What does natural selection mean in biology. Report: Natural selection

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

How natural selection works

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

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

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

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

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

Principles of classification of natural selection

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

Positive

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

Negative

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

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

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

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

Moving

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

stabilizing

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

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

Destabilizing

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

Disruptive (tearing)

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

Sexual

Standing apart in this series of logically substantiated processes is sexual selection. Its essence lies in the fact that representatives of the same species (usually males) compete with each other in the struggle for the right to procreate. . However, they often develop the same symptoms. which adversely affect their viability. A classic example is a peacock with its magnificent tail, which does not have any practical use, moreover, making it visible to predators and able to interfere with movement. Its only function is to attract a female, and it successfully performs this function. There are two hypotheses explaining the mechanism of female selection:

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

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

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

Living in natural conditions, there is individual variability that can manifest itself in three types useful, neutral and harmful. Usually, organisms with harmful variability die at various stages of individual development. The neutral variability of organisms does not affect their viability. Individuals with beneficial variability survive by virtue of an advantage in intraspecific, interspecific, or against adverse conditions environment.

driving selection

When environmental conditions change, those individuals of the species survive in which hereditary variability has manifested itself and, in connection with this, signs and properties have developed that correspond to new conditions, and those individuals that did not have such variability die. During his voyage, Darwin discovered that on oceanic islands where strong winds prevail, there are few long-winged insects and many insects with rudimentary wings and wingless insects. As Darwin explains, insects with normal wings could not withstand the strong winds on these islands and died. And insects with rudimentary wings and wingless did not rise at all into the air and hid in the cracks, finding shelter there. This process, which was accompanied by hereditary variability and natural selection and continued for many thousands of years, led to a decrease in the number of long-winged insects on these islands and the appearance of individuals with rudimentary wings and wingless insects. Natural selection, which ensures the emergence and development of new features and properties of organisms, is called motive selection.

Disruptive selection

Disruptive selection- this is a form of natural selection, leading to the formation of a number of polymorphic forms that differ from each other within the same population.

Among organisms of a certain species, individuals with two or more different forms are sometimes found. This is the result of a special form of natural selection, disruptive selection. Yes, at ladybugs there are two forms of rigid wings - with a dark red and reddish color. Beetles with reddish wings rarely die from cold in winter, but give few offspring in summer, and with dark red wings, on the contrary, they die more often in winter, being unable to withstand the cold, but give numerous offspring in summer. Consequently, these two forms of ladybugs, due to their different adaptability to different seasons, managed to keep their offspring for centuries.

Natural selection is the main, leading, guiding factor in evolution, underlying the theory of Ch. Darwin. All other factors of evolution are random, only natural selection has a direction (in the direction of adapting organisms to environmental conditions).

Definition: selective survival and reproduction of the fittest organisms.

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

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

Forms:

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

Consequences of natural selection:

Evolution (change, complication of organisms)
Emergence of new species (increase in the number [diversity] of species)
The adaptation of organisms to environmental conditions. Any fit is relative., i.e. adapts the body to only one specific conditions.

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 rate
3) the emergence of new species
4) preservation of individuals with altered traits

Answer

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

Answer

Choose three options. What are the characteristics of motive selection?
1) operates under relatively constant living conditions
2) eliminates individuals with an average value of the trait
3) promotes the reproduction of individuals with a modified genotype
4) preserves individuals with deviations from the average values of the trait
5) preserves individuals with the established norm of the reaction of the trait
6) contributes to the appearance of mutations in the population

Answer

Choose three features that characterize the driving form of natural selection
1) provides the appearance of a new species
2) manifests itself in changing environmental conditions
3) the adaptability of individuals to the original environment is improved
4) individuals with a deviation from the norm are culled
5) the number of individuals with the average value of the trait increases
6) individuals with new traits are preserved

Answer

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

Answer

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

Answer

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

Answer

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

Answer

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

3) natural selection
4) artificial selection

Answer

Choose one, the most correct option. Insect pests acquire resistance to pesticides over time as a result of
1) high fecundity
2) modification variability
3) preservation of mutations by natural selection
4) artificial selection

Answer

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

Answer

Choose one, the most correct option. Are the following statements about the forms of natural selection correct? 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 a trait
1) only A is true
2) only B is true
3) both statements are correct
4) both judgments are wrong

Answer

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

Answer

1. Establish a correspondence between the characteristic of natural selection and its form: 1) driving, 2) stabilizing. Write the numbers 1 and 2 in the correct order.
A) preserves the mean value of the feature
B) contributes to adaptation to changing environmental conditions
C) retains individuals with a trait that deviates from its average value
D) contributes to an increase in 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 the 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 the 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 down the numbers 1 and 2 in the order corresponding to the letters.
A) operates in changing environmental conditions
B) operates in constant environmental conditions
C) is aimed at maintaining the previously established average value of the trait
D) leads to a shift in the average value of the trait in the population
D) under its action, both an increase in a sign and a weakening can occur

Answer

4. Establish a correspondence between the signs and forms of natural selection: 1) stabilizing, 2) driving. Write down the 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
B) maintains the average norm of the trait
D) culls individuals with deviations from the average norm of signs
D) increases the heterozygosity of the population

Answer

Establish a correspondence between examples and forms of natural selection, which are illustrated by these examples: 1) driving, 2) stabilizing. Write down the 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 insect pest resistance to pesticides
C) the preservation of the reptile tuatara living in New Zealand to the present day
D) a decrease in the size of the cephalothorax in crabs that live in muddy water
E) in mammals, the mortality of newborns with an average weight is less than with very low or very high
E) the death of winged ancestors and the preservation of insects with reduced wings on islands with strong winds

Answer

Establish a correspondence between the forms of the struggle for existence and examples illustrating them: 1) intraspecific, 2) interspecific. Write down the 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
C) capercaillie lekking
D) nosed monkeys try to shout down each other, puffing out huge noses
D) chaga mushroom settles on a birch
E) the main prey of the marten is squirrel

Answer

Analyze the table "Forms of natural selection". For each letter, select the appropriate 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 sign
6) the formation of bacterial resistance to antibiotics
7) preservation of the relict plant species Gingko biloba 8) increase in the number of heterozygous organisms

Answer

It is a holistic doctrine of the historical development of the organic world.

The essence of evolutionary teaching lies in the following basic provisions:

1. All kinds of living beings inhabiting the Earth have never been created by someone.

2. Arising naturally, organic forms were slowly and gradually transformed and improved in accordance with the surrounding conditions.

3. The transformation of species in nature is based on such properties of organisms as heredity and variability, as well as natural selection constantly occurring in nature. Natural selection is carried out through the complex interaction of organisms with each other and with factors inanimate nature; this relationship Darwin called the struggle for existence.

4. The result of evolution is the adaptability of organisms to the conditions of their habitat and the diversity of species in nature.

Natural selection. However, Darwin's main merit in creating the theory of evolution lies in the fact that he developed the doctrine of natural selection as the leading and guiding factor in evolution. Natural selection, according to Darwin, is a set of changes occurring in nature that ensure the survival of the fittest individuals and their predominant offspring, as well as the selective destruction of organisms that are unadapted to existing or changing environmental conditions.

In the process of natural selection, organisms adapt, i.e. they develop the necessary adaptations to the conditions of existence. As a result of competition different types having similar vital needs, less adapted species die out. Improving the mechanism of adaptation of organisms leads to the fact that the level of their organization is gradually becoming more complicated and thus the evolutionary process is carried out. At the same time, Darwin paid attention to such characteristics natural selection, as the gradualness and slowness of the process of change and the ability to summarize these changes into large, decisive causes leading to the formation of new species.

Based on the fact that natural selection acts among diverse and unequal individuals, it is considered as the total interaction of hereditary variability, preferential survival and reproduction of individuals and groups of individuals better adapted than others to given conditions of existence. Therefore, the doctrine of natural selection as a driving and guiding factor historical development The organic world is central to Darwin's theory of evolution.

Forms of natural selection:

Driven selection is a form of natural selection that operates in a directed change in environmental conditions. Described by Darwin and Wallace. In this case, individuals with traits that deviate in a certain direction from the average value receive advantages. At the same time, other variations of the trait (its deviations in the opposite direction from the average value) are subjected to negative selection.

As a result, in the population from generation to generation, there is a shift in the average value of the trait in 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).

In the 20th century, in a number of areas, the proportion of dark-colored butterflies in some well-studied populations of the birch moth in England reached 95%, while the first dark butterfly (morfa carbonaria) was captured in 1848.

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

Stabilizing selection- a form of natural selection, in which its action is directed against individuals with extreme deviations from the average norm, in favor of individuals with an average severity of the trait. The concept of stabilizing selection was introduced into science and analyzed by I. I. Shmalgauzen.

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

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

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

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

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

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

Sexual selection 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 rivalry between individuals of one sex, usually males, for the possession of individuals of the other sex."

Traits that reduce the viability of their carriers can emerge and spread if the advantages they provide in breeding success are significantly greater than their disadvantages for survival. 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 the watering place because it feels thirsty.

In the same way, females, choosing bright males, follow their instincts - they like bright tails. Those who instinctively prompted a 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 that, acting constantly from generation to generation, has formed that amazing variety of forms, colors and instincts that we observe in the world of living nature.

When analyzing the causes of an increase in the organization of organisms or their adaptability to living conditions, Darwin drew attention to the fact that selection does not necessarily require the selection of the best, it can only be reduced to the destruction of the worst. This is exactly what happens in unconscious selection. But the destruction (elimination) of the worst, less adapted to the existence of organisms in nature, can be observed at every step. Consequently, natural selection can be carried out by the "blind" forces of nature.

Darwin emphasized that the expression "natural selection" should in no case be understood in the sense that someone conducts this selection, since this term speaks of the action of the elemental forces of nature, as a result of which organisms adapted to given conditions survive and die. unadapted. The accumulation of useful changes leads first to small, and then to large changes. This is how new varieties, species, genera and other systematic units of a higher rank appear. This is the leading creative role natural selection in evolution.

Elementary evolutionary factors. Mutation process and genetic combinatorics. Population waves, isolation, genetic drift, natural selection. Interaction of elementary evolutionary factors.

Elementary evolutionary factors are stochastic (probabilistic) processes occurring in populations that serve as sources of primary intrapopulation variability.

3. Periodic with high amplitude. Found in a wide variety of organisms. Often they are periodic in nature, for example, in the "predator-prey" system. May be associated with exogenous rhythms. It is this type of population waves that plays the greatest role in evolution.

History reference. The expression “waves of life” (“Wave of life”) was probably used for the first time by the explorer of the South American pampas Hudson (W.H. Hudson, 1872-1873). Hudson noted that under favorable conditions (light, frequent showers) the vegetation that usually burns out has been preserved; an abundance of flowers gave birth to an abundance of bumblebees, then mice, and then birds that fed on mice (including cuckoos, storks, short-eared owls).

S.S. Chetverikov drew attention to the waves of life, noting the appearance in 1903 in the Moscow province of some species of butterflies that had not been found there for 30 ... 50 years. Before that, in 1897 and somewhat later, there was a mass appearance of the gypsy moth, which denuded vast areas of forests and caused significant damage to orchards. In 1901, the admiral butterfly appeared in significant numbers. He reported the results of his observations in short essay"Waves of Life" (1905).

If during the period of maximum population size (for example, a million individuals) a mutation appears with a frequency of 10-6, then the probability of its phenotypic manifestation will be 10-12. If, during the period of decline in the population to 1000 individuals, the carrier of this mutation survives by chance, then the frequency of the mutant allele will increase to 10-3. The same frequency will remain in the period of the subsequent increase in the number, then the probability of the phenotypic manifestation of the mutation will be 10-6.

Insulation. Provides manifestation of the Baldwin effect in space.

In a large population (for example, one million diploid individuals), a mutation rate of 10-6 means that about one in a million individuals is a carrier of the new mutant allele. Accordingly, the probability of the phenotypic manifestation of this allele in a diploid recessive homozygote is 10-12 (one trillionth).

If this population is divided into 1000 small isolated populations of 1000 individuals, then one of the isolated populations will most likely contain one mutant allele, and its frequency will be 0.001. The probability of its phenotypic manifestation in the next subsequent generations will be (10 - 3) 2 = 10 - 6 (one millionth). In ultra-small populations (tens of individuals), the probability of a mutant allele in the phenotype increases to (10 - 2)2 = 10 - 4 (one ten-thousandth).

Thus, only due to the isolation of small and ultra-small populations, the chances of a phenotypic manifestation of a mutation in the next generations will increase thousands of times. At the same time, it is difficult to assume that the same mutant allele appears in the phenotype by chance in different small populations. Most likely, each small population will be characterized by a high frequency of one or a few mutant alleles: either a, or b, or c, etc.

Natural selection is a process originally defined by Charles Darwin as leading to the survival and preferential reproduction of individuals who are more adapted to given environmental conditions and have useful hereditary traits. In accordance with Darwin's theory and the modern synthetic theory of evolution, the main material for natural selection is random hereditary changes - recombination of genotypes, mutations and their combinations.

The idea of comparing artificial and natural selection is that in nature the most “successful”, “best” organisms are also selected, but in the role of an “appraiser” of the usefulness of properties in this case it is not the person who acts, but the environment. In addition, the material for both natural and artificial selection are small hereditary changes that accumulate from generation to generation.

Mechanism of natural selection

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

Organisms produce more offspring than can survive;
In the population of these organisms, there is hereditary variability;
Organisms that have different genetic traits have different survival rates and ability to reproduce.

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 offspring, but the number of offspring with a given genotype (relative fitness). For example, if the offspring of a successful and rapidly reproducing organism are weak and do not reproduce well, then the genetic contribution and, accordingly, the fitness of this organism will be low.

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

Directed Selection- changes in the average value of the trait over time, for example, an increase in body size;
Disruptive selection- selection for the extreme values of the trait and against the average values, for example, large and small body sizes;
Stabilizing selection- selection against the extreme values of the 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 an individual for potential partners. Traits that have evolved through sexual selection are particularly evident in the males of certain animal species. Traits such as large horns, bright colors, on the one hand, can attract predators and reduce the survival rate of males, and on the other hand, this is balanced by the reproductive success of males with similar pronounced traits.

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

Forms of natural selection

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

driving selection

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

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

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

Stabilizing selection

Stabilizing selection- a form of natural selection, in which its action is directed against individuals with extreme deviations from the average norm, in favor of individuals with an average severity of the trait. The concept of stabilizing selection was introduced into science and analyzed by I. I. Shmalgauzen.

Disruptive selection

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

sexual selection

sexual selection This is natural selection for success in reproduction. The survival of organisms is an important but not the only component of natural selection. Another important component is attractiveness to members of the opposite sex. Darwin called this phenomenon sexual selection. "This form of selection is determined not by the struggle for existence in the relations of organic beings among themselves or with external conditions, but by the rivalry between individuals of one sex, usually males, for the possession of individuals of the other sex." Traits that reduce the viability of their carriers can emerge and spread if the advantages they provide in breeding success are significantly greater than their disadvantages for survival.

Two hypotheses about the mechanisms of sexual selection are common.

According to the “good genes” hypothesis, the female “reasons” in the following way: “If this male, despite the bright plumage and long tail, managed not to die in the clutches of a predator and live to puberty, 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 bright males, females choose good genes for their offspring.
According to the “attractive sons” hypothesis, the logic of female selection is somewhat different. If bright males, for whatever reason, are attractive to females, it is worth choosing a bright father for your future sons, because his sons will inherit the bright color genes and will be attractive to females in the next generation. Thus, a positive feedback occurs, which leads to the fact that from generation to generation the brightness of the plumage of males increases more and more. The process goes on increasing 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 the watering hole because it feels thirsty. In the same way, females, choosing bright males, follow their instincts - they like bright tails. Those who instinctively prompted a 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 that, acting constantly from generation to generation, has formed that amazing variety of forms, colors and instincts that we observe in the world of wildlife.

Selection methods: positive and negative selection

There are two forms of artificial selection: Positive and Clipping (negative) selection.

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

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

The role of natural selection in evolution

In the example of the worker ant, we have an insect extremely different from its parents, yet absolutely barren and therefore unable to transmit from generation to generation acquired modifications of structure or instincts. One can ask a good question - to what extent is it possible to reconcile this case with the theory of natural selection?
- Origin of Species (1859)

Darwin assumed that selection could be applied not only to the individual organism, but also to the family. He also said that, perhaps, to one degree or another, this can also explain the behavior of people. He turned out to be right, but it was not until the advent of genetics that it became possible to provide a more extended view of this concept. The first outline of the "kind selection theory" 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 a whole family, but also at the level of a gene.

Notes

, With. 43-47.
, p. 251-252.
Orr H.A. Fitness and its role in evolutionary genetics // Nature Reviews Genetics. - 2009. - Vol. 10, no. 8. - P. 531-539. - DOI:10.1038/nrg2603. - PMID 19546856 .
Haldane J.B.S. The theory of natural selection today // Nature. - 1959. - Vol. 183, no. 4663. - P. 710-713. - PMID 13644170 .
Lande R., Arnold S. J. The measurement of selection on correlated characters // Evolution. - 1983. - Vol. 37, no. 6. - P. 1210-1226. -

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