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The driving force of evolution: what forms of natural selection exist. Natural selection

In this lesson, you will learn what a natural look is, what are its types. How does natural selection affect populations of living organisms? What are the similarities and differences between natural and artificial selection? What exactly is selected in the process of natural selection and how does this process occur? You will get acquainted with stabilizing, moving and tearing (disruptive) selection, find out the nature of sexual selection discovered by Ch. Darwin. Perhaps this lesson will help you in your personal struggle for existence. You will learn how natural selection affects modern man.

Topic: Evolutionary doctrine

Lesson: Types of natural selection

1. Natural selection and its types

Natural selection is the main driving force of evolution.

The notion of natural selection has deepened greatly thanks to modern ideas genetics and the works of domestic scientists I. I. Shmalgauzen and S. S. Chetverikov (Fig. 1), as well as many of their foreign colleagues.

According to modern ideas about natural selection, three forms of it can be distinguished.

2. Driving selection

The first form of natural selection is motive selection. It occurs when environmental conditions change and leads to a shift in the average value of the manifestation of a trait in a population under the influence of environmental factors (Fig. 2). The new feature or its value should be better suited to the changed conditions than the old ones.

Rice. 2. Scheme of the influence of driving selection on the value of the representation of a trait in a population

For example, when the climate becomes colder, individuals with warmer coats are selected.

A classic example of motive selection is the evolution of color in the birch moth. The color of the wings of this butterfly imitates the color of trees covered with gray bark. Atmospheric pollution associated with emissions from factories and factories has led to the darkening of tree trunks. Light butterflies on a dark background became easily visible to birds. From the middle of the 17th century, mutant dark forms of butterflies began to appear in the populations of the birch moth. The frequency of this allele increased rapidly, and by the end of the 19th century, some urban populations of the birch moth almost entirely consisted of dark forms. While in rural populations, where the level of pollution was lower, light forms still dominated.

A change in a trait can occur both in the direction of its strengthening and in the direction of weakening, up to a complete reduction. Thus, for example, there was a disappearance of the visual organs in moles and other burrowing animals, or a reduction in wings in flightless birds and insects (see Fig. 3).

Rice. 3. Examples of long-term exposure to motive selection: lack of eyes in a mole (left) and wings in an ostrich (right)

3. Disruptive selection

The second type of selection is disruptive (tearing) selection. In this case, individuals with several extreme variants of the trait leave offspring, and individuals with an average value of the trait are eliminated (Fig. 4).

Rice. 4. Scheme of the influence of disruptive (tearing) selection on the representation of a trait among individuals in a population

Darwin believed that disruptive selection leads to divergence, i.e., to the divergence of characters, and serves to maintain the polymorphism of the population. In the course of disruptive selection, two forms of butterflies emerged from a common light yellow ancestor: white and yellow. Different colors lead to different heating of the wings. It is convenient for white butterflies to fly at noon, and for yellow butterflies in the morning. It is inconvenient for light yellow butterflies to fly both during the day and in the morning, so the selection acts just against the average value of the trait.

4. Stabilizing selection

The third form of natural selection is stabilizing selection. It operates under constant conditions external environment, by culling individuals with significant deviations of the trait (Fig. 5).

Rice. 5. Scheme of stabilizing selection

It is aimed at maintaining and consolidating the average value of the attribute. For example, the flowers of plants that are pollinated by insects are very conservative, that is, their shape rarely changes. This is due to the fact that pollinating insects cannot penetrate the corolla of a flower that is too deep or too narrow (see video).

Therefore, the genes that lead to such changes in the structure of flowers are not passed on and are forced out of the gene pool.

Thanks to stabilizing selection, the so-called. living fossils.

6. Living fossils

To this day, some species of living beings have survived unchanged, millions of years ago, the former typical representatives flora and fauna of a bygone era.

For example, horseshoe crabs (see Fig. 6), ancient arthropods that lived half a billion years ago, successfully exist today thanks to stabilizing selection. This species is almost twice as old as extinct dinosaurs.

The coelacanth fish coelacanth, whose ancestors were widespread in the Paleozoic era, clearly shows how the transformation of fish fins into the paws of future amphibians could occur.

Stabilizing selection stopped the further evolution of its limbs due to the transition of these fish to life in the depths of the ocean (see video).

5. Sexual selection

There is another concept sexual selection. It has nothing to do with the above classification, and represents the struggle of males or females for the opportunity to leave offspring. That is, this is an example of an intraspecific struggle for existence.

Most often, an individual simply chooses the most powerful and viable partner for itself. Sexual competition leads to the emergence of complex behavioral mechanisms: singing, demonstrative behavior, courtship (see video). Often, fights arise between males, which can end in injury or death of the participants.

Characteristic cat screams at night usually accompany just such fights of competing males.

Sexual selection promotes sexual dimorphism, i.e., differences in external structure males and females. You can remember how roosters and hens, ducks and drakes, male and female deer and walruses differ (see video).

As a result of sexual selection, the strongest, most viable and healthy individuals leave offspring. The rest are removed from reproduction, and their genes disappear from the gene pool of the population.

Homework:

1. What is natural selection? Why is it happening?

2. What is the difference between natural and artificial selection?

3. What is the difference between driving and stabilizing selection?

4. What is tearing selection?

5. Where is natural selection directed?

6. What is sexual selection?

7. What types of natural selection operate in human populations?

8. Give examples of influence different types natural selection on populations of living beings. Is it possible to observe the action of natural selection in nature?

9. What experiments can confirm or disprove the existence of natural selection?

1. Laboratory of Protein Ptysics.

2. Enough. com.

3. Afonin-59-bio. people. ru.

Bibliography

1. Kamensky A. A., Kriksunov E. A., Pasechnik V. V. General biology 10-11 class Bustard, 2005.

2. Belyaev D.K. Biology grade 10-11. General biology. A basic level of. - 11th ed., stereotype. - M.: Education, 2012. - 304 p.

3. Biology grade 11. General biology. Profile level / V. B. Zakharov, S. G. Mamontov, N. I. Sonin and others - 5th ed., stereotype. - Bustard, 2010. - 388 p.

4. Agafonova I. B., Zakharova E. T., Sivoglazov V. I. Biology 10-11 class. General biology. A basic level of. - 6th ed., add. - Bustard, 2010. - 384 p.

Natural selection is the driving force behind evolution. Selection mechanism. Forms of selection in populations (I.I. Shmalgauzen).

Natural selection- the process by which the number of individuals with the maximum fitness (the most favorable traits) increases in the population, 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 adaptations, but not the only cause of evolution. Non-adaptive causes include genetic drift, gene flow, and mutations.

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

Organisms produce more offspring than can survive;

In the population of these organisms, there is hereditary variability;

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

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

The central concept of the concept of natural selection is the fitness of organisms. Fitness is defined as the ability of an organism to survive and reproduce, 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 offspring, but the number of offspring with a given genotype (relative fitness). For example, if the offspring of a successful and rapidly reproducing organism are weak and do not reproduce well, then the genetic contribution and, accordingly, the fitness of this organism will be low.

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

Directed Selection- changes in the average value of the trait over time, for example, an increase in body size;

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

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

A special case of natural selection is sexual selection, the substrate of which is any trait that increases mating success by increasing the individual's attractiveness to potential partners. Traits that have evolved through sexual selection are particularly evident in the males of certain animal species. Such traits as large horns, bright coloration, 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 traits.

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

Forms of natural selection

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

driving selection- a form of natural selection that operates under directed changing environmental conditions. Described by Darwin and Wallace. In this case, individuals with traits that deviate in a certain direction from the average value receive advantages. At the same time, other variations of the trait (its deviations in the opposite direction from the average value) are subjected 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. At the same time, 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 motive selection is the evolution of color in the birch moth. The color of the wings of this butterfly imitates the color of the bark of trees covered with lichens, on which it spends daylight hours. Obviously, such a protective coloration was formed over many generations of previous evolution. However, with the beginning of the industrial revolution in England, this device began to lose its importance. Atmospheric pollution has led to the mass death of lichens and the darkening of tree trunks. Light butterflies on a dark background became easily visible to birds. Since the middle of the 19th century, mutant dark (melanistic) forms of butterflies began to appear in populations of the birch moth. Their frequency increased rapidly. By the end of the 19th century, some urban populations of the moth were almost entirely composed of dark forms, while light forms still predominated in rural populations. This phenomenon has been called industrial melanism. Scientists have found that in polluted areas, birds are more likely to eat light forms, and in clean areas - dark ones. The imposition of restrictions on atmospheric pollution in the 1950s caused natural selection to change direction again, and the frequency of dark forms in urban populations began to decline. They are almost as rare today as they were before the Industrial Revolution.

Driving selection is carried out when the environment changes or adapts to new conditions with the expansion of the range. It preserves hereditary changes in a certain direction, shifting the rate of reaction accordingly. For example, during the development of the soil as a habitat for various unrelated groups of animals, the limbs turned into burrowing ones.

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 severity 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 individuals with maximum fecundity should make the greatest contribution to the gene pool of the next generation. 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. As a result, individuals with average fecundity turn out to be the most adapted.

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

Most widely famous example such a polymorphism is sickle cell anemia. This severe blood disease occurs in people homozygous for a 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 a higher resistance to malaria than homozygotes for the normal allele. Due to this, heterozygosity for this lethal allele in the homozygote is created and stably maintained in populations inhabiting malaria areas.

Stabilizing selection is a mechanism for the accumulation of variability in natural populations. The outstanding scientist I. I. Shmalgauzen was the first to pay attention to this feature of stabilizing selection. He showed that even under stable conditions of existence, neither natural selection nor evolution ceases. Even remaining phenotypically unchanged, the population does not cease to evolve. Its genetic makeup is constantly changing. Stabilizing selection creates such genetic systems that provide 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 concealing genetic variation owe their existence to stabilizing selection.

Thus, stabilizing selection, sweeping aside deviations from the norm, actively forms 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 (tearing) 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 the trait. As a result, several new forms may appear from one initial one. Darwin described the operation of disruptive selection, believing that it underlies divergence, although he could not provide evidence for its existence in nature. Disruptive selection contributes to the emergence and maintenance of population polymorphism, and in some cases can cause speciation.

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

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 - stretched almost all summer, and most of the plants bloom and bear fruit in the middle of summer. However, in hay meadows, those plants that have time to bloom and produce seeds before mowing, and those that produce seeds at the end of summer, after mowing, receive advantages. 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 setae, leaving only individuals with a small and large number of setae. As a result, from about the 30th generation, the two lines diverged very strongly, 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.

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 the genes that let him do it. So, he should be chosen as a father for his children: he will pass on his good genes to them. By choosing bright males, females choose good genes for their offspring.

According to the “attractive sons” hypothesis, the logic of female selection 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 bright color genes and will be attractive to females in next generation. Thus, there is a positive Feedback, which leads to the fact that from generation to generation the brightness of the plumage of males is more and more enhanced. The process goes on increasing until it reaches the limit of viability.

In choosing males, females are no more and no less logical than in all 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 the watering hole because it feels thirsty. In the same way, females, choosing bright males, follow their instincts - they like bright tails. All those who instinctively prompted a different behavior, all of them left no offspring. Thus, we discussed 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 that amazing variety of shapes, colors and instincts that we observe in the world of wildlife. .

positive and negative selection

There are two forms of natural selection: Positive And Clipping (negative) selection.

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

Cut-off selection culls out from the population the vast majority of individuals that carry traits that sharply reduce viability under given environmental conditions. With the help of cut-off selection, strongly harmful alleles are removed from the population. Also, individuals with chromosomal rearrangements and a set of chromosomes that sharply disrupt the normal operation of the genetic apparatus can be subjected to cutting selection.

The role of natural selection in evolution

Charles Darwin considered 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 for the emergence of species and supraspecific taxa, although accumulation in late XIX- 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 alternatively proposed concepts based on the assessment of the genotype mutation factor as extremely important. The authors of such theories postulated not a gradual, but a very rapid (over several generations) spasmodic nature of evolution (the mutationism of Hugo de Vries, the saltationism of Richard Goldschmitt, and other less well-known concepts). The discovery of well-known correlations among the traits 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, batmogenesis, autogenesis, ontogenesis, and others. In the 1920s and 1940s in evolutionary biology, those who rejected Darwin's idea of evolution by natural selection (sometimes called "selectionist" theories that emphasized natural selection) revived interest in this theory due to the revision of classical Darwinism in the light of relatively young science of genetics. The resulting synthetic theory of evolution, often incorrectly referred to as neo-Darwinism, relies, among other things, on the quantitative analysis of allele frequencies in populations as they change 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 "the discoveries of the last decades in various fields of scientific knowledge - from molecular biology with her theory of neutral mutationsMotoo Kimura And paleontology with her theory of punctuated equilibrium Stephen Jay Gould And Niles Eldredge (wherein view understood as a relatively static phase of the evolutionary process) until mathematics with her theorybifurcations And phase transitions- testify to the insufficiency of the classical synthetic theory of evolution for an adequate description of 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."

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

driving selection

An example of the action of motive selection is "industrial melanism" in insects. "Industrial melanism" is a sharp increase in the proportion of melanistic (having a dark color) individuals in those populations of insects (for example, butterflies) that live in industrial areas. Due to industrial impact, tree trunks darkened significantly, and light lichens also died, which made light butterflies more visible to birds, and dark ones worse. In the 20th century, in a number of areas, the proportion of dark-colored butterflies in some well-studied populations of the birch moth in England reached 95%, while for the first time a dark butterfly ( Morfa carbonaria) was captured in 1848.

Stabilizing selection

Disruptive selection

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

sexual selection

sexual selection This is natural selection for success in reproduction. 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 rivalry between individuals of the same sex, usually males, for the possession of individuals of the other sex." Traits that reduce the viability of their carriers can emerge and spread if the advantages they provide in breeding success are significantly greater than their disadvantages for survival. Two main hypotheses about the mechanisms of sexual selection have been proposed. According to the “good genes” hypothesis, the female “reasons” as follows: “If this male, despite his bright plumage and long tail, somehow managed not to die in the clutches of a predator and survive to puberty, then, therefore, he has good the genes that let him do it. So, he should be chosen as a father for his children: he will pass on his good genes to them. By choosing bright males, females choose good genes for their offspring. According to the “attractive sons” hypothesis, the logic of female selection 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 bright color genes and will be attractive to females in the next generation. Thus, a positive feedback occurs, which leads to the fact that from generation to generation the brightness of the plumage of males is more and more enhanced. The process goes on increasing until it reaches the limit of viability. In choosing males, females are no more and no less logical than in all 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 the watering hole because it feels thirsty. In the same way, females, choosing bright males, follow their instincts - they like bright tails. All those who instinctively prompted a different behavior, all of them left no offspring. Thus, we discussed 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 that amazing variety of shapes, colors and instincts that we observe in the world of wildlife. .

38. Physiological adaptation: the concept of how it arises and what underlies it.

Biological adaptation(from lat. adaptatio- adaptation) - the adaptation of the body to the conditions of existence. “[Life] is a constant adaptation ... to the conditions of existence,” said the outstanding Russian physiologist I. M. Imanalieva. - An organism without an external environment supporting its existence is impossible; therefore, the scientific definition of an organism should also include the environment that influences it. "At the same time:" ... Each organism is a dynamic combination of stability and variability, in which variability serves its adaptive reactions and, consequently, the protection of its hereditarily fixed constants ". The organism even in extremely short periods of time is changeable due to the dynamics of its functional states and with the homeothetical variability of its “homeostatic constants” (K. Waddington, 1964, 1970). modern knowledge about the mechanisms and essence of the adaptation process: “... A person is ... a system ..., like any other in nature, obeying the laws that are inevitable and common to all nature ...” (IP Pavlov, 1951).

Currently, there are several forms of natural selection, the main of which are stabilizing, moving, or directed, and disruptive.

Stabilizing selection contributes to the maintenance of an average, previously established trait in the population. It occurs in cases where phenotypic traits optimally match environmental conditions and competition between individuals is relatively weak. Such selection operates in all populations, while individuals with extreme deviations of characters are destroyed.

In any population, due to its genetic heterogeneity, individuals with varying degrees of expression of one or another trait are born. Such a variety of individuals for any trait is provided by genetic and environmental factors that affect populations over many generations. If we count the number of individuals with this or that expression of this trait, it turns out that the majority will approach a certain average value, the average norm.

Stabilizing selection leads to the destruction of extreme deviations and, as it were, stabilizes the average norm of the trait's severity, leading to a narrowing of the reaction norm (Fig. 4.1). It is observed in environmental conditions, which long time remain constant. In a relatively unchanged environment, typical individuals with an average expression of a trait, well adapted to it, have an advantage, and mutants that differ from them die. We can give the following example of stabilizing selection. In 1898, the American ornithologist G. Bypas, after strong winds and snowfall found 136 stunned and half-dead house sparrows. During warming, 72 of them survived, 64 died. It turned out that the dead sparrows had either very long or very short wings.

Rice. 4.1 . Scheme of action of stabilizing (a), driving (b) and disruptive (c) natural selection (according to N.V. Timofeev-Resovsky et al., 1977), F- generations. Eliminated variants are shaded on the population curves. The size of the arc during selection within one offspring corresponds to the norm of the reaction.

driving selection lies in the fact that with a slow change in environmental conditions in a new direction, the average norm is steadily shifting in one direction or another. In other words, during driving selection, mutations with one trait value are eliminated, which are replaced by mutations with a different average trait value. Propulsive selection thus leads to evolutionary change by exerting such pressure on a population that favors an increase in the frequency of new alleles in it (see Fig. 4.1). After the new average norm of the trait expression (average phenotype) comes into optimal correspondence with the new environmental conditions, stabilizing selection comes into play.

A classic example of evolutionary change according to the type of motive selection is the appearance of dark-colored butterflies under the influence of chemical pollution of the atmosphere (industrial melanism). Over the past 100 years, more than 80 species of butterflies have developed dark-colored forms. Previously, for example, the birch moth had a pale cream color with black dots. In the middle of the XIX century. in England, dark-colored specimens of this butterfly were found, which by the end of the century made up 98%. The melanic form is the result of random mutations and has a great advantage in industrial areas compared to light-colored ones. Light-colored butterflies 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, the dark bark of trees was exposed, which became even darker due to the soot covering it. On a dark background, light-colored moths were pecked by robins and thrushes, while melanic forms survived and successfully reproduced, which are less noticeable against a dark background.

Disruptive (tearing) selection carried out in cases where two or more genetically various forms have an advantage in different conditions, for example, in different seasons of the year. Disruptive selection favors more than one phenotype and is directed against intermediate forms. It sort of breaks the population according to a given trait into several groups found in the same territory, and, with the participation of isolation, can lead to the division of the population into two or more (see Fig. 4.1).

A model of disruptive selection can be the situation of the emergence of dwarf races of predatory fish in a water body with little food. Often, juveniles of the year do not have enough food in the form of fish fry. In this case, the fastest growing individuals, which for very a short time reach sizes that allow them to eat their fellows. On the other hand, squints with the maximum delay in growth rate will be in an advantageous position, since their small size allows them to feed on small planktonic crustaceans for a long time. Such a situation, through stabilizing selection, can lead to the emergence of two races of fish.

Source : ON THE. Lemeza L.V. Kamlyuk N.D. Lisov "Biology manual for applicants to universities"

Evolution is a story of winners, and natural selection is an impartial judge who decides who lives and who dies. Examples of natural selection are everywhere: the entire variety of living beings on our planet is a product of this process, and man is no exception. However, one can argue about a person, because he has long been accustomed to intervene in a businesslike manner in those areas that used to be the sacred secrets of nature.

How natural selection works

This fail-safe mechanism is the fundamental process of evolution. Its action ensures growth in the population the number of individuals that have a set of the most favorable traits that ensure maximum adaptability to living conditions in environment, and at the same time - a decrease in the number of less adapted individuals.

Science owes the very term "natural selection" to Charles Darwin, who compared this process with artificial selection, that is, selection. The difference between these two species is only in who acts as a judge in choosing certain properties of organisms - a person or a habitat. As for the “working material”, in both cases these are small hereditary mutations that accumulate or, conversely, are eradicated in the next generation.

The theory developed by Darwin was incredibly bold, revolutionary, even scandalous for its time. But now natural selection does not cause in scientific world doubt, moreover, it is called a "self-evident" mechanism, since its existence logically follows from three indisputable facts:

Living organisms obviously produce more offspring than they can survive and reproduce further;
All organisms are susceptible hereditary variability;
Living organisms endowed with different genetic characteristics survive and reproduce with unequal success.

All this causes fierce competition between all living organisms, which drives evolution. The evolutionary process in nature, as a rule, proceeds slowly, and the following stages can be distinguished in it:

Principles of classification of natural selection

According to the direction of action, positive and negative (cutting off) types of natural selection are distinguished.

Positive

Its action is aimed at the consolidation and development of useful traits and contributes to an increase in the population of the number of individuals with these traits. Thus, within specific species, positive selection works to increase their viability, and on the scale of the entire biosphere, to gradually complicate the structure of living organisms, which is well illustrated by the entire history of the evolutionary process. For example, the transformation of the gills that took millions of years in some species of ancient fish, in the middle ear of amphibians, it accompanied the process of “landing” of living organisms under conditions of strong ebbs and flows.

Negative

In contrast to positive selection, cut-off selection forces out of the population those individuals that carry harmful traits that can significantly reduce the viability of the species in the existing environmental conditions. This mechanism acts like a filter that does not allow the most harmful alleles to pass through and does not allow their further development.

For example, when, with the development of the thumb on the hand, the ancestors of Homo sapiens learned to fold the brush into a fist and use it in fights against each other, individuals with fragile skulls began to die from head injuries (as evidenced by archaeological finds), giving up living space to individuals with stronger skulls. skulls.

A very common classification, based on the nature of the influence of selection on the variability of a trait in a population:

moving;
stabilizing;
destabilizing;
disruptive (tearing);
sexual.

Moving

The driving form of natural selection weeds out mutations with one value of the average trait, replacing them with mutations with another average value of the same trait. As a result, for example, one can trace the increase in the size of animals from generation to generation - this happened with mammals that gained terrestrial dominance after the death of dinosaurs, including human ancestors. Other forms of life, on the contrary, have significantly decreased in size. Thus, ancient dragonflies in conditions of high oxygen content in the atmosphere were gigantic in comparison with modern sizes. The same goes for other insects..

stabilizing

In contrast to the driving one, it tends to preserve existing features and manifests itself in cases of long-term preservation of environmental conditions. Examples are species that have come down to us from antiquity almost unchanged: crocodiles, many types of jellyfish, giant sequoias. There are also species that have existed, practically unchanged, for millions of years: this is the oldest ginkgo plant, a direct descendant of the first lizards of the hatteria, coelacanth (a brush-finned fish, which many scientists consider an “intermediate link” between fish and amphibians).

Stabilizing and driving selection act in conjunction and are two sides of the same process. The mover strives to keep the mutations that are most beneficial in changing environmental conditions, and when these conditions stabilize, the process will end with the creation the best way adapted form. Here comes the turn of stabilizing selection- it preserves these time-tested genotypes and does not allow mutant forms that deviate from the general norm to multiply. There is a narrowing of the reaction norm.

Destabilizing

It often happens that the ecological niche occupied by a species expands. In such cases, a wider reaction rate would be beneficial to the survival of that species. Under conditions of a heterogeneous environment, a process occurs that is opposite to stabilizing selection: traits with a wider reaction rate gain an advantage. For example, the heterogeneous illumination of a reservoir causes wide variability in the color of the frogs living in it, and in reservoirs that do not differ in a variety of color spots, all frogs are approximately the same color, which contributes to their camouflage (the result of stabilizing selection).

Disruptive (tearing)

There are many populations that are polymorphic - coexistence within one species of two or even several forms on any basis. This phenomenon can be caused by various reasons, both natural and anthropogenic origin. For example, droughts unfavorable for mushrooms, falling in the middle of summer, determined the development of their spring and autumn species, and haymaking, also occurring at this time in other areas, led to the fact that inside some types of grasses, seeds ripen early in some individuals, and late in others, that is before and after haymaking.

Sexual

Standing apart in this series of logically substantiated processes is sexual selection. Its essence lies in the fact that representatives of the same species (usually males) compete with each other in the struggle for the right to procreate. . However, they often develop the same symptoms. which adversely affect their viability. A classic example is the peacock with its luxurious tail, which has no practical use, moreover, makes it visible to predators and can interfere with movement. Its only function is to attract a female, and it successfully performs this function. There are two hypotheses explaining the mechanism of female selection:

The hypothesis of "good genes" - the female chooses a father for future offspring, based on his ability to survive even with such difficult secondary sexual characteristics;
The Attractive Son Hypothesis - A female tends to produce successful male offspring that retain the father's genes.

Sexual selection is of great importance for evolution, because the main goal for individuals of any species is not to survive, but to leave offspring. Many species of insects or fish die as soon as they complete this mission - without this there would be no life on the planet.

The considered tool of evolution can be characterized as an endless process of moving towards an unattainable ideal, because the environment is almost always a step or two ahead of its inhabitants: what was achieved yesterday is changing today to become obsolete tomorrow.

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