Examples of a stabilizing form of natural selection. other forms of natural selection

Natural selection favors the survival and increase in the number of individuals in the population, carriers of some genotypes to the detriment of carriers of others. This contributes to the accumulation in the population of traits that have adaptive value.

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

Moving;
stabilizing;
disruptive.

Driving Form (with examples)

The manifestation of driving selection occurs when the resulting changes in the new environment are more useful. The 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 color change of moths in the vicinity of industrial cities in Europe and America. If previously light coloration was typical for them, then as the tree trunks were contaminated with soot and soot, the light variants that became noticeable on the bark of the trees were primarily eaten by birds and the dark variants gained more and more advantage, it was they who were preserved by natural selection. This led to a change in color.

Evolution, the emergence of new adaptations, is 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 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 ancestors of new races.

Stabilizing form (with examples)

Stabilizing selection occurs under relatively constant conditions. Here, deviations from the average value of the trait may already turn out to be unfavorable and are swept aside. In these cases, selection is aimed at preserving mutations that lead to less variability of 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 an average wing length survive the winter more easily than long-winged or short-winged ones. Also, a constant body temperature in homoiothermic animals is a consequence of stabilizing selection.

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

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

When the conditions of existence change, the 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 dominant one, which is associated with the heterogeneity of the territory they inhabit. Under the action of certain factors, some signs contribute to survival, when conditions change, others.

Disruptive selection is directed against those representatives of the species that have average manifestations of the 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 trait. 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 snail shell. The color of the shell depends on the environmental conditions in which the snail enters. In the forest zone, where the surface layer of the earth is colored brown, snails with brown shells live. In the steppe region, where the grass is dry and yellow, they have yellow shells. The difference in the color of the shells is adaptive in nature, since it protects the snails from being eaten by birds of prey.

Table of the main types of natural selection

Characteristic	driving form	Stabilizing form	Disruptive form
Action	Occurs under gradually changing living conditions of the individual.	The living conditions of the body do not change for a long time.	With a sharp change in the living conditions of the body.
Orientation	Aimed at the conservation of organisms with characteristics that contribute to the survival of the species.	Maintaining the homogeneity of the population, the destruction of extreme forms.	The action is aimed at the survival of individuals in heterogeneous conditions, through the manifestation of different phenotypes.
Outcome	The appearance of an average norm, which comes to replace the old one, which is not suitable in the new environment.	Saving the average indicators of the norm.	Formation of several average norms necessary for survival.

Other types of natural selection

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

Destabilizing;
sexual;
group.

Destabilizing form in action it is opposite to the stabilizing one, while the reaction rate expands, but the average indicators are also preserved.

So frogs that live in swamps, in an environment with different illumination, differ significantly in the color of their skin - this is a manifestation of destabilizing selection. Frogs inhabiting a territory 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 choose a pair for crossing. For example, the bright color of feathers and the singing of birds, a loud voice, mating dances or the release of odorous substances to attract the opposite side of insects, and more.

group form aimed at the survival of the population, not individuals. The death of several members of the group for the sake of saving the species will be justified. So, in a herd of wild animals at the genetic level, it is laid 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, while it will die, but save the rest.

Rice. Stabilizing form of natural selection

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

Stabilizing selection preserves the state of the population, which ensures its maximum fitness under constant conditions of existence. In each generation, individuals that deviate from the average optimal value in terms of adaptive characteristics are removed.
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 birds that died after the storm 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.

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

The most widely known example of such a polymorphism is sickle cell anemia. This severe blood disease occurs in people homozygous for the mutated hemoglobin alley (HbS) 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 HbS have a higher resistance to malaria than homozygotes for the normal alley. Due to this, in populations inhabiting malarial areas, heterozygosity is created and stably maintained for this lethal alley in the homozygote.

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 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 the tuatara and ginkgo.
Thanks to stabilizing selection, "living fossils" that live in relatively constant environmental conditions have survived to this day:

1. tuatara, bearing the features of reptiles of the Mesozoic era;
2. coelacanth, a descendant of lobe-finned fish, widespread in the Paleozoic era;
3. the North American opossum, a marsupial known from the Cretaceous period;
4. Ginkgo gymnosperm, similar to tree forms that became extinct in the Jurassic period of the Mesozoic era.

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

It is important to note here that the constancy of conditions does not mean their immutability. During the year, environmental conditions change regularly. Stabilizing selection adapts populations to these seasonal changes. Breeding cycles are timed to them, so that the young are born in that season of the year when food resources are maximum. All deviations from this optimal cycle, reproducible from year to year, are eliminated by stabilizing selection. Descendants born too early die from starvation, too late - they do not have time to prepare for winter. How do animals and plants know when winter is coming? On the onset of frost? No, it's not a very reliable pointer. Short-term temperature fluctuations can be very deceptive. If in some year it gets warmer earlier than usual, this does not mean at all 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 - an increase in 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 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.

Driving selection. Natural selection always leads to an increase in the average fitness of populations. Changes in external conditions can lead to changes in the fitness of individual genotypes. In response to these changes, natural selection, using a huge store of genetic diversity for many different traits, leads to significant shifts in the genetic structure of the population. If the external environment is constantly changing in a certain direction, then natural selection changes the genetic structure of the population in such a way that its fitness in these changing conditions remains maximum. In this case, the frequencies of individual alleles in the population change. The average values of adaptive traits in populations also change. In a number of generations, their gradual shift in a certain direction can be traced. This form of selection is called driving selection.

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 brings the genetic composition of populations in line with changes in the external environment so that the average fitness of populations is maximum. On the island of Trinidad, guppy fish live in different water bodies. Many of those that live in the lower reaches of the rivers and in the ponds perish in the teeth of predatory fish. In the upper reaches, life for guppies is much calmer - there are few predators. These differences in environmental conditions led to the fact that the "top" and "grassroots" guppies evolved in different directions. The "grassroots", which are under constant threat of extermination, begin to breed at an earlier age and produce many very small fry. The chance of survival of each of them is very small, but there are a lot of them and some of them have time to multiply. "Horse" reach puberty later, their fertility is lower, but the offspring are larger. When the researchers transferred the "grassroots" guppies to uninhabited reservoirs in the upper reaches of the rivers, they observed a gradual change in the type of development of the fish. 11 years after the move, they became much larger, entered breeding later and produced fewer but larger offspring.

The rate of change in allele frequencies in the population and the average values of traits under the action of selection depends not only on the intensity of selection, but also on the genetic structure of traits, on which there is a turnover. Selection against recessive mutations is much less effective than against dominant ones. In the heterozygote, the recessive allele does not appear in the phenotype and therefore eludes selection. Using the Hardy-Weinberg equation, one can estimate the rate of change in the frequency of a recessive allele in a population depending on the intensity of selection and the initial ratio of frequencies. The lower the allele frequency, the slower its elimination occurs. In order to reduce the frequency of recessive lethality from 0.1 to 0.05, only 10 generations are needed; 100 generations - to reduce it from 0.01 to 0.005 and 1000 generations - from 0.001 to 0.0005.

The driving form of natural selection plays a decisive role in the adaptation of living organisms to external conditions that change over time. It also ensures the wide distribution of life, its penetration into all possible ecological niches. It is a mistake to think, however, that under stable conditions of existence, natural selection ceases. Under such conditions, it continues to act in the form of stabilizing selection.

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

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.

The most widely known example of such a polymorphism is sickle cell anemia. This severe blood disease occurs in people homozygous for a mutant hemoglobin allele ( HbS) 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 HbS 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.

disruptive selection. With stabilizing selection, individuals with an average manifestation of traits have an advantage, with driving selection - one of the extreme forms. Theoretically, another form of selection is conceivable - disruptive or tearing selection, when both extreme forms gain an advantage.

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.

In certain situations, disruptive selection for traits related to ecological features (breeding time, preference for different types of food, different habitats) can lead to the formation of ecologically separate races within a species and then to speciation.

sexual selection. In males of many species, pronounced secondary sexual characteristics are found that at first glance seem maladaptive: the tail of a peacock, the bright feathers of birds of paradise and parrots, the scarlet combs of roosters, the enchanting colors of tropical fish, the songs of birds and frogs, etc. Many of these features make life difficult for their carriers, making them easily visible to predators. It would seem that these signs do not give 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 origin 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 members of the opposite sex. Ch. Darwin called this phenomenon sexual selection. He first mentioned this form of selection in The Origin of Species and later 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 relationship 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."

Sexual selection is natural selection for success in reproduction.. 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. A male that lives a short time but is liked by females and therefore produces many offspring has a much higher cumulative fitness than one that lives long but leaves few offspring. In many animal species, the vast majority of males do not participate in reproduction at all. In each generation, fierce competition for females arises between males. This competition can be direct, and manifest itself in the form of a struggle for territories or tournament fights (Fig. XI.15.2). It can also occur in an indirect form and be determined by the choice of females. In cases where females choose males, male competition is shown in displaying their flamboyant appearance or complex courtship behavior. Females choose those males that they like the most. As a rule, these are the brightest males. But why do females like bright males?

The fitness of the 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 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. 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. In the end, the tail reaches such a length that its harm to the viability of the male is balanced by its attractiveness in the eyes of females.

In explaining these hypotheses, we tried to understand the logic of the action of female birds. It may seem that we expect too much from them, that such complex fitness calculations are hardly accessible to them. In fact, in choosing males, females are no more and no less logical than in all other behaviors. 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 place because it feels thirsty. When a worker bee stings a predator attacking a hive, she does not calculate how much by this self-sacrifice she increases the cumulative fitness of her sisters - she follows instinct. 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. .

1. Compare the selection forms and highlight the similarities and differences between them.

2. Give examples of different forms of selection in nature.

3. Is it true that in changing environmental conditions only motive selection acts, and in unchanged conditions - only stabilizing selection?

4. In what cases does selection lead to a decrease in the genetic variability of populations, and in which cases to its accumulation?

5. Give examples of sexual dimorphism in animals and try to explain their evolution using the mechanisms of driving and sexual selection.

This form of selection was discovered by C. Darwin and was called the driving one. Stabilizing selection contributes to the maintenance of an average, previously established trait in a population. What exactly is selected in the process of natural selection and how does this process occur? What are the similarities and differences between natural and artificial selection?

The answer to this question is given by the doctrine of stabilizing selection, developed by the Russian evolutionist I.I. Schmalhausen. Stabilizing selection occurs when environmental conditions remain fairly constant for a long time. Many examples of stabilizing selection are known. So, after a snowfall and strong winds in North America, 136 stunned, half-dead house sparrows were found, 72 of them survived, and 64 died.

As a result of the action of the stabilizing form of selection, mutations with a wide reaction rate are replaced by mutations with the same average value, but a narrower reaction rate.

The central concept of the concept of natural selection is the fitness of organisms

Stabilizing and driving selections are interrelated and represent two sides of the same process. The term "natural selection" was popularized by Charles Darwin, comparing this process with artificial selection, the modern form of which is selection. In addition, the material for both natural and artificial selection are small hereditary changes that accumulate from generation to generation. Such conditions create competition between organisms for survival and reproduction and are the minimum necessary conditions for evolution through natural selection.

The survival of organisms is an important but not the only component of natural selection.

And vice versa, for less beneficial or harmful alleles, their share in populations will decrease, that is, selection will act against these alleles. Traits that have evolved through sexual selection are particularly evident in the males of certain animal species.

Moreover, selection can act simultaneously at different levels. Selection at levels above the individual, such as group selection, can lead to cooperation (see Evolution#Cooperation). Driving selection is a form of natural selection that operates under a directed change in environmental conditions. Described by Darwin and Wallace. At the same time, other variations of the trait (its deviations in the opposite direction from the average value) are subjected to negative selection.

In this case, the pressure of driving selection must correspond to the adaptive capabilities of the population and the rate of mutational changes (otherwise, environmental pressure can lead to extinction). An example of the action of motive selection is "industrial melanism" in insects.

8. Give examples of the influence of different types of natural selection on populations of living beings

Stabilizing selection is 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 a trait. The concept of stabilizing selection was introduced into science and analyzed by I. I. Shmalgauzen. Selection in favor of averages has been found for a variety of traits. Darwin described the operation of disruptive selection, believing that it underlies divergence, although he could not provide evidence for its existence in nature.

We can give the following example of stabilizing selection

One of the possible situations in nature in which disruptive selection comes into play is when a polymorphic population occupies a heterogeneous habitat. 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. In a number of other experiments (with plants), intensive crossing prevented the effective action of disruptive selection.

Another important component is attractiveness to members of the opposite sex. Darwin called this phenomenon sexual selection. Two hypotheses about the mechanisms of sexual selection are common. 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. Darwin assumed that selection could be applied not only to the individual organism, but also to the family.

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. Driving selection consists 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 the course of disruptive selection, two forms of butterflies appeared from a common light yellow ancestor: white and yellow

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). 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.

Since selection is based on phenotypes, the individuals of a given group must differ from each other, i.e., the group must be of different quality. Different phenotypes under the same conditions can be provided by different genotypes. For millions of years, stabilizing selection protects species from significant changes, but only as long as the conditions of life do not change significantly.

Currently, there are several forms of natural selection, the main of which are stabilizing, moving, or directed, and disruptive. You will learn how natural selection affects modern man. Many examples of the action of stabilizing selection in nature have been described.

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