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

Definition: selective survival and reproduction of the fittest organisms.

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

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

Shapes:

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

Consequences of natural selection:

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

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

Answer

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

Answer

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

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

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

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

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

3) natural selection
4) artificial selection

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

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

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

Answer

© D.V. Pozdnyakov, 2009-2019

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

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

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

There are two types of driving selection:

• transitive (transitive);
• directed.

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

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

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

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

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

Rice. 1. Driving selection.

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

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

Stabilizing selection

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

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

Rice. 2. Stabilizing selection.

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

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

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

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

Disruptive selection

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

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

Rice. 3. Three forms of female African swallowtails.

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

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

Table “Forms of natural selection”

What have we learned?

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

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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 resistant 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 proto-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 different characteristics. 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) version 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).

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, drawing on the enormous pool 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. At the same time, the frequencies of individual alleles in the population change. The average values ​​of adaptive traits in populations also change. In a series of generations, their gradual shift in a certain direction can be traced. This form of selection is called driving selection.

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

Driving selection brings the genetic composition of populations into line with changes in the external environment so that the average fitness of populations is maximized. On the island of Trinidad, guppy fish live in different bodies of water. Many of those that live in the lower reaches of rivers and ponds die in the teeth of predatory fish. In the upper reaches, life for guppies is much calmer - there are few predators there. These differences in external conditions led to the fact that the “top” and “bottom” guppies evolved in different directions. The "lower ones", under constant threat of extermination, begin to reproduce at an earlier age and produce many very small fry. The chance of survival for each of them is very small, but there are a lot of them and some of them manage to reproduce. The “mountains” reach sexual maturity later, their fertility is lower, but their offspring are larger. When researchers transferred “low-growth” guppies to uninhabited reservoirs in the upper reaches of rivers, they observed a gradual change in the type of development of the fish. Eleven years after the move, they became significantly larger, began breeding later, and produced fewer but larger offspring.

Rate of change in allele frequencies in the population and average trait values under the influence of selection depends not only on the intensity of selection, but also on the genetic structure of traits, along which the turnover goes. Selection against recessive mutations turns out to be much less effective than against dominant ones. In a heterozygote, the recessive allele does not appear in the phenotype and therefore escapes 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 frequency ratio. 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 in 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 that ensures its maximum fitness under constant conditions of existence. In each generation, individuals that deviate from the average optimal value for adaptive traits are removed.

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

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

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

The most widely known example of such a polymorphism is sickle cell anemia. This severe blood disease occurs in people homozygous for the mutant hemoglobin 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 higher resistance to malaria than homozygotes for the normal allele. Thanks to this, in populations inhabiting malarial areas, heterozygosity for this allele, which is lethal in homozygotes, is created and stably maintained.

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

It is important to note here that the constancy of conditions does not mean their immutability. Environmental conditions change regularly throughout the year. Stabilizing selection adapts populations to these seasonal changes. Reproduction cycles are timed to coincide with them, so that young animals are born at that season of the year when food resources are maximum. All deviations from this optimal cycle, which is reproduced from year to year, are eliminated by stabilizing selection. Descendants born too early die from lack of food; offspring born too late do not have time to prepare for winter. How do animals and plants know that winter is coming? Upon the onset of frost? No, this is not a very reliable pointer. Short-term temperature fluctuations can be very misleading. If in some year it gets warmer earlier than usual, this does not mean that spring has come. Those who react too quickly to this unreliable signal risk being left without offspring. It is better to wait for a more reliable sign of spring - increasing daylight hours. In most animal species, it is this signal that triggers the mechanisms of seasonal changes in vital functions: cycles of reproduction, molting, migration, etc. I.I. Schmalhausen convincingly showed that these universal adaptations arise as a result of stabilizing selection.

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

Disruptive selection. With stabilizing selection, individuals with an average manifestation of traits have an advantage; with driving selection, one of the extreme forms has an advantage. Theoretically, another form of selection is conceivable - disruptive or discontinuous selection, when both extreme forms gain 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 - is extended almost throughout the summer, with most of the plants flowering and fruiting in mid-summer. However, in hay meadows, those plants that have time to flower and produce seeds before mowing, and those that produce seeds at the end of summer, after mowing, benefit. As a result, two races of rattle are formed - early and late flowering.

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

Sexual selection. Males of many species display clearly expressed secondary sexual characteristics that at first glance seem non-adaptive: the tail of a peacock, the bright feathers of birds of paradise and parrots, the scarlet crests of roosters, the enchanting colors of tropical fish, the songs of birds and frogs, etc. Many of these features complicate the life of their carriers and make them easily noticeable to predators. It would seem that these characteristics do not provide any advantages to their carriers in the struggle for existence, and yet they are very widespread in nature. What role did natural selection play in their emergence and spread?

We already know that the survival of organisms is an important, but not the only component of natural selection. Another important component is attractiveness to individuals of the opposite sex. Charles Darwin called this phenomenon sexual selection. He first mentioned this form of selection in On the Origin of Species and then analyzed it in detail in The Descent of Man and Sexual Selection. He believed that “this form of selection is determined not by the struggle for existence in the relations of organic beings among themselves or with external conditions, but by the competition between individuals of one sex, usually males, for the possession of individuals of the other sex.”

Sexual selection is natural selection for reproductive success. Traits that reduce the viability of their hosts can emerge and spread if the advantages they provide for reproductive success are significantly greater than their disadvantages for survival. A male who lives short but is liked by females and therefore produces many offspring has much higher overall fitness than one who lives long but produces few offspring. In many animal species, the vast majority of males do not participate in reproduction at all. In each generation, fierce competition arises between males for females. This competition can be direct, and manifest itself in the form of struggle for territory or tournament battles (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 manifests itself through displays of flamboyant appearance or complex courtship behavior. Females choose the males they like best. As a rule, these are the brightest males. But why do females like bright males?

The fitness of a female depends on how objectively she is able to assess the potential fitness of the future father of her children. She must choose a male whose sons will be highly adaptable and attractive to females.

Two main hypotheses about the mechanisms of sexual selection have been proposed.

According to the “attractive sons” hypothesis, the logic of female choice is somewhat different. If brightly colored males, for whatever reason, are attractive to females, then it is worth choosing a brightly colored father for his future sons, because his sons will inherit the brightly colored genes and will be attractive to females in the next generation. Thus, a positive feedback arises, which leads to the fact that from generation to generation the brightness of the plumage of males becomes more and more intense. The process continues to grow until it reaches the limit of viability. Let's imagine a situation where females choose males with a longer tail. Long-tailed males produce more offspring than males with short and medium tails. From generation to generation, the length of the tail increases because females choose males not with a certain tail size, but with a larger than average size. Eventually, the tail reaches a length where its detriment to the male's vitality is balanced by its attractiveness in the eyes of females.

In explaining these hypotheses, we tried to understand the logic of the actions of female birds. It may seem that we expect too much from them, that such complex calculations of fitness are hardly possible for them. In fact, females are no more or less logical in their choice of males than in all their other behavior. When an animal feels thirsty, it does not reason that it should drink water in order to restore the water-salt balance in the body - it goes to a watering hole because it feels thirsty. When a worker bee stings a predator attacking a hive, she does not calculate how much with this self-sacrifice she increases the overall fitness of her sisters - she follows instinct. In the same way, females, when choosing bright males, follow their instincts - they like bright tails. All those to whom instinct suggested a different behavior, all of them did not leave offspring. Thus, we were discussing not the logic of females, but the logic of the struggle for existence and natural selection - a blind and automatic process that, acting constantly from generation to generation, has formed all the amazing diversity of shapes, colors and instincts that we observe in the world of living nature .

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

Mechanism of natural selection

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

1. Organisms produce more offspring than can survive;
2. There is heritable variation in the population of these organisms;
3. Organisms with different genetic traits have different survival rates and ability to reproduce.

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 in its existing environment. This determines the size of his genetic contribution to the next generation. However, the main thing in determining fitness is not the total number of descendants, but the number of descendants with a given genotype (relative fitness). For example, if the offspring of a successful and rapidly reproducing organism are weak and do not reproduce well, then the genetic contribution and therefore the fitness of that organism will be low.

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:

1. Directional selection- changes in the average value of a trait over time, for example an increase in body size;
2. Disruptive selection- selection for extreme values ​​of a trait and against average values, for example, large and small body sizes;
3. Stabilizing selection- selection against extreme values ​​of a trait, which leads to a decrease in the variance of the trait.

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

Selection can operate at 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, for example, group selection, can lead to cooperation (see Evolution#Cooperation).

Forms of natural selection

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

Driving selection

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 a number of areas, the proportion of dark-colored butterflies in some well-studied moth populations in England reached 95%, while for the first time a 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 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.

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 members of the opposite sex. Darwin called this phenomenon sexual selection. “This form of selection is determined not by the struggle for existence in the relations of organic beings among themselves or with external conditions, but by the competition between individuals of one sex, usually males, for the possession of individuals of the other sex.” Traits that reduce the viability of their hosts can emerge and spread if the advantages they provide for reproductive success are significantly greater than their disadvantages for survival.

Two hypotheses about the mechanisms of sexual selection are common.

• According to the “good genes” hypothesis, the female “reasons” as follows: “If a given male, despite his bright plumage and long tail, managed not to die in the clutches of a predator and survive to sexual maturity, then he has good genes that allowed him to do this . Therefore, he should be chosen as the father of his children: he will pass on his good genes to them.” By choosing colorful males, females are choosing good genes for their offspring.
• According to the “attractive sons” hypothesis, the logic of female choice is somewhat different. If brightly colored males, for whatever reason, are attractive to females, it is worth choosing a brightly colored father for his future sons, because his sons will inherit the brightly colored genes and will be attractive to females in the next generation. Thus, a positive feedback occurs, which leads to the fact that from generation to generation the brightness of the plumage of males becomes more and more intense. The process continues to grow until it reaches the limit of viability.

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

The role of natural selection in evolution

In the example of the worker ant we have an insect extremely different from its parents, yet absolutely sterile and, therefore, unable to transmit from generation to generation acquired modifications of structure or instincts. A good question to ask is how reconcilable is this case with the theory of natural selection?

- Origin of Species (1859)

Darwin assumed that selection could apply not only to an individual organism, but also to a family. He also said that perhaps, to one degree or another, this could explain people's behavior. He was right, but it was only with the advent of genetics that it became possible to provide a more expanded view of the concept. The first sketch of the “theory of kin selection” was made by the English biologist William Hamilton in 1963, who was the first to propose considering natural selection not only at the level of an individual or an entire family, but also at the level of the gene.

see also

Notes

1. , With. 43-47.
2. , p. 251-252.
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4. Haldane J.B.S. The theory of natural selection today // Nature. - 1959. - Vol. 183, no. 4663. - P. 710-713. - PMID 13644170.
5. Lande R., Arnold S. J. The measurement of selection on correlated characters // Evolution. - 1983. - Vol. 37, no. 6. - P. 1210-1226. - DOI:10.2307/2408842.