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

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

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

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

Forms of natural selection

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

Stabilizing natural selection

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

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

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

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

Driving selection

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

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

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

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

Disruptive or disruptive selection

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

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

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

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

Struggle for existence

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

Charles Darwin drew attention to the fact that in nature there are always two opposing development trends:

1. the desire for unlimited reproduction and dispersal and
2. overpopulation, large crowding, the influence of other populations and living conditions, which inevitably lead to the emergence of a struggle for existence and limiting the development of species and their populations.

That is, the species strives to occupy all possible habitats for its existence. But the reality is often harsh, resulting in species numbers and habitats being significantly limited. It is the struggle for existence against the background of high mutagenesis and combinative variability during sexual reproduction that leads to the redistribution of characteristics, and its direct consequence is natural selection.

There are three main forms of struggle for existence.

Interspecies fight

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

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

Intraspecific struggle

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

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

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

Combating abiotic environmental factors

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

1. artificial selection is carried out by humans, and natural selection is spontaneously realized in nature under the influence of external environmental factors;
2. the result of artificial selection is new breeds of animals, plant varieties and strains of microorganisms with traits useful for human economic activity, and with natural selection new (any) organisms arise with traits that allow them to survive in strictly defined environmental conditions;
3. during artificial selection, the traits that arise in organisms may not only not be useful, they may be harmful for a given organism (but they are useful for human activity); with natural selection, the resulting traits are useful for a given organism in a given, specific environment of its existence, since they contribute to its better survival in this environment;
4. natural selection has been carried out since the appearance of organisms on Earth, and artificial selection has been carried out only since the domestication of animals and the advent of agriculture (growing plants in special conditions).

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

NATURAL SELECTION is the result of the struggle for existence; it is based on the preferential survival and leaving of offspring by the most adapted individuals of each species and the death of less adapted organisms

IN Under conditions of constant environmental change, natural selection eliminates unadapted forms and preserves hereditary deviations that coincide with the direction of the changed conditions of existence. There is either a change in the reaction norm or its expansion (norm of reaction called the ability of the body to respond with adaptive changes to the action of environmental factors; reaction norm is the limits of modification variability controlled by the genotype of a given organism). This form of selection was discovered by Charles Darwin and was called driving .

An example is the displacement of the original light-colored form of the birch moth butterfly by a dark-colored form. In the southeast of England, in the past, along with the light-colored form of the butterfly, dark-colored ones were occasionally found. In rural areas, light coloring on birch bark turns out to be protective; they are invisible, while dark-colored ones, on the contrary, stand out against a light background and become easy prey for birds. In industrial zones, due to environmental pollution with industrial soot, dark-colored forms gain an advantage and quickly replace light-colored ones. Thus, out of 700 species of butterflies in this country, over the past 120 years, 70 species of moths have changed their light color to dark. The same picture is observed in other industrial zones of Europe. Similar examples include the emergence of insecticide-resistant insects, antibiotic-resistant forms of microorganisms, the spread of poison-resistant rats, etc.

Domestic scientist I. I. Shmalgauzen discovered stabilizing form selection, which operates under constant conditions of existence. This form of selection is aimed at maintaining the existing norm. In this case, the constancy of the reaction norm is maintained as long as the environment remains stable, while individuals deviating from the average norm disappear from the population. For example, during snowfall and strong winds, short-winged and long-winged sparrows died, but individuals with average wing sizes survived. Or another example: the stable constancy of the parts of a flower in comparison with the vegetative organs of the plant, since the proportions of the flower are adapted to the size of pollinating insects (a bumblebee cannot penetrate a too narrow corolla of a flower, the proboscis of a butterfly cannot touch the too short stamens of flowers with a long corolla). Over millions of years, stabilizing selection protects species from significant changes, but only as long as living conditions do not change significantly.

Also distinguished tearing, ordisruptive , selection operating in a diverse environment: not just one trait is selected, but several different ones, each of which favors survival within narrow limits of the population’s range. Because of this, the population is divided into several groups. For example, some wolves in the Kitskill Mountains of the USA look like a light greyhound and hunt deer, while other wolves in the same area, heavier, with short legs, usually attack herds of sheep. Disruptive selection operates under conditions of a sharp change in the environment: forms with multidirectional changes survive on the periphery of the population; they give rise to a new group in which stabilizing selection comes into effect. Neither form of selection occurs naturally in pure form, since environmental factors change and act together as a whole. However, at certain historical periods of time, one of the forms of selection may become leading.

All forms of natural selection constitute a single mechanism, which, acting on a statistical basis as a cybernetic regulator, maintains the balance of populations with the surrounding environmental conditions. The creative role of natural selection is not only to eliminate the unadapted, but also to direct the emerging adaptations (the result of mutations and recombinations), “selecting” in a long series of generations only those of them that are most suitable in the given conditions of existence , which leads to the emergence of more and more new life forms.

Forms of natural selection (T.A. Kozlova, V.S. Kuchmenko. Biology in tables. M., 2000)

The driving form of natural selection begins to act in changing environmental conditions. With it, individuals with any deviation of the trait from the value that is characteristic of the majority of individuals, i.e., from the average value, receive advantages. As they reproduce, individuals with a trait deviation from the previous average value themselves become the majority and carriers of the new average value. Thus, the trait changes under the influence of a changing environment.

An example of driving selection is the change in color of the birch moth butterfly from predominantly white to predominantly black in England in the 18th and 19th centuries. At this time, there was a rapid development of production there, coal was used, and a lot of soot was released into the atmosphere. It settled on trees, including birches, causing their trunks to turn black. Birch moths are food for birds. The coloring of butterflies allows them to camouflage themselves while sitting on trees. However, white butterflies became noticeable and were pecked at more often by birds. While black butterflies became less visible, they survived and left offspring. After some time, the entire population of moths became predominantly black. Thus, while the birches were white, stabilizing selection acted, destroying deviations from the norm (black butterflies). But as soon as conditions changed, the deviant trait gained an advantage, which caused a change in the entire population.

Another example of the driving form of natural selection is the emergence of insect resistance to pesticides. Insect populations almost always contain individuals that are resistant to one or another poison. After the death of the bulk of the individuals in the population, they reproduce, as a result of which the entire population becomes resistant to a specific poison.

Insects living in windy areas have reduced wings. Because otherwise they would have been blown away by the wind. Their winged ancestors, who found themselves in such a habitat, perished. However, among them there were short-winged ones that survived. They left offspring, which gradually became predominantly wingless.

The giraffe's ancestors had shorter necks. However, in places with prolonged droughts and insufficient leaves in the lower part of the tree crowns, individuals with longer necks gained an advantage; they could reach high-lying leaves. Such animals survived and gave birth to offspring. Gradually, the entire population began to consist of individuals with long necks.

Forms of natural selection

The intensity of selection pressure is its quantitative characteristic; the direction of natural selection determines its qualitative influence on evolution. Depending on the direction, different forms of natural selection are distinguished.

The genetic basis of any form of natural selection is hereditary variability, and the cause is the influence of environmental conditions. Mutants that were previously less adapted compared to the normal genotype, with a favorable change in environmental conditions, gain an advantage and gradually displace the previous norm. The result of long-term selection is the transformation of the population gene pool, the replacement of some quantitatively dominant genotypes with others.

The driving form of natural selection

Driving selection was described by Charles Darwin. The very name “motive” suggests that such selection acts as the creative force of evolution. In the driving form of selection, mutations with one value of the average trait are eliminated, which are replaced by mutations with a different average value of the trait. This form of selection is easier to detect than others. As a result of the action of the driving form of selection, for example, an increase in the size of the descendants occurs in comparison with the ancestors (in the evolutionary series of equines from the fox-sized fossil Phenacodus to the modern donkey, zebra, and horse). Other forms may shrink in size. Thus, elephants came to the islands of the Mediterranean Sea at the end of the Tertiary period. In conditions of limited resources of island forests, individuals with small sizes had an advantage.

Coelacanth. Photo: sybarite48

Rice. 24. Above are 4 types of relict forms

Mutations of dwarfism were picked up by the driving form of selection, and the original alleles that determined the normal size for elephants were eliminated due to the death of large individuals. As a result, dwarf elephants up to one and a half meters tall appeared on the Mediterranean islands (they were exterminated by the first hunters who settled these islands). Charles Darwin explained the origin of many wingless insects living on oceanic islands by the action of driving selection.

A classic example of the action of driving selection in nature is the so-called industrial melanism. In areas that have not undergone industrialization, the birch moth butterfly has a white color that matches the light birch bark. Among the light butterflies on the trunks of birches there were also dark ones, but they were clearly visible and were pecked by birds. Industrial development led to air pollution, and white birch trees became covered with a layer of soot. Now, on dark trunks, birds noticed not dark, but typical light butterflies much more easily. Gradually, in contaminated areas, the frequency of occurrence of dark (mutant) individuals increased sharply and they became predominant, although relatively recently they were extremely rare.

A convincing example of driving selection is the development of resistance to antibiotics and pesticides in microorganisms, insects, and mouse-like rodents. Numerous studies have established that exposure of microorganisms to various antibiotics causes, in a relatively short period of time, resistance to doses many times higher than the initial one. This is explained by the fact that antibiotics act as a selection factor that promotes the survival of mutant forms resistant to it. Due to the rapid proliferation of microorganisms, mutant individuals increase in number and form new populations that are immune to the action of antibiotics. Increasing the dose or using stronger drugs again creates the conditions for the action of driving selection, as a result of which more and more stable populations of microorganisms are formed. That is why medicine is steadily searching for new forms of antibiotics to which pathogenic microbes have not yet acquired resistance.

In countries with advanced agricultural culture, chemical plant protection products against pests (insects, fungi) are increasingly being abandoned. Since, after a limited number of generations, mutations of resistance to chemicals are fixed in pests by driving selection. Instead of chemical treatment, it is considered advisable to replace the old variety with a new one after 10-12 years, which has not yet been “found” by pests.

Stabilizing selection

It is known that the relict plant Ginkgo and the descendant of the first lizards, Hatteria, as well as the lobe-finned fish coelacanth, have existed almost unchanged for millions of years (Fig. 24). How to explain such stability of species if a mutation process is constantly taking place in nature? The answer to this question is given by the doctrine of stabilizing selection, developed by the largest evolutionist I. I. Shmalgauzen.

Stabilizing selection is observed if environmental conditions remain fairly constant for a long time. In a relatively unchanged environment, typical, well-adapted individuals with average expression of the trait have an advantage, and mutants that differ from them die. There are many known examples of stabilizing selection.

So, after snowfall and strong winds in North America, 136 stunned, half-dead house sparrows were found, 72 of them survived, and 64 died. The dead birds had either very long or very short wings. Individuals with medium, “normal” wings turned out to be more resilient.

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

Stabilizing selection leads to greater phenotypic homogeneity of the population. If it lasts for a long time, it appears that the population or species is not changing. However, this immutability is apparent and concerns only the external appearance of the population, while its gene pool continues to change based on the appearance of mutations with the same average value, but with a narrower reaction rate.

The stabilizing form of selection is also characteristic of humans. It is known that disturbances in the smallest 21-22nd pairs of chromosomes lead to the most severe hereditary disease - Down syndrome. If deviations occur in the number and shape of larger chromosomes, this will lead to the death of fertilized eggs. Spontaneous abortions are often caused by the death of embryos with abnormalities in medium-sized chromosomes.

Thus, the stabilizing form of selection over hundreds of thousands and millions of generations protects species from significant changes, from the destructive influence of the mutation process, culling mutant forms. Without stabilizing selection there would be no stability (stability) in living nature.

Stabilizing and driving selections are interconnected and represent two sides of the same process. Populations are constantly forced to adapt to changes in environmental conditions. Driving selection will preserve genotypes that are most consistent with changes in the environment. When environmental conditions are stabilized, selection will lead to the creation of a form well adapted to it. From this moment, stabilizing selection comes into play, which will maintain typical, predominant genotypes and eliminate mutant forms that deviate from the average norm from reproduction.

Destabilizing selection

Stabilizing selection narrows the reaction norm. However, in nature there are often cases when the ecological niche of a species may become wider over time. In this case, individuals and populations with a wider reaction norm receive a selective advantage, while at the same time maintaining the average value of the trait. As a result, a process occurs that is the opposite of stabilizing selection: mutations with a wider reaction rate receive an advantage. Thus, populations of lake frogs living in ponds with heterogeneous illumination, with alternating areas overgrown with duckweed, reeds, cattails, and with “windows” of open water, are characterized by a wide range of color variability (the result of destabilizing selection). On the contrary, in water bodies with uniform illumination and color (ponds completely overgrown with duckweed, or open ponds), the range of color variability of frogs is narrow (the result of the action of stabilizing selection). Thus, destabilizing form of selection leads to an expansion of the reaction norm.

Disruptive selection

Characteristic of many populations polymorphism - the existence of two or more forms based on one or another characteristic. Polymorphism cannot be explained solely by the occurrence of new mutations. The reasons for it may be different. In particular, it may be due to the increased relative viability of heterozygotes. In other cases, polymorphism may be the result of a special form of selection, called tearing or disruptive. This form of selection occurs when two or more genetically different forms have an advantage under different conditions, such as different seasons of the year.

Stabilizing Moving Disruptive

Rice. 25. Scheme of action of different forms of selection

The case of the predominant survival of “red” forms of the two-spotted ladybug in the winter season and “black” forms of the two-spotted ladybug in the summer season has been well studied. Disruptive selection favors more than one phenotype and is directed against intermediate intermediate forms. It seems to tear the population according to this characteristic into several groups found in the same territory, and can, with the participation of isolation, lead to the division of the population into two or more (Fig. 25).

The creative role of natural selection. Critics of Darwinism attributed to selection the role of a “sieve” or “gravedigger”, eliminating or sorting out changes in populations. This result of selection actually exists in nature, but selection not only eliminates individuals less adapted to the environment, but also determines the direction of evolution, successively accumulating numerous hereditary changes. As mentioned above, the mutation process, waves of numbers and other evolutionary factors supply material for evolution. The same material (hereditary changes), depending on the direction of selection, can lead to different results. Acting indefinitely (millions and billions of years), natural selection, together with other evolutionary factors, genetic drift and isolation, has created a huge diversity of species in nature, adapted to life in different parts of our planet.

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

As a result, in a population from generation to generation there is a shift in the average value of the trait in a certain direction. In this case, the pressure of driving selection must correspond to the adaptive capabilities of the population and the rate of mutational changes (otherwise, environmental pressure can lead to extinction).

An example of the action of driving selection is “industrial melanism” in insects. “Industrial melanism” is a sharp increase in the proportion of melanistic (dark-colored) individuals in those populations of insects (for example, butterflies) that live in industrial areas. Due to industrial impact, the tree trunks darkened significantly, and light-colored lichens also died, which is why light-colored butterflies became better visible to birds, and dark-colored ones became less visible. In the 20th century, in some areas, the proportion of dark-colored butterflies in some well-studied moth populations in England reached 95%, while for the first time the dark-colored butterfly ( morpha carbonaria) was captured in 1848.

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

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Forms of natural selection

I.I. Schmalhausen defined the forms of natural selection:

Stabilizing - aimed at maintaining the average norm of reaction of a trait of an organism and the deviation of individuals with an extreme norm of reaction in constant environmental conditions. Selection operates under constant environmental conditions, conservative, aimed at preserving the basic characteristics of the species in an unchanged state.
2. Driving - leads to the consolidation of signs that deviate from the norm. Selection operates under changing environmental conditions, leading to changes in the average reaction rate and the evolution of the species.
3. Disruptive (tearing) - selection aimed at preserving individuals with extreme characteristics and destroying individuals with average characteristics. Acts in changing conditions, leads to the separation of a single population and the formation of two new populations with excellent signs. Selection can lead to the emergence of new populations and species. For example, populations of wingless and winged forms of insects.

Any form of selection does not act by chance, but passes through the preservation and accumulation of useful characteristics. Selection occurs successfully when there is a greater range of variability and more diverse genotypes of species.

Manifests itself in the form of a stable and, to a certain extent, directional change in the frequency of the allele (genotype, phenotype) in the population. The end result of the driving form of selection is the complete replacement of an allele (genotype, phenotype) by another allele (genotype, phenotype). Thus, driving selection leads to changes in the genetic and phenotypic structure of the population.

During driving selection, the average fitness of a population (but not necessarily all its members!) increases.

The mechanism of driving selection is the accumulation and strengthening of deviations from the original (normal) variant of the trait. These deviations appear during the action of elementary evolutionary factors. In the future, the original version of the symptom may become a deviation from the norm.

Driving selection leads to the appearance of transitive or transitional polymorphism in a population. Polymorphism is the simultaneous coexistence in a population of two or more alleles of one gene, two or more genotypes or phenotypes. It is difficult to identify this type of polymorphism, since it exists in the population for a few (several tens) generations.

In order to find out how many generations it takes to change the frequency of a recessive allele, you can use the formula:

t =1/q2 – 1/q1

For example, the albinism allele occurs in a population with a frequency of q1 = 0.007, and it is desirable to reduce this frequency to q2 = 0.005. Then

t =1/0.005- 1/0.007 =200 – 143 = 57 (generations)

Stabilizing selection (centripetal selection) is the total result of the action of two or more directions of driving selection in favor of one geno/phenotype or a group of genotypes with a similar phenotype. Stabilizing selection is aimed at preserving the genetic and phenotypic structure of the population.

Stabilizing selection manifests itself in the form of preservation of allele frequencies (genotypes, phenotypes) in a population. The result of stabilizing selection is the preservation of a population state in which its average fitness is maximum.

There are two forms of stabilizing selection: purifying selection and selection for diversity.

During purifying selection, the original (normal) variant of the trait is preserved.

Deviations from the normal variant of the trait reduce the fitness of individuals and are removed (eliminated) from the population. In this case, the frequency of one of the alleles tends to 1, and the frequencies of other alleles of a given gene tend to zero.

When selecting for diversity, selection often acts in favor of heterozygotes (the superiority of heterozygotes over homozygotes is called overdominance). Then two or more alleles of one gene remain in a constant ratio for a long time in the population. Stabilizing selection for diversity leads to the emergence and maintenance of balanced (stable) polymorphism in the population. This type of polymorphism persists in populations indefinitely.

Powerful stabilizing selection promotes the conservation of taxa. Numerous persistent forms are known - “living fossils” (brachiopods, horseshoe crabs, hatteria, coelacanth, ginkgo). In horseshoe crabs, intrapopulation polymorphism is no less than in young arthropod species, however, any deviation from the average value of a trait (from the adaptive norm) leads to a decrease in fitness.

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

Stabilizing selection often includes canalizing selection - selection for stability of development, for the autonomization of ontogenesis (this issue will be discussed in more detail in the corresponding lecture).

Disruptive selection (centrifugal selection) is the total result of the action of two or more directions of driving selection in favor of two or more equally adapted genotypes/phenotypes or groups of genotypes with similar phenotypes.

Disruptive selection leads to the appearance of unbalanced (unstable) polymorphism in the population. For long-term persistence of this type of polymorphism in a population, a number of conditions must be met:

a) all forms must be truly equally adapted: w (AA) = w (Aa) = w (aa);

b) both forms must not cross with each other: k (aa × AA) → 0;

c) the habitat must be heterogeneous in space and/or time.

Fulfillment of even one of the conditions is quite rare, so unbalanced polymorphism within a population is a rare occurrence. The most common are seasonal polymorphism in insects (butterflies, ladybugs), environmentally determined polymorphism in large populations of plants, polymorphism with zero fitness of heterozygotes (tropical butterflies).

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

Driving selection

Driving selection- a form of natural selection that operates when directed changing environmental conditions. Described by Darwin and Wallace. In this case, individuals with traits that deviate in a certain direction from the average value receive advantages. In this case, other variations of the trait (its deviations in the opposite direction from the average value) are subject to negative selection. As a result, in a population from generation to generation there is a shift in the average value of the trait in a certain direction. In this case, the pressure of driving selection must correspond to the adaptive capabilities of the population and the rate of mutational changes (otherwise, environmental pressure can lead to extinction).

An example of the action of driving selection is “industrial melanism” in insects. “Industrial melanism” is a sharp increase in the proportion of melanistic (dark-colored) individuals in those populations of insects (for example, butterflies) that live in industrial areas. Due to industrial impact, the tree trunks darkened significantly, and light-colored lichens also died, which is why light-colored butterflies became better visible to birds, and dark-colored ones became less visible. In the 20th century, in some areas, the proportion of dark-colored butterflies in some well-studied moth populations in England reached 95%, while for the first time the dark-colored butterfly ( morpha carbonaria) was captured in 1848.

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

Stabilizing selection

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.

Disruptive selection

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

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

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

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

Sexual selection

Sexual selection- This is natural selection for reproductive success. The survival of organisms is an important, but not the only component of natural selection. Another important component is attractiveness to individuals of the opposite sex. Darwin called this phenomenon sexual selection. “This form of selection is determined not by the struggle for existence in the relations of organic beings among themselves or with external conditions, but by the competition between individuals of one sex, usually males, for the possession of individuals of the other sex.” Traits that reduce the viability of their hosts can emerge and spread if the advantages they provide for reproductive success are significantly greater than their disadvantages for survival. Two main hypotheses about the mechanisms of sexual selection have been proposed. According to the “good genes” hypothesis, the female “reasons” as follows: “If this male, despite his bright plumage and long tail, somehow managed not to die in the clutches of a predator and survive to puberty, then, therefore, he has good genes.” genes that allowed him to do this. This means that he should be chosen as a father for his children: he will pass on his good genes to them.” By choosing colorful males, females are choosing good genes for their offspring. According to the “attractive sons” hypothesis, the logic of female choice is somewhat different. If 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 feedback, 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 .

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

Biological adaptation(from lat. adaptatio- adaptation) - adaptation of the organism 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 that supports its existence is impossible; therefore, the scientific definition of an organism must also include the environment influencing it." At the same time: "...Each organism is a dynamic combination of stability and variability, in which variability serves its adaptive reactions and, therefore, the protection of its hereditarily fixed constants." The organism even in extremely short periods of time is variable due to the dynamics of its functional states and the homeoretic variability of its “homeostatic constants” (K. Waddington, 1964, 1970). systematic approach should form the basis of modern knowledge about the mechanisms and essence of the adaptation process: “...Man is... a system..., like any other in nature, subject to inevitable and uniform laws for all of nature...” (I. P. Pavlov, 1951).

Anthropology and concepts of biology Kurchanov Nikolay Anatolievich

Natural selection

Natural selection

Natural selection is the most important factor in evolution. Darwinism (namely, STE is built on the basis of Darwinism), as noted above, is called the theory of natural selection.

A brief and successful definition of selection can be formulated by I. Lerner: “Selection is the differential reproduction of genotypes”(Lerner J., 1958). This definition shows that reproduction does not mean more intense, but more efficient reproduction. Natural selection was very well characterized by one of the founders of modern cytogenetics, S. Darlington (1903–1981), as a process of transfer “...from the chemical level of mutation to biological level adaptation"(Darlington S., 1958).

The role of natural selection is one of the key controversial issues in evolutionary biology throughout its history.

By the middle of the 20th century, thanks to the fundamental theoretical developments of I. I. Shmalhausen and J. Simpson, the idea of ​​three forms of selection was formed in evolutionary biology.

Stabilizing selection- this is the preferential survival of organisms that have characteristics that do not have noticeable deviations from the norm characteristic of a given population. The most obvious result of the action of stabilizing selection is the stabilization of the already existing norm of reaction for a given trait.

Driving selection– contributes to a shift in the average value of the characteristic. With a directed change in the environment, individuals with individual characteristics corresponding to this change. Such selection contributes to the consolidation of a new form to replace the old one, which has come into conflict with the changed environmental conditions.

Disruptive selection– selection directed against individuals with an average value of traits and leading to a break in the population into several groups for a given trait.

This division was well confirmed in subsequent experimental studies.

The variability of a trait in a population is described by a normal distribution curve. A normal genotype leads to the development of an individual whose characteristics are close to the average norm ( fashion) variation curve of this characteristic. The more the genotype of an individual is changed, the less common such individuals are. If the genotype is changed so much that ontogeny cannot lead to the development of a sexually mature individual, then such an individual is outside the variation curve (lethal mutations).

In addition to the three forms of selection noted, there are many other classifications. In population genetics, attention is focused on changes in the frequency of alleles in a population and the following selection options are highlighted (Ayala F., Kaiger J., 1988):

– selection against a recessive allele;

– selection against the dominant allele;

– selection against the neutral allele;

– selection in favor of heterozygotes;

– selection against heterozygotes;

– frequency-dependent selection.

The last option is quite interesting. It is characterized by an increase in the probability of crossing depending on the frequency of the genotype, and often selection occurs in favor of a rare allele.

An important role in nature is played by selection in favor of heterozygotes, leading to stable polymorphism of populations. In evolutionary ecology special meaning is given by selection for reproductive strategies, which we will consider below. A very specific type of selection is sexual selection.

There are many other classifications of natural selection, and there is not always a consensus among evolutionists.

Chapter IV. Natural selection, or survival of the most