Natural selection is the driving factor of evolution. Types of natural selection

The situation, or you can act at random. It is enough to create a wide range of diverse individuals - and, ultimately, the fittest will survive.

1. At first an individual appears with new, completely random properties
2. After she is or is not able to leave offspring, depending on these properties
3. Finally, if the outcome of the previous stage is positive, then she leaves offspring and her descendants inherit the newly acquired properties

Currently, the somewhat naive views of Darwin himself have been partially reworked. Thus, Darwin imagined that changes should occur very smoothly, and the spectrum of variability should be continuous. Today, however, mechanisms natural selection are explained using genetics, which brings some originality to this picture. Mutations in the genes that operate in the first step of the process described above are essentially discrete. It is clear, however, that the basic essence of Darwin's idea remains unchanged.

Forms of natural selection

Driving selection

Driving selection- a form of natural selection when environmental conditions contribute to a certain direction of change in a characteristic or group of characteristics. At the same time, other possibilities for changing the trait are subject to negative selection. As a result, a shift occurs in the population from generation to generation average size sign in a certain direction. In this case, the pressure of driving selection must correspond to the adaptive capabilities of the population and the rate of mutational changes (otherwise, environmental pressure can lead to extinction).

A modern case of driving selection is the "industrial melanism of English butterflies." “Industrial melanism” is a sharp increase in the proportion of melanistic (dark-colored) individuals in butterfly populations that live in industrial areas. Due to industrial impact, 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 reached 95%, while the first dark-colored butterfly (Morfa carbonaria) was captured in 1848.

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

Stabilizing selection

Stabilizing selection- a form of natural selection in which the action is directed against individuals with extreme deviations from the average norm, in favor of individuals with an average expression of the trait.

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.

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. Disruptive selection contributes to the emergence and maintenance of population polymorphism, and in some cases can cause speciation.

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

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

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

Cutting selection

Cutting selection- a form of natural selection. Its action is the opposite of positive selection. 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.

Positive selection

Positive selection- a form of natural selection. Its action is the opposite of cutting 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. With the help of positive selection and cutting selection, species are changed (and not only through the destruction of unnecessary individuals, then any development should stop, but this does not happen).

Examples of positive selection include: a stuffed Archeopteryx can be used as a glider, but a stuffed swallow or seagull cannot. But the first birds flew better than Archeopteryx. Another example of positive selection is the emergence of predators that are superior in their “mental abilities” to many other warm-blooded animals. Or the appearance of reptiles such as crocodiles, which have a four-chambered heart and are able to live both on land and in water.

Particular directions of natural selection

• The survival of the most adapted species and populations, such as those with gills in water, because fitness wins the fight for survival.
• Survival of physically healthy organisms.
• Survival of the physically strongest organisms, since physical competition for resources is an integral part of life. It is important in intraspecific struggle.
• Survival of the most sexually successful organisms, as sexual reproduction is the dominant mode of reproduction. IN in this case sexual selection comes into play.

However, all these cases are special, and the main thing remains successful preservation over time. Therefore, sometimes these directions are violated in order to pursue the main goal.

The role of natural selection in evolution

Darwin hesitated for a long time to publish his theory, because... I saw a problem with ants, which could only be explained from the standpoint of genetics.

see also

Links

• “Problems of macroevolution” - website of paleontologist A.V. Markov
• “Forms of Natural Selection” - an article with well-known examples: the color of butterflies, human resistance to malaria, etc.
• “Evolution based on patterns” - an article about whether the role of mutations in the process of evolution is great, or whether some characteristics exist in advance and then develop under the influence of driving selection

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

Answer

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

Answer

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

Answer

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 characteristic values
5) preserves individuals with an established norm of reaction of the trait
6) promotes the appearance of mutations in the population

Answer

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 rejected
5) the number of individuals with the average value of the trait increases
6) individuals with new characteristics are preserved

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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) a decrease in the size of the cephalothorax in crabs living in muddy 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

Answer

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

Answer

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

Answer

© D.V. Pozdnyakov, 2009-2019

Natural selection is the basis of evolution. It can be considered a process as a result of which in populations of living organisms the number of individuals that are better adapted to environmental conditions increases. While the number of individuals less adapted for certain characteristics decreases.

Since the environmental conditions of populations are not the same (in some places the conditions are stable, in others they are variable), there are several different forms of natural selection. Typically, three main forms are distinguished: stabilizing, driving and disruptive selection. There is also sexual natural selection.

Stabilizing form of natural selection

Mutations always occur in populations of organisms, and there is also combinative variability. They lead to the appearance of individuals with new characteristics or their combinations. However, if environmental conditions remain constant and the population has already been well adapted to them, then the new values ​​of traits that appear usually become irrelevant. The individuals in which they arose turn out to be less adapted to the existing conditions, lose the struggle for existence and leave fewer offspring. As a result, new characteristics are not fixed in the population, but are removed from it.

Thus, the stabilizing form of natural selection operates under constant environmental conditions and maintains average, widespread values ​​of traits in the population.

An example of stabilizing selection is the maintenance of average fertility in many animals. Individuals giving birth a large number of cubs, they cannot feed them well. As a result, the offspring turn out to be weak and die in the struggle for existence. Individuals that give birth to a small number of cubs cannot fill the population with their genes in the same way as individuals that give birth to an average number of cubs can.

Red shows the distribution of the trait in the old population, blue - in the new one.

The driving form of natural selection

The driving form of natural selection begins to act in changing environmental conditions. For example, with a gradual cooling or warming, a decrease or increase in humidity, the appearance of a new predator that slowly increases its number. Also, the environment may change as a result of expansion of the population's range.

It should be noted that a gradual change in conditions is important for natural selection, since the emergence of new adaptations in organisms is a long process that occurs over many generations. If conditions change sharply, then populations of organisms usually simply die out or move to new habitats with the same or similar conditions.

Under new conditions, some previously harmful and neutral mutations and combinations of genes may turn out to be useful, increasing the adaptability of organisms and their chances of survival in the struggle for existence. Consequently, such genes and the traits they define will become fixed in the population. As a result, each new generation of organisms will move further and further away from the original population in some way.

It is important to understand that with the driving form of natural selection, only a certain value of a trait from previously unuseful ones turns out to be useful, and not all. For example, if previously only individuals with average height survived, and large and small ones died, then with driving selection, individuals with only small height will survive better, but those with average and especially large height will find themselves in worse conditions and gradually disappear from the population .

A disruptive form of natural selection

The disruptive form of natural selection is similar in its mechanism to driving form. However, there is a significant difference. Driving selection favors only one value of a particular trait, removing from the population not only the average value of this trait, but also all other extremes. Disruptive selection acts only against the average value of a trait, usually favoring two extreme values ​​of the trait. For example, on the islands with strong wind

Insects survive without wings (they do not fly) or with powerful wings (they can resist the wind when flying). Insects with medium wings are carried into the ocean. Disruptive natural selection leads to the emergence polymorphism

in populations, when, according to some characteristic, two or more varieties of individuals are formed, sometimes occupying slightly different ecological niches.

In sexual selection, individuals in populations choose as partners those individuals of the opposite sex who possess some trait (for example, a bright tail, large horns) that is not directly related to increased survival or even harmful to this. Possessing such a trait increases the chances of reproduction and, therefore, consolidation of one’s genes in the population.

There are several hypotheses regarding the reasons for the emergence of sexual selection.

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

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

with artificial selection, the modern form of which is selective breeding. The idea of ​​comparing artificial and natural selection is that in nature the selection of the most “successful”, “best” organisms also occurs, but in this case the role of “evaluator” of the usefulness of properties is not a person, but the environment. In addition, the material for both natural and artificial selection is small hereditary changes that accumulate from generation to generation.

Mechanism of natural selection

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

Organisms produce more offspring than can survive;

There is heritable variation in the population of these organisms;

Organisms with different genetic traits have different survival rates and ability to reproduce. high probability pass them on to their descendants than organisms with hereditary traits that do not have a similar advantage.

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

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

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

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

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

Stabilizing selection- selection against extreme values ​​of a trait, which leads to a decrease in the variance of the trait.

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

Selection can operate at different levels of organization, such as genes, cells, individual organisms, groups of organisms, and species. Moreover, selection can simultaneously act on different levels. Selection at levels above the individual, such as group selection, can lead to cooperation.

Forms of natural selection

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

Driving selection- a form of natural selection that operates when directed changing conditions external environment. Described by Darwin and Wallace. In this case, individuals with traits that deviate in a certain direction from the average value receive advantages. In this case, other variations of the trait (its deviations in the opposite direction from the average value) are subject to negative selection. As a result, in the 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).

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

Driving selection occurs when 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. Shmalgauzen.

Many examples of the action of stabilizing selection in nature have been described. For example, at first glance it seems that the greatest contribution to the gene pool of the next generation should be made by individuals with maximum fertility. However, observations of natural populations of birds and mammals show that this is not the case. The more chicks or cubs in the nest, the more difficult it is to feed them, the smaller and weaker each of them is. As a result, individuals with average fertility are the most fit.

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

Most widely famous example This polymorphism is sickle cell anemia. This severe blood disease occurs in people homozygous for the mutant hemoglobin allele ( Hb S) and leads to their death at an early age. In most human populations, the frequency of this allele is very low and approximately equal to the frequency of its occurrence due to mutations. However, it is quite common in areas of the world where malaria is common. It turned out that heterozygotes for Hb S have higher resistance to malaria than homozygotes for the normal allele. Thanks to this, in populations inhabiting malarial areas, heterozygosity for this allele, which is lethal in homozygotes, is created and stably maintained.

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

Thus, stabilizing selection, sweeping aside deviations from the norm, actively shapes genetic mechanisms that ensure the stable development of organisms and the formation of optimal phenotypes based on various genotypes. It ensures the stable functioning of organisms in a wide range of fluctuations in external conditions familiar to the species.

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

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

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

Disruptive selection was carried out artificially in experiments with Drosophila. 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 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 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 bright males, for whatever reason, are attractive to females, then it is worth choosing a bright father for your future sons, because his sons will inherit the genes for bright colors and will be attractive to females in the next generation. Thus, there is a positive, which leads to the fact that from generation to generation the brightness of the plumage of males increases more and more. The process continues to grow until it reaches the limit of viability.

In choosing 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, 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 variety of shapes, colors and instincts that we observe in the world of living nature .

Positive and negative selection

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

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

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

The role of natural selection in evolution

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

Natural selection favors the survival and increase in numbers in a population of individuals carrying some genotypes to the detriment of carriers of others. This contributes to the accumulation in the population of traits that have adaptive significance.

IN different conditions environment, natural selection has a different character. There are three main forms of natural selection:

• Moving;
• stabilizing;
• disruptive.

Motion form (with examples)

The manifestation of driving selection is felt when the emerging changes in the new environment turn out to be more useful. Selection will be aimed at their preservation. This will entail gradual changes in the phenotype of individuals in the population, a change in the reaction norm and a change in the average value of the trait.

A classic example of driving selection is the change in color of moths in the vicinity of industrial cities in Europe and America. If previously a light color was typical for them, then as tree trunks became contaminated with soot and soot, the light variants that became noticeable on the tree bark were primarily eaten by birds and gained more and more advantage dark options, it was they who were preserved by natural selection. This led to a change in color.

Evolution and the emergence of new adaptations are associated with driving selection. In recent decades, many species of insects have developed races that are resistant to insecticides (drugs that are poisonous to insects). Insects sensitive to the poison died, but in some individuals a new mutation arose or they previously had a neutral gene for insensitivity to any insecticides. Under changed conditions, the gene ceased to be neutral. Driving selection has preserved the carriers of this gene. They became the founders of new races.

Stabilizing form (with examples)

Stabilizing selection occurs under relatively constant conditions. Here, deviations from the average value of the attribute may already turn out to be unfavorable and are discarded. In these cases, selection is aimed at preserving mutations leading to less variability in the trait.

It has been established that representatives of the population with an average manifestation of the trait are more resistant to extreme changes in conditions, so sparrows with medium length winged ones survive winter more easily than long- or short-winged ones. Also, constant body temperature in homeothermic animals is a consequence of stabilizing selection.

In plants pollinated by certain types of insects, the structure of the flower corolla cannot vary; it corresponds in shape and size to the size and shape of the pollinators. Any deviations from the “standard” are immediately rejected by selection, since they do not leave offspring.

Stabilizing selection occurs most often and is considered central in the development of organisms, when improvement of average indicators leads to evolutionary progress.

When conditions of existence change, driving and stabilizing selection can replace each other.

Disruptive form (with examples)

Disruptive selection can be observed when, among all variants of the genotype, there is no predominant one, which is associated with the heterogeneity of the territory they inhabit. Under the influence of certain factors, some traits contribute to survival, and when conditions change, others do.

Disruptive selection is directed against those representatives of a species that have average manifestations of a trait, which leads to the appearance of polymorphism among one population. The disruptive form is also called tearing, because the population is divided into separate parts according to the current characteristic. Thus, the disruptive form is responsible for the development of extreme phenotypes and is directed against the average forms.

An example of disruptive selection is the color of the shell of a grape snail. The color of the shell depends on the environmental conditions in which the snail finds itself. In the forest zone, where the surface layer of the earth is colored Brown color, live snails with brown shells. In the steppe region, where the grass is dry and yellow, they have yellow shells. The difference in shell color is adaptive in nature, as it protects the snails from being eaten by birds of prey.

Table of the main types of natural selection

Other types of natural selection

The main forms of selection are described above; there are also additional ones:

• Destabilizing;
• sexual;
• group.

Destabilizing Form the action is opposite to the stabilizing one, while the norm of the reaction expands, but the average indicators are preserved.

Thus, frogs that live in swamps, in environments with different light levels, differ significantly in the color of their skin - this is a manifestation of destabilizing selection. Frogs inhabiting an area that is completely shaded or, conversely, with good access to light, have a uniform color - this is a manifestation of stabilizing selection.

Sexual form of natural selection is aimed at the formation of secondary sexual characteristics, which help to select a pair for crossing. For example, the bright color of feathers and the singing of birds, loud voice, mating dances or the release of odorous substances to attract the opposite mate in insects and more.

Group form aimed at the survival of the population, not individuals. The death of several members of the group to save the species would be justified. Thus, in a herd of wild animals, it is determined at the genetic level that the life of the group is more important than one’s own. When danger approaches, the animal will make loud noises to warn its relatives, in which case it will die, but will save the rest.