animal and plant diseases to drugs and various pesticides. This is how forms resistant to these substances emerged.

During driving selection, divergence (branching) of characters usually does not occur, and some characters and the genotypes that carry them are smoothly replaced by others, without forming transitional or deviating forms.

Disruptive or disruptive selection

With this form of selection, extreme variants of adaptations receive advantages, and intermediate traits that have developed under conditions of stabilizing selection become inappropriate in new conditions, and their carriers die out.

Under the influence of disruptive selection, two or more forms of variability are formed, often leading to polymorphism - the existence of two or more phenotypic forms. This can be facilitated by different living conditions within the range, leading to the emergence of several local populations within the species (the so-called ecotypes).

For example, constant mowing of plants led to the appearance of a large rattle of two populations in the plant, actively reproducing in June and August, since regular mowing caused the extermination of the average July population.

With prolonged action of disruptive selection, the formation of two or more species may occur, inhabiting the same territory, but being active at different times. For example, frequent droughts in mid-summer, unfavorable for fungi, led to the appearance of spring and autumn species and forms.

Struggle for existence

The struggle for existence is the main operating mechanism of natural selection.

Charles Darwin drew attention to the fact that in nature there are constantly two opposing development trends: 1) the desire for unlimited reproduction and settlement and 2) overpopulation, large crowding, the influence of other populations and living conditions, which inevitably lead to the emergence of a struggle for existence and limitation development of species and their populations. That is, the species strives to occupy all possible habitats for its existence. But the reality is often harsh, resulting in species numbers and habitats being significantly limited. It is the struggle for existence against the background of high mutagenesis and combinative variability during sexual reproduction that leads to the redistribution of characteristics, and its direct consequence is natural selection.

There are three main forms of struggle for existence.

Interspecies fight

This form, as the name suggests, is carried out at the interspecific level. Its mechanisms are complex biotic relationships that arise between species:

Amensalism is the infliction of damage by one population on another population (for example, the release of antibiotics, trampling of grass and nests of small animals by large animals without any gain for themselves);

Competition is the struggle for common sources of nutrition and resources (for food, water, light, oxygen, etc.;

Predation - feeding at the expense of other species, but the development cycles of predators and prey are unrelated or poorly related;

Commensalism (freeloading) - a commensal lives at the expense of another organism, without affecting the latter (for example, many bacteria and fungi live on the surface of the roots, leaves and fruits of plants, feeding on their secretions);

Protocooperation is a mutually beneficial relationship for both species, but not obligatory (random) for them (for example, some birds brush the teeth of crocodiles, using the remains of their food and the protection of a large predator; the relationship between hermit crabs and sea anemones, etc.);

Mutualism is a positive and obligatory relationship for both types (for example, mycorrhizae, lichen symbioses, intestinal microbiota, etc.). Partners either cannot develop without each other, or their development is worse in the absence of a partner.

Combinations of these connections can improve or worsen living conditions and the rate of reproduction of populations in nature.

Intraspecific struggle

This form of struggle for existence is associated with overpopulation of populations, when competition arises between individuals of the same species for a place to live - for nesting, for light (in plants), moisture, nutrients, territory for hunting or grazing (in animals), etc. It manifests itself, for example, in skirmishes and fights in animals and in shading of rivals due to faster growth in plants.

This same form of struggle for existence also includes the struggle for females (mating tournaments) in many animals, when only the strongest male can leave offspring, and weak and inferior males are excluded from reproduction and their genes are not passed on to offspring.

Part of this form of struggle is caring for the offspring, which exists in many animals and helps reduce mortality among the younger generation.

Combating abiotic environmental factors

This form of struggle is most acute in years with extreme weather conditions - severe droughts, floods, frosts, fires, hail, eruptions, etc. Under these conditions, only the strongest and hardiest individuals can survive and leave offspring.

The role of selection of organisms in the evolution of the organic world

The most important factor in evolution (along with heredity, variability and other factors) is selection.

Evolution can be divided into natural and artificial. Natural evolution is called evolution that occurs in nature under the influence of natural environmental factors, excluding the direct direct influence of humans.

Artificial evolution is called evolution carried out by man in order to develop forms of organisms that satisfy his needs.

Selection plays an important role in both natural and artificial evolution.

Selection is either the survival of organisms more adapted to a given environment, or the culling of forms that do not meet certain criteria.

In this regard, two forms of selection are distinguished - artificial and natural.

The creative role of artificial selection is that a person creatively approaches the breeding of a plant variety, an animal breed, a strain of microorganisms, combining different methods of selection and selection of organisms in order to form characteristics that best suit human needs.

Natural selection is the survival of individuals most adapted to specific conditions of existence, and their ability to leave offspring that are fully functional under given conditions of existence.

As a result of genetic research, it became possible to distinguish two types of natural selection - stabilizing and driving.

Stabilizing is a type of natural selection in which only those individuals survive whose characteristics strictly correspond to given specific environmental conditions, and organisms with new characteristics resulting from mutations die or do not produce full-fledged offspring.

For example, a plant is adapted to pollination by a given specific type of insect (it has strictly defined sizes of flower elements and their structure). A change occurred - the cup size increased. The insect freely penetrates inside the flower without touching the stamens, due to which pollen does not fall on the body of the insect, which prevents the possibility of pollinating the next flower. This will lead to the fact that the plant will not produce offspring and the resulting trait will not be inherited. If the calyx size is very small, pollination is generally impossible, since the insect will not be able to penetrate the flower.

Stabilizing selection makes it possible to lengthen the historical period of existence of a species, since it does not allow the characteristics of the species to be “eroded.”

Driving selection is the survival of those organisms that develop new characteristics that allow them to survive in new environmental conditions.

An example of driving selection is the survival of dark-colored butterflies against a background of smoked birch trunks in a population of light-colored butterflies.

The role of driving selection is the possibility of the emergence of new species, which, along with other factors of evolution, made possible the emergence of the modern diversity of the organic world.

The creative role of natural selection is that through various forms of struggle for existence, organisms develop characteristics that allow them to most fully adapt to given environmental conditions. These useful traits are fixed in organisms due to the survival of individuals that have such traits and the extinction of those individuals that do not have useful traits.

For example, reindeer are adapted to life in the polar tundra. He can survive there and give birth to normal fertile offspring if he can get his food normally. The deer's food is moss (reindeer moss, a lichen). It is known that the tundra has a long winter and food is hidden under the snow cover, which the deer needs to destroy. This will become possible only if the deer has very strong legs equipped with wide hooves. If only one of these signs is realized, then the deer will not survive. Thus, in the process of evolution, only those individuals survive that possess the two characteristics described above (this is the essence of the creative role of natural selection in relation to reindeer).

It is important to understand the differences between natural and artificial selection. They are:

1) artificial selection is carried out by humans, and natural selection is spontaneously realized in nature under the influence of external environmental factors;

2) the result of artificial selection is new breeds of animals, plant varieties and strains of microorganisms with traits useful for human economic activity, and with natural selection, new (any) organisms arise with traits that allow them to survive in strictly defined environmental conditions;

3) during artificial selection, the traits that arise in organisms can not only be not useful, they can be harmful for a given organism (but they are useful for human activity); with natural selection, the resulting traits are useful for a given organism in a given, specific environment of its existence, since they contribute to its better survival in this environment;

4) natural selection has been carried out since the appearance of organisms on Earth, and artificial selection has been carried out only since the domestication of animals and the advent of agriculture (growing plants in special conditions).

So, selection is the most important driving force of evolution and is realized through the struggle for existence (the latter refers to natural selection).

Living in natural conditions, there is individual variability, which can manifest itself in three types - beneficial, neutral and harmful. Typically, organisms with harmful variability die at various stages of individual development. Neutral variability of organisms does not affect their viability. Individuals with beneficial variation survive due to advantages in intraspecific, interspecific, or environmental struggles.

Driving selection

When environmental conditions change, those individuals of the species that have exhibited hereditary variability and, as a result, developed characteristics and properties corresponding to the new conditions, survive, and those individuals that did not have such variability die. During his voyage, Darwin discovered that on oceanic islands, where strong winds prevail, there are few long-winged insects and many insects with vestigial wings and wingless insects. As Darwin explains, insects with normal wings could not withstand the strong winds on these islands and died. But insects with rudimentary wings and wingless ones did not rise into the air at all and hid in crevices, finding shelter there. This process, which was accompanied by hereditary variability and natural selection and continued for many thousands of years, led to a reduction in the number of long-winged insects on these islands and to the appearance of individuals with vestigial wings and wingless insects. Natural selection, which ensures the emergence and development of new characteristics and properties of organisms, is called driving selection.

Disruptive selection

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

Natural selection 3 types. How to pass her natural selection

Mechanism of natural selection

Forms of natural selection

Driving selection

Stabilizing selection

Disruptive selection

Sexual selection

Selection methods: positive and negative selection

The role of natural selection in evolution

see also

Notes

Forms of natural selection

Stabilizing natural selection

Driving selection

Disruptive or disruptive selection

Struggle for existence

Interspecies fight

Intraspecific struggle

Combating abiotic environmental factors

The role of selection of organisms in the evolution of the organic world

Driving selection

Disruptive selection