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Forms of selection: driving, stabilizing, disruptive. Driving selection. An example of a driving form of selection. What is natural selection

Driving selection. Exists many forms of natural selection. The initial (leading) form is driving selection, which leads to changes in the genetic and phenotypic structure of the population. The essence of driving selection is the accumulation and strengthening of deviations from the original (normal) variant of a trait. In the future, the original version of the sign may become a deviation from the norm. Thus, driving selection manifests itself in the form of a stable and, to a certain extent, directional change in the frequencies of alleles (genotypes, phenotypes) in the population. The result of driving selection is the achievement of a population state in which its average fitness reaches a maximum. In other words, during driving selection the average fitness population, but not necessarily all its members. The appearance in a population of a trait (phenotype and corresponding genotype) that ensures an increase in the fitness of individual individuals or the population as a whole is called an elementary adaptive phenomenon. Sustainable and, to a certain extent, a directed change in the genetic structure of the population, in particular, a change in the frequency of the allele (haplotype) is called an elementary evolutionary phenomenon.

Transitive (transitional) polymorphism. Polymorphism is an external manifestation of intrapopulation diversity, the coexistence in a population of two or more forms (variants) of a trait. Evolutionary (genetic) polymorphism is the coexistence of two or more alleles (genotypes, phenotypes) in a population, and the presence of a rare allele or variant of a trait cannot be explained only by the mutation process or migrations. At intermediate stages of driving selection, the population is polymorphic, but this transitive or transitional polymorphism there is a limited number of generations until the original allele (genotype, phenotype) is replaced by a new allele (genotype, phenotype).

8. Stabilizing selection. Stable polymorphism. Channel selection
Stabilizing selection– the total result of the action of two or more directions of driving selection in favor of one genotype/phenotype or a group of genotypes with a similar phenotype. Stabilizing selection is aimed at preserving the genetic and phenotypic structure of the population.

Stabilizing selection manifests itself in the form of preservation of allele frequencies (genotypes, phenotypes) in a population. The result Stabilizing selection is the preservation of a population state in which its average fitness is maximum. There are two forms stabilizing selection: purifying selection and selection for diversity.

At purifying selection the original (normal) version of the trait is preserved. Deviations from the normal variant of the trait reduce the fitness of individuals and are removed (eliminated) from the population. In this case, the frequency of one of the alleles tends to 1, and the frequencies of other alleles of a given gene tend to zero. When selecting for diversity, selection often acts in favor of heterozygotes (the superiority of heterozygotes over homozygotes is called overdominance). Then two or more alleles of one gene remain in a constant ratio for a long time in the population. Stabilizing selection for diversity leads to the emergence and maintenance of a balanced (stable) polymorphism. This type of polymorphism persists in populations for an indefinite period of time. Powerful stabilizing selection contributes to the preservation of taxa. Numerous persistent forms are known - “living fossils” (brachiopods, horseshoe crabs, hatteria, coelacanth, ginkgo). In horseshoe crabs, intrapopulation polymorphism is no less than in young arthropod species, however, any deviation from the average value of a trait (from the adaptive norm) leads to a decrease in fitness.

The theory of stabilizing selection was developed by Ivan Ivanovich Shmalhausen.

Stabilizing selection is often referred to as and channel selection– selection for stability of development (homeorez), for autonomization of ontogenesis.

9. Disruptive selection. Consequences of disruptive selection. The role of disruptive selection in speciation
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. 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. ( Ecological niche- the place occupied by a species in a biocenosis, including a complex of its biocenotic connections and requirements for environmental factors.)

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.

Evolutionary consequences of disruptive selection:

1. If homozygotes are characterized by equal fitness, but allele frequencies are unequal, then selection acts in favor of the allele with an initially higher frequency. As a result, of two equal alleles, the one that was initially characterized by a higher frequency is fixed.

2. With unequal fitness of homozygotes and equal initial frequencies of alleles, selection acts in favor of the allele that provides maximum fitness. As a result, such an allele is fixed.

3. With unequal fitness of homozygotes and unequal initial frequencies of alleles, selection acts so that the average fitness of the population does not decrease. As a result, an allele may be fixed in the population, which reduced the fitness of genotypes, but had a fairly high frequency. Then the average fitness of the population turns out to be less than the maximum possible.

10. Sexual selection, its specifics
Sexual selection- a process based on competition for a sexual partner between individuals of the same sex, which entails selective mating and production of offspring

This mechanism may cause the evolution of certain characteristic traits and lead to their strengthening. Within a species, one of the sexes (almost always female) plays the role of a limited resource for the other (almost always male). Sexual selection is natural selection for reproductive success. The survival of organisms is an important, but not the only component of natural selection. Another important component is attractiveness to members of the opposite sex. Darwin called this phenomenon sexual selection. “This form of selection is determined not by the struggle for existence in the relations of organic beings among themselves or with external conditions, but by the competition between individuals of one sex, usually males, for the possession of individuals of the other sex.” Traits that reduce the viability of their hosts can emerge and spread if the advantages they provide for reproductive success are significantly greater than their disadvantages for survival.

Two hypotheses about the mechanisms of sexual selection are common.

According to the “good genes” hypothesis, the female “reasons” as follows: “If a given male, despite his bright plumage and long tail, managed not to die in the clutches of a predator and survive to sexual maturity, then he has good genes that allowed him to do this. Therefore, he should be chosen as the father of his children: he will pass on his good genes to them.” By choosing colorful males, females are choosing good genes for their offspring.

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

When choosing males, females do not think about the reasons for their behavior. When an animal feels thirsty, it does not reason that it should drink water in order to restore the water-salt balance in the body - it goes to a watering hole because it feels thirsty. In the same way, females, when choosing bright males, follow their instincts - they like bright tails. Those for whom instinct suggested different behavior did not leave offspring. The logic of the struggle for existence and natural selection is the logic of a blind and automatic process, which, acting constantly from generation to generation, has formed the amazing variety of forms, colors and instincts that we observe in the world of living nature.

11. Kin selection. The action of kin selection in humans and social insects
Selection of relatives– selection in colonies, families and ultra-small populations. Leads to the accumulation of altruistic characteristics (altruism - self-sacrifice).

If an “altruism allele” appears in a family (in reality, altruism is determined by complex genetic complexes), then half of the relatives will have it. The carrier of this allele, by taking care of his grandchildren or nephews, contributes to their survival and the accumulation of the “altruism allele” in subsequent generations.

Altruism is manifested in its highest form in social Hymenoptera (bees, ants), since these insects have sisters who are closer relatives than mother and daughters. This effect is due to the haploidity of males - all siblings carry half of the exact same alleles.

Kin selection has played a significant role in human evolution, along with sexual selection.

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.

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- 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

Natural selection tests organisms for compliance with living conditions and is carried out in different forms that have their own characteristics. What form or mechanism of selection acts on a given group of organisms depends on climatic, geological and other conditions.

The driving form of natural selection preserves useful deviations from the average norm.

This deviation can be any trait that increases the survival and fertility of some organisms compared to others.

There are two types of driving selection:

transitive (transitive);
directed.

Transitional selection is the development of an initially small form that gains an advantage under changed environmental conditions.

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An example of such selection is the development of industrial melanism in Lepidoptera.

Thus, the birch moth previously had about 98% of light-colored individuals in populations. As the bark of trees darkened in industrial areas, dark-colored moths began to predominate because they became less noticeable to birds.

The action of transitive selection is reversible, and when external conditions change, the ratio of dark and light individuals will also change.

With directed selection, the formation and reproduction of forms differ in some characteristic from the original form. Such selection occurs under conditions of unidirectional environmental changes.

Rice. 1. Driving selection.

Unlike transitional selection, with this type of selection there is no ready-made different form and useful changes accumulate in ordinary representatives of the species.

For example, bacteria can mutate when exposed to antibiotics. The resulting mutants are resistant to doses much higher than the original.

Stabilizing selection

If we talk briefly about the stabilizing form of natural selection, it is the preservation of average norms.

The condition for stabilizing selection is constant environmental parameters, and in this it is the opposite of driving selection.

Rice. 2. Stabilizing selection.

Each species has an optimal average rate of fertility and weight of the cubs born.

If birds lay fewer eggs than normal, this may not be enough to maintain the population. If the chicks hatch more than the average norm, then the parents risk not feeding them.

In this case we see the action of stabilizing selection. Increased fertility is not an advantage in conditions of competition and lack of food.

Driving and stabilizing are the two main forms of natural selection, which are essentially two sides of the same process.

Disruptive selection

A disruptive, or disruptive, form of selection splits a previously single population into two or more new ones.

Thus, female African swallowtail butterflies have evolved into three forms, imitating three different inedible butterfly species.

Rice. 3. Three forms of female African swallowtails.

Having such similarities is more beneficial to a population than imitating just one species.

Disruptive selection drives stratifying evolution , as a result of which new groups of organisms are formed, for example, many orders in the class of mammals.

Table “Forms of natural selection”

	*Propulsion*	*Stabilizing*	*Tearing*
Terms of action	Gradual environmental changes	Constant environmental conditions	Availability of several options for adapting to the environment
Focus	In favor of individuals with useful traits different from the original ones	Against extreme values of a characteristic in favor of the average	Against the average values of the characteristic in favor of the extreme ones
Result	Creating a new average rate	Maintaining the average rate	Creation of two or more new norms
	The emergence of populations resistant to pesticides, antibiotics, etc.	Preservation of flower shape and size in insect-pollinated plants to match the size of the pollinator; relict species	Preservation of groups of insects with either highly developed or small wings in conditions of frequent windy weather

What have we learned?

While studying the three forms of natural selection in biology, we gave them a brief description. Forms of selection differ in: conditions, focus, results. Stabilizing selection preserves old adaptations, while disruptive and driving selection preserves new ones. At the same time, the purpose of all forms is to adapt organisms to the conditions of existence.

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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.

Among organisms of a certain species, individuals with two or more different forms are sometimes found. This is the result of a special form of natural selection - disruptive selection. Thus, ladybugs have two forms of hard wings - with a dark red and a reddish color. Beetles with reddish wings are less likely to die from the cold in winter, but produce few offspring in the summer, while those with dark red wings, on the contrary, more often die in winter, being unable to withstand the cold, but produce numerous offspring in the summer. Consequently, these two forms of ladybugs, due to their different adaptations to different seasons, managed to preserve their offspring for centuries.

Natural selection is the main, leading, guiding factor of evolution, which underlies the theory of Charles Darwin. All other factors of evolution are random; only natural selection has a direction (towards the adaptation of organisms to environmental conditions).

Definition: selective survival and reproduction of the fittest organisms.

Creative role: By selecting useful traits, natural selection creates new ones.

Efficiency: The more different mutations there are in a population (the higher the heterozygosity of the population), the greater the efficiency of natural selection, the faster evolution proceeds.

Shapes:

Stabilizing - acts under constant conditions, selects average manifestations of the trait, preserves the characteristics of the species (coelacanth fish)
Driving - acts in changing conditions, selects extreme manifestations of a trait (deviations), leads to changes in traits (birch moth)
Sexual - competition for a sexual partner.
Tearing - selects two extreme forms.

Consequences of natural selection:

Evolution (change, complication of organisms)
Emergence of new species (increase in the number [diversity] of species)
Adaptation of organisms to environmental conditions. All fitness is relative, i.e. adapts the body to only one specific condition.

Choose one, the most correct option. The basis of natural selection is
1) mutation process
2) speciation
3) biological progress
4) relative fitness

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Choose one, the most correct option. What are the consequences of stabilizing selection?
1) preservation of old species
2) change in reaction norm
3) the emergence of new species
4) preservation of individuals with altered characteristics

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Choose one, the most correct option. In the process of evolution, a creative role plays
1) natural selection
2) artificial selection
3) modification variability
4) mutational variability

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Choose three options. What features characterize driving selection?
1) operates under relatively constant living conditions
2) eliminates individuals with an average trait value
3) promotes the reproduction of individuals with an altered genotype
4) preserves individuals with deviations from the average values of the trait
5) preserves individuals with an established norm of reaction of the trait
6) promotes the appearance of mutations in the population

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Select three traits that characterize the driving form of natural selection
1) ensures the emergence of a new species
2) manifests itself in changing environmental conditions
3) the adaptability of individuals to the original environment improves
4) individuals with deviations from the norm are discarded
5) the number of individuals with the average value of the trait increases
6) individuals with new characteristics are preserved

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Choose one, the most correct option. The starting material for natural selection is
1) struggle for existence
2) mutational variability
3) change in the habitat of organisms
4) adaptability of organisms to their environment

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Choose one, the most correct option. The starting material for natural selection is
1) modification variability
2) hereditary variability
3) the struggle of individuals for survival conditions
4) adaptability of populations to their environment

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Choose three options. The stabilizing form of natural selection manifests itself in
1) constant environmental conditions
2) change in the average reaction rate
3) preservation of adapted individuals in their original habitat
4) culling of individuals with deviations from the norm
5) preservation of individuals with mutations
6) preservation of individuals with new phenotypes

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Choose one, the most correct option. The efficiency of natural selection decreases when
1) the occurrence of recessive mutations
2) an increase in homozygous individuals in the population
3) change in the reaction norm of the trait
4) increasing the number of species in the ecosystem

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Choose one, the most correct option. In arid conditions, in the process of evolution, plants with pubescent leaves were formed due to the action of
1) relative variability

3) natural selection
4) artificial selection

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Choose one, the most correct option. Pests become resistant to pesticides over time as a result of
1) high fertility
2) modification variability
3) preservation of mutations by natural selection
4) artificial selection

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Choose one, the most correct option. The material for artificial selection is
1) genetic code
2) population
3) genetic drift
4) mutation

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Choose one, the most correct option. Are the following statements about the forms of natural selection true? A) The emergence of resistance to pesticides in insect pests of agricultural plants is an example of a stabilizing form of natural selection. B) Driving selection contributes to an increase in the number of individuals of a species with an average value of the trait
1) only A is correct
2) only B is correct
3) both judgments are correct
4) both judgments are wrong

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Establish a correspondence between the results of the action of natural selection and its forms: 1) stabilizing, 2) driving, 3) disruptive (tearing). Write the numbers 1, 2 and 3 in the correct order.
A) Development of antibiotic resistance in bacteria
B) The existence of fast and slow growing predatory fish in the same lake
C) Similar structure of the visual organs in chordates
D) The appearance of flippers in waterfowl mammals
E) Selection of newborn mammals with average weight
E) Preservation of phenotypes with extreme deviations within one population

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1. Establish a correspondence between the characteristics of natural selection and its form: 1) driving, 2) stabilizing. Write numbers 1 and 2 in the correct order.
A) preserves the average value of the characteristic
B) promotes adaptation to changed environmental conditions
C) retains individuals with a trait that deviates from its average value
D) helps to increase the diversity of organisms
D) contributes to the preservation of species characteristics

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2. Compare the characteristics and forms of natural selection: 1) Driving, 2) Stabilizing. Write numbers 1 and 2 in the correct order.
A) acts against individuals with extreme values of traits
B) leads to a narrowing of the reaction norm
B) usually operates under constant conditions
D) occurs during the development of new habitats
D) changes the average values of a trait in the population
E) can lead to the emergence of new species

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3. Establish a correspondence between the forms of natural selection and their characteristics: 1) driving, 2) stabilizing. Write numbers 1 and 2 in the order corresponding to the letters.
A) acts in changing environmental conditions
B) operates under constant environmental conditions
C) aimed at preserving the previously established average value of the characteristic
D) leads to a shift in the average value of a trait in the population
D) under its influence, both strengthening and weakening of the characteristic can occur

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4. Establish a correspondence between the characteristics and forms of natural selection: 1) stabilizing, 2) driving. Write numbers 1 and 2 in the order corresponding to the letters.
A) forms adaptations to new environmental conditions
B) leads to the formation of new species
C) maintains the average norm of the trait
D) rejects individuals with deviations from the average norm of characteristics
D) increases the heterozygosity of the population

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5. Establish a correspondence between the characteristics and forms of natural selection: 1) stabilizing, 2) driving. Write numbers 1 and 2 in the order corresponding to the letters.
A) manifestation in constant living conditions
B) death of individuals with new characteristics
C) preservation of individuals with new mutations
D) preservation of individuals with an aromorphic trait
D) an increase in the number of individuals with an established reaction norm

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Establish a correspondence between the examples and the forms of natural selection that these examples illustrate: 1) driving, 2) stabilizing. Write numbers 1 and 2 in the order corresponding to the letters.
A) an increase in the number of dark butterflies in industrial areas compared to light ones
B) the emergence of resistance to pesticides in insect pests
C) the preservation to this day of the reptile tuateria, which lives in New Zealand
D) reduction in the size of the cephalothorax in crabs living in turbid water
E) in mammals, the mortality rate of newborns with an average birth weight is lower than with very low or very high birth weights
E) the death of winged ancestors and the preservation of insects with reduced wings on islands with strong winds

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Establish a correspondence between the examples and types of natural selection: 1) driving, 2) tearing. Write numbers 1 and 2 in the order corresponding to the letters.
A) a giraffe has a long neck
B) white and orange wings of yellow butterflies
C) different beak shapes of finches
D) the presence of early and late flowering forms of rattle
D) an increase in the number of light butterflies in the birch forest
E) an increase in average human height from generation to generation

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Establish a correspondence between the forms of struggle for existence and examples illustrating them: 1) intraspecific, 2) interspecific. Write numbers 1 and 2 in the order corresponding to the letters.
A) fish eat plankton
B) seagulls kill chicks when there are a large number of them
B) mating of wood grouse
D) big-nosed monkeys try to outshout each other, inflating their huge noses
D) the chaga mushroom settles on a birch tree
E) the main prey of the marten is squirrel

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Analyze the table “Forms of Natural Selection.” For each letter, select the corresponding concept, characteristic and example from the list provided.
1) sexual
2) driving
3) group
4) preservation of organisms with two extreme deviations from the average value of the trait
5) the emergence of a new feature
6) formation of bacterial resistance to antibiotics
7) preservation of a relict species of the plant Ginkgo biloba 8) increase in the number of heterozygous organisms

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