Introduction
The theory of Charles Darwin, known as the theory of natural selection, is one of the pinnacles of scientific thought in the 19th century. However, its significance goes far beyond its age and beyond the scope of biology: Darwin's theory has become the natural-historical basis of the materialistic worldview.
We, of course, cannot give a complete exposition of Darwin's grandiose work on the pages of this work and will restrict ourselves to a brief review of the main provisions of his theory, which is necessary to understand its relationship with the modern theory of evolution.
Darwin's theory is a wonderful example scientific research based on a large number of reliable scientific facts, the analysis of which leads Darwin to a coherent system of commensurate conclusions. Darwin collected data to substantiate his theory for many years. The first outline of the theory was already written in 1842, but (a striking example of scientific caution and conscientiousness!) was not published for many years, during which Darwin continued to collect and analyze new data. Darwin's great work, The Origin of Species by Means of Natural Selection, or the Preservation of Favored Species in the Struggle for Life, did not appear until 1859.
It is known that the stimulus that accelerated the publication of Darwin's work was the work of A. Wallace (1823-1913), who independently came to close evolutionary conclusions. Both papers were jointly reported in 1858 at a meeting of the Linnean Society in London, and Wallace, having familiarized himself with Darwin's work, fully recognized its priority. Darwin analyzed the evolutionary process much broader and deeper than Wallace, and, paying due respect to the latter, we rightly call Ch. Darwin the author of the theory of natural selection.
2. Natural selection
2.1. The variability of organisms in nature
Darwin collected numerous data indicating that the variability of the most diverse types of organisms in nature is very great, and its forms are fundamentally similar to the forms of variability of domestic animals and plants.
Darwin believed that there is no qualitative difference between a species and a variety - these are just different stages of the gradual accumulation of differences between groups of individuals of different scales.
Greater variability is characteristic of more widespread species living in more diverse conditions. In nature, as well as in the domesticated state, the main form of variability of organisms is indefinite, which serves as a universal material for the process of speciation.
Jordan N.N. The evolution of life. - M .: "Academy", 2001. - P.19.
Varied and fluctuating differences between individuals of the same species form, as it were, a smooth transition to more stable differences between varieties of this species; in turn, the latter just as gradually pass into clearer differences in even larger groupings - subspecies, and differences between subspecies - into well-defined interspecific differences. Thus, individual variability smoothly turns into group differences. From this Darwin concluded that the individual differences of individuals are the basis for the emergence of varieties. Varieties with the accumulation of differences between them turn into subspecies, and those, in turn, into certain types. Therefore, a clearly expressed variety can be considered as the first step towards the isolation of a new species (a variety is a "beginning species").
.2. Struggle for existence
Jordan N.N. The evolution of life. - M .: "Academy", 2001. - P.21.
Comparing all the information collected about the variability of organisms in the wild and domesticated state and the role of artificial selection for breeding breeds and varieties of domesticated animals and plants, Darwin approached the discovery of that creative force that drives and directs the evolutionary process in nature - natural selection:“Since far more individuals of each species are born than how many of them can survive, and since, consequently, the struggle for existence constantly arises, it follows from this that any creature that, in the complex and often changing conditions of its life, although insignificantly, will change in a direction that is beneficial to him, will have a better chance of surviving and. thus subject to natural selection. By virtue of the strict principle of heredity, the selected variety will tend to reproduce in its new and altered form."
2.3. The results of natural selection
Jordan N.N. The evolution of life. - M .: "Academy", 2001. - P.27.
Natural selection is the inevitable result of the struggle for existence and the hereditary variability of organisms. According to Darwin, natural selection is the most important creative force that directs the evolutionary process and naturally determines the emergence of adaptations of organisms, progressive evolution and an increase in the diversity of species.
emergence devices (adaptation) organisms to the conditions of their existence, which gives the structure of living beings the features of "expediency", is a direct result of natural selection, since its very essence is differentiated survival and the predominant leaving of offspring precisely by those individuals who, due to their individual characteristics, are better adapted to environmental conditions than others. The accumulation by selection from generation to generation of those traits that give an advantage in the struggle for existence, and gradually leads to the formation of specific adaptations.
Darwin emphasized that any particular degree of fitness of organisms relative- usually more possible perfect forms adaptations to this environment. This is proved by numerous examples of extremely rapid reproduction and wide distribution of a number of animal and plant species in areas completely new to them. the globe where they were accidentally or deliberately introduced by humans (rabbits in Australia, rats, cats, dogs, pigs on the islands of Oceania, Canadian elodea in European waters, etc.). All these species, which arose in completely different geographical areas, turned out to be better adapted to the conditions of new areas for themselves than the species of animals and plants that had inhabited these areas for a long time and had sufficiently perfect adaptations to their conditions.
3. The origin of man
Man belongs to the order of primates, and the evolutionary history of man is part of the phylogeny of this group.
Grant V. Evolution of organisms. - M.: Enlightenment, 1992. - P.103.
For a long time, scientific knowledge was too abrupt and incomplete to solve the problem of the origin of man. Only in 1857, Charles Darwin made a hypothesis, and in 1871, in his work “The Origin of Man and Sexual Selection”, he convincingly proved that people originated from apes, and were not created by an act of divine creation, as the church teaches. “If we do not deliberately close our eyes, then with the current level of knowledge we will be able to approximately recognize our ancestors, and we have no reason to be ashamed of them,” Charles Darwin wrote. Role social factors, which was also pointed out by Ch. Darwin, was revealed by F. Engels in the work "The role of labor in the process of turning a monkey into a man" (1896).
The commonality of man and vertebrates is confirmed by the common plan of their structure: the skeleton, nervous system, circulatory, respiratory, digestive systems. The relationship between man and animals is especially convincing when comparing their embryonic development. On his early stages the human embryo is difficult to distinguish from the embryos of other vertebrates. At the age of 1.5 - 3 months, it has gill slits, and the spine ends in a tail. For a very long time, the similarity of human embryos and monkeys remains. Specific (species) human features appear only at the latest stages of development.
Rudiments and atavisms serve as important evidence of the kinship of man with animals. There are about 90 rudiments in the human body: coccygeal bone (remainder of a reduced tail); crease in the corner of the eye (remnant of the nictitating membrane); thin hair on the body (the rest of the wool); a process of the caecum - the appendix, etc. All these rudiments are useless for humans and are the legacy of animal ancestors. Atavisms (unusually highly developed rudiments) include an external tail, with which people are very rarely born; abundant hair on the face and body; polynipple, strongly developed fangs, etc.
The commonality of the structural plan, the similarity of embryonic development, rudiments, atavisms are indisputable evidence of the animal origin of man and evidence that man, like animals, is the result of a long historical development of the organic world.
According to the structure and physiological characteristics, the closest relatives of man are the great apes, or anthropoids (from the Greek anthropos - man). These include chimpanzee, gorilla, orangutan. Similar structural details testify to the close relationship between humans and anthropoids: general character physique, tail reduction, prehensile hand with flat nails and opposed thumb, shape of eyes and ears, equal number of incisors, canines and molars; complete change of milk teeth and much more.
Conclusion
Summing up, we can list the main conclusions of this work.
2. The focus of evolutionary theory should be not individual organisms, but biological species and intraspecific groups (populations).
4. The inevitable result of the hereditary variability of organisms and the struggle for existence is natural selection - preferential survival and the provision of offspring for better adapted individuals. Less adapted organisms (and whole species) die out without leaving offspring.
1. Organisms, both in a tamed and in a wild state, are characterized by hereditary variability. The most common and important form of variability is indeterminate. Changes in the external environment serve as a stimulus for the emergence of variability in organisms, but the nature of variability is determined by the specifics of the organism itself, and not by the direction of changes in external conditions.
3. All types of organisms in nature are forced to wage a fierce struggle for their existence. The struggle for existence for individuals of a given species consists of their interaction with unfavorable biotic and abiotic factors external environment, as well as from their competition with each other. The latter is a consequence of the tendency of every species to unlimited reproduction and the huge "overproduction" of individuals in each generation. According to Darwin, the intraspecific struggle is the most important.
5. The consequences of the struggle for existence and natural selection are: the development of adaptations of species to the conditions of their existence (determining the "expediency" of the structure of organisms), divergence (development from a common ancestor of several species, an increasing divergence of their characteristics in evolution) and progressive evolution (complication and organizational improvement).
All ingenious is simple, but not all simple is ingenious. There are only two criteria for a brilliant discovery. First, it must touch upon the fundamental foundations of our knowledge. Second: it should be so simple that, on the one hand, it is clear that there is no more concise explanation, and on the other hand, bewilderment arises, as it was not noticed earlier. If we approach with such a measure, then, perhaps, one of the most ingenious discoveries of mankind is the evolutionary teaching of Charles Darwin. The theory of natural selection is familiar, of course, to everyone. But, since we will have to refer to it very often, let us recall its main provisions.
Charles Darwin
The idea that a mechanism similar to man-made artificial selection operates in living nature was first expressed by English scientists Charles Darwin and Alfred Wallace. The meaning of their idea is that in order to create more perfect organisms, it is not at all necessary for nature to understand and analyze what it does, but it is possible to act at random. It is enough to constantly create in individuals a wide range of various qualities - and in the end the fittest will survive, preserving and passing on to their descendants those properties that turn out to be useful.
According to Darwin, evolution is described by three principles: heredity, variability and natural selection. According to them:
First, an individual appears with new, completely random properties.
Interacting with the external environment and competing with others, an individual either gives offspring or dies earlier.
Finally, if the outcome of the previous stage is positive, and she leaves offspring, her offspring inherit the newly acquired properties and the test of natural selection continues on the offspring.
As we now know, all the properties of a living organism are encoded in its set of chromosomes, which is called the genome. Each chromosome is made up of a sequence of genes. What properties genes code for is determined by their type and location on the chromosome.
During asexual reproduction, the genes of the parent are copied, and the offspring receives all the same properties as its ancestor. However, under the influence of the external environment (natural background radiation, chemical substances and viruses) mutations occur, that is, changes in the genome. Changing genes leads to the emergence of new, sometimes completely unexpected properties. If these properties turn out to be non-negative, then the creature survives and passes them on to offspring. If the mutation turns out to be detrimental, then the creature most likely dies. The habitat creates food restrictions, and many creatures have enemies for whom they themselves are food. Naturally, in such conditions of competition, the one who is the most fit survives.
The main mechanism for the emergence of new genes is duplication. Random doubling of the nucleotide sequence leads to the fact that one of the copies of the gene continues to perform its original function, while the other copy goes into standby mode and can accumulate mutations without harm to the body. Generations later, the cumulative changes can lead to the appearance of a new function beneficial for the organism in this copy. Myoglobin, whose ancestor is hemoglobin, is usually cited as an example of such evolution. Myoglobin also binds to oxygen, but is adapted for this function in skeletal and cardiac muscles.
Evolution goes faster if, in addition to mutations, there is an exchange of genes between different individuals. So, among plants there is cross-pollination, and the offspring, respectively, receives hereditary properties from two parents - partly from one, partly from the other. The exchange of genes significantly increases the rate of evolution. If someone has a useful trait, his descendants get it. If another creature of the same species develops another useful trait, then the gene swap gives a chance to create a creature in which these two useful traits intersect.
In bacteria, there is a so-called horizontal gene transfer, when one bacterium transfers genetic material to another, which is not its descendant. This phenomenon was discovered when studying the transmission between different types of bacteria of resistance to antibiotics. It is now believed that horizontal transfer plays a huge role in bacterial evolution, as it allows a valuable trait that appeared in one population of bacteria to spread very quickly among a large number of species.
Sexual reproduction, which is also characteristic of humans, in addition to ensuring the exchange of genes, creates additional tools for competition within the species, which has far-reaching consequences.
In 1859, Charles Darwin published his seminal work, The Origin of Species by Means of Natural Selection, or the Preservation of Favorable Races in the Struggle for Life. From that moment began the drama in the minds of people. On the one hand, absolutely everything in nature seems to confirm the correctness of the evolutionary doctrine. But on the other hand, how to believe that such an incredible complexity of living beings is the result of just random experiments of nature. This conflict of faith has long tormented people, causing some to doubt Darwin's theory, while others desperately deny it. Thank God, now it is no longer accepted to doubt globally. But the conflict of faith did not disappear, it turned into a widespread belief that, in addition to natural selection, there are mechanisms that allow us to speed up and optimize evolution. In this book, we will proceed solely from the principles evolutionary theory, suggesting that if something accelerates natural selection, then these are properties in its process and acquired.
Each quality that is acquired in the course of evolution turns out to be beneficial for its carriers at the moment when it arises. But, having arisen and consolidated, it serves as the foundation for the emergence of new qualities. When things become very complicated, many things become difficult to explain. And then there is a temptation to explain what is being explained, and write off the rest as "random exceptions that confirm the rules." We must always remember that there is nothing accidental, absolutely everything that evolution has brought has a rational explanation. And a good theory should explain everything, even seemingly insignificant nuances.
If a husband brings flowers for no reason, then there is a reason after all.
We will refer to natural selection very often in this book. In this regard, we will immediately make a reservation. It is clear that evolution is not a directed movement, subordinated to some higher goal. There is no higher power, which would set tasks or punish for disobedience. There is a global statistical “it happened”. But, in order for the narrative not to turn out dry and boring, we will use not entirely correct and not at all correct figurative formulations. We will say something like: “it is beneficial for nature”, “evolution created”, “nature invented”. All this must be taken as figures of speech and always remember the statistical essence of natural selection.
The idea of comparing artificial and natural selection is that in nature the selection of the most “successful”, “best” organisms also takes place, but in this case it is not a person who acts as an “appraiser” of the usefulness of properties, 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 total number descendants, and 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, 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, whose substrate 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 changing environment or adaptation 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.
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 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.
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 the 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
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 with one another or with external conditions, but by 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” as follows: “If this male, despite the bright plumage and long tail, managed not to die in the clutches of a predator and survive 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 expanded 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.
see also
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. -
The idea of comparing artificial and natural selection is that in nature the selection of the most “successful”, “best” organisms also takes place, but in this case it is not a person who acts as an “appraiser” of the usefulness of properties, 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:
- Driving (directional) 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, whose substrate 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 act simultaneously at 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 areas, the proportion of dark-colored butterflies in some well-studied populations of the birch moth in England reached 95%, while for the first time a dark butterfly ( Morfa carbonaria) was captured in 1848.
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 reaction rate 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.
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 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.
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 the 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
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” as follows: “If this male, despite the bright plumage and long tail, managed not to die in the clutches of a predator and survive 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.
The role of natural selection in evolution
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 expanded 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. see alsoNotes
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Natural selection is a natural process in which, of all living organisms, only those that have qualities that contribute to the successful reproduction of their own kind are preserved in time. According to the synthetic theory of evolution, natural selection is one of the most important factors in evolution.
Mechanism of natural selectionThe idea that a mechanism similar to artificial selection operates in living nature was first expressed by English scientists Charles Darwin and Alfred Wallace. The essence of their idea is that for the appearance of successful creatures, nature does not have to understand and analyze the situation at all, but you can act at random. It is enough to create a wide range of diverse individuals - and, ultimately, the fittest will survive.
1. First, an individual appears with new, completely random properties.
2. Then she is or is not able to leave offspring, depending on these properties.
3. Finally, if the outcome of the previous stage is positive, then she leaves offspring and her descendants inherit the newly acquired properties
At present, the partly naive views of Darwin himself have been partly reworked. So, Darwin imagined that changes should occur very smoothly, and the spectrum of variability is continuous. Today, however, the mechanisms of natural selection are explained with the help of genetics, which brings some originality to this picture. Mutations in the genes that operate in the first step of the process described above are essentially discrete. It is clear, however, that the basic essence of Darwin's idea has remained unchanged.
Forms of natural selection
driving selection- a form of natural selection, when environmental conditions contribute to a certain direction of change in any trait or group of traits. At the same time, other possibilities for changing the trait are subjected 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).
A modern case of motive selection is the "industrial melanism of English butterflies". "Industrial melanism" is a sharp increase in the proportion of melanistic (having a dark color) individuals in those butterfly populations 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 reached 95%, while for the first time a dark butterfly (Morfa carbonaria) was caught 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, when developing the soil as a habitat for various unrelated groups of animals, the limbs turned into burrowing ones.
Stabilizing selection- a form of natural selection, in which the action is directed against individuals with extreme deviations from the average norm, in favor of individuals with an average severity of the trait.
Many examples of the action of stabilizing selection in nature have been described. For example, at first glance it seems that individuals with maximum fecundity should make the greatest contribution to the gene pool of the next generation. However, observations of natural populations of birds and mammals show that this is not the case. The more chicks or cubs in the nest, the more difficult it is to feed them, the smaller and weaker each of them. As a result, individuals with average fecundity turn out to be the most adapted.
Selection in favor of averages has been found for a variety of traits. In mammals, very low and very high birth weight newborns are more likely to die at birth or in the first weeks of life than middle weight newborns. Accounting for the size of the wings of birds that died after the storm showed that most of them had too small or too large wings. And in this case, the average individuals turned out to be the most adapted.
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. 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 meadow rattle in hay meadows. Under normal conditions, the flowering and seed ripening periods of this plant cover the whole summer. But in hay meadows, seeds are produced mainly by those plants that have time to bloom and ripen either before the mowing period, or bloom at the end of summer, after mowing. As a result, two races of the rattle are formed - early and late flowering.
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.
Cut-off selection is a form of natural selection. Its action is opposite to positive selection. 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.
positive selection is a form of natural selection. Its action is the opposite of cutting 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. With the help of positive selection and cutting selection, a change in species is carried out (and not only through the destruction of unnecessary individuals, then any development should stop, but this does not happen). Examples of positive selection include: a stuffed Archeopteryx can be used as a glider, but a stuffed swallow or seagull cannot. But the first birds flew better than Archeopteryx.
Another example of positive selection is the emergence of predators that outperform many other warm-blooded creatures in their "mental abilities". Or the emergence of reptiles such as crocodiles, which have a four-chambered heart and are able to live both on land and in water.
Paleontologist Ivan Efremov argued that man was not only selected for the best adaptability to environmental conditions, but also "selected for sociality" - those communities survived, whose members supported each other better. This is another example of positive selection.
Private directions of natural selection
· Survival of the most adapted species and populations, for example, species with gills in the water, because fitness allows you to win the fight for survival.
Survival of physically healthy organisms.
· Survival of the physically strongest organisms, since the physical struggle for resources is an integral part of life. It is important in intraspecific struggle.
· Survival of the most sexually successful organisms, since sexual reproduction is the dominant mode of reproduction. This is where sexual selection comes into play.
However, all these cases are particular, and the main thing is the successful preservation in time. Therefore, sometimes these directions are violated in order to follow the main goal.
The role of natural selection in evolutionC. Darwin considered natural selection to be a fundamental factor in the evolution of living things (selectionism in biology). Accumulation in late XIX- at the beginning of the 20th century, information on genetics, in particular, the discovery of the discrete nature of the inheritance of phenotypic traits, prompted many researchers to revise Darwin's thesis: genotype mutations began to be considered as extremely important evolutionary factors (G. de Vries' mutationism, R. Goldschmitt's saltationism, etc. ). On the other hand, the discovery of known correlations among the characters of related species (the law of homologous series) by N. I. Vavilov led to the formulation of hypotheses about evolution based on regularities, and not random variability (L. S. Berg’s nomogenesis, E. D. Kop’s bathmogenesis and etc.). In the 1920s-1940s, interest in selectionist theories in evolutionary biology was revived due to the synthesis of classical genetics and the theory of natural selection.
The resulting synthetic theory of evolution (STE), often referred to as neo-Darwinism, is based on a quantitative analysis of the frequency of alleles in populations, changing under the influence of natural selection. However, the discoveries of recent decades in various fields of scientific knowledge - from molecular biology with its theory of neutral mutations by M. Kimura and paleontology with its theory of punctuated equilibrium by S. J. Gould and N. Eldridge (in which the species is understood as a relatively static phase of the evolutionary process) to mathematics with its theory of bifurcations and phase transitions - indicate the insufficiency of the classical STE for an adequate description of all aspects of biological evolution. Role discussion various factors in evolution continues today, and evolutionary biology has come to the need for its next, third synthesis.
The emergence of adaptations as a result of natural selection
Adaptations are the properties and characteristics of organisms that provide adaptation to the environment in which these organisms live. Adaptation is also called the process of adaptation. We have seen above how some adaptations arise as a result of natural selection. Populations of the birch moth have adapted to the changed external conditions due to the accumulation of dark color mutations. In human populations inhabiting malarial areas, adaptation has arisen due to the spread of the sickle cell mutation. In both cases, adaptation is achieved through the action of natural selection.
In this case, the hereditary variability accumulated in populations serves as the material for selection. Since different populations differ from each other in the set of accumulated mutations, they adapt differently to the same environmental factors. Thus, African populations have adapted to life in malarial areas due to the accumulation of mutations of sickle cell anemia Hb S, and in populations inhabiting Southeast Asia, resistance to malaria has formed on the basis of the accumulation of a number of other mutations, which in the homozygous state also cause blood diseases, and in the heterozygous, they provide protection against malaria.
These examples illustrate the role of natural selection in shaping adaptations. However, it must be clearly understood that these are special cases of relatively simple adaptations that arise due to the selective reproduction of carriers of single "beneficial" mutations. It is unlikely that most adaptations arose in this way.
Protective, warning and imitative coloring. Consider, for example, such widespread adaptations as patronizing, warning, and imitative coloration (mimicry). Protective coloring allows animals to become invisible, merging with the substrate. Some insects are strikingly similar to the leaves of the trees on which they live, others resemble dried twigs or thorns on tree trunks. These morphological adaptations are complemented by behavioral adaptations. Insects choose to hide exactly those places where they are less visible.
Inedible insects and poisonous animals - snakes and frogs - have a bright, warning color. A predator, once faced with such an animal, associates this type of coloration with danger for a long time. This is used by some non-poisonous animals. They acquire a striking resemblance to poisonous ones, and thereby reduce the danger from predators. Already imitates the color of the viper, the fly imitates the bee. This phenomenon is called mimicry.
How did all these amazing devices come about? It is unlikely that a single mutation could provide such a precise correspondence between an insect wing and a living leaf, between a fly and a bee. It's incredible that a single mutation would cause a patronizingly colored insect to hide on exactly the leaves it looks like. Obviously, such adaptations as protective and warning coloration and mimicry arose by the gradual selection of all those small deviations in body shape, in the distribution of certain pigments, in innate behavior that existed in the populations of the ancestors of these animals. One of the most important characteristics of natural selection is its cumulative nature, i.e., its ability to accumulate and intensify these deviations in a number of generations, composing changes in individual genes and the systems of organisms controlled by them.
The most interesting and difficult problem is the initial stages of the emergence of adaptations. It is clear what advantages the almost perfect resemblance of a praying mantis to a dry branch gives. But what advantages could his distant ancestor, who only remotely resembled a twig, have? Are predators so stupid that they can be fooled so easily? No, predators are by no means stupid, and natural selection from generation to generation "teaches" them to better and better recognize the tricks of their prey. Even the perfect resemblance of a modern praying mantis to a knot does not give him a 100% guarantee that not a single bird will ever notice him. However, its chances of eluding a predator are higher than those of an insect with a less perfect protective coloration. In the same way, his distant ancestor, who only slightly looks like a knot, had a slightly higher chance of life than his relative who did not look like a knot at all. Of course, the bird that sits next to him will easily notice him on a clear day. But if the day is foggy, if the bird does not sit nearby, but flies by and decides not to waste time on what may be a praying mantis, or may be a knot, then the minimal similarity saves the life of the bearer of this barely noticeable similarity. His descendants who inherit this minimal resemblance will be more numerous. Their share in the population will increase. This will make life difficult for the birds. Among them, those who will more accurately recognize camouflaged prey will become more successful. The same principle of the Red Queen, which we discussed in the paragraph on the struggle for existence, comes into play. In order to maintain the advantage in the struggle for life, achieved through minimal similarity, the prey species has to change.
Natural selection picks up all those minute changes that increase the similarity in color and shape with the substrate, the similarity between the edible species and the inedible species that it imitates. It should be borne in mind that different types of predators use different methods of finding prey. Some pay attention to shape, others to color, some have color vision, others do not. So natural selection automatically enhances, as far as possible, the similarity between imitator and model, and leads to those amazing adaptations that we see in nature.
The emergence of complex adaptations
Many adaptations come across as elaborate and purposefully planned devices. How could such a complex structure as the human eye have arisen by natural selection of randomly occurring mutations?
Scientists suggest that the evolution of the eye began with small groups of light-sensitive cells on the surface of the body of our very distant ancestors, who lived about 550 million years ago. The ability to distinguish between light and dark was certainly useful for them, increasing their chances of life compared to their completely blind relatives. An accidental curvature of the "visual" surface improved vision, this made it possible to determine the direction to the light source. An eyecup appeared. Newly emerging mutations could lead to narrowing and widening of the optic cup opening. The narrowing gradually improved vision - the light began to pass through a narrow aperture. As you can see, each step increased the fitness of those individuals that changed in the “right” direction. Light-sensitive cells formed the retina. Over time, a lens has formed in the front of the eyeball, which acts as a lens. It appeared, apparently, as a transparent two-layer structure filled with liquid.
Scientists have tried to simulate this process on a computer. They showed that an eye like the compound clam eye could have evolved from a layer of photosensitive cells with relatively mild selection in just 364,000 generations. In other words, animals that change generations every year could form a fully developed and optically perfect eye in less than half a million years. This is a very short period for evolution, given that average age species in molluscs is several million years old.
All the supposed stages in the evolution of the human eye can be found among living animals. The evolution of the eye has followed different paths in different types of animals. Through natural selection, many different forms of the eye have independently evolved, and the human eye is only one of them, and not the most perfect.
If you carefully consider the design of the human eye and other vertebrates, you can find a number of strange inconsistencies. When light enters the human eye, it passes through the lens and onto the light-sensitive cells in the retina. Light has to travel through a dense network of capillaries and neurons to reach the photoreceptor layer. Surprisingly, but the nerve endings approach the photosensitive cells not from behind, but from the front! Moreover, the nerve endings are collected in the optic nerve, which extends from the center of the retina, and thus creates a blind spot. To compensate for the shadowing of photoreceptors by neurons and capillaries and get rid of the blind spot, our eye is constantly moving, sending a series of different projections of the same image to the brain. Our brain performs complex operations, adding these images, subtracting the shadows, and calculating the real picture. All these difficulties could be avoided if the nerve endings approached the neurons not from the front, but from behind, as, for example, in an octopus.
The very imperfection of the vertebrate eye sheds light on the mechanisms of evolution by natural selection. We have already said more than once that selection always operates “here and now”. He sorts different variants already existing structures, choosing and putting together the best of them: the best of the "here and now", regardless of what these structures may become in the distant future. Therefore, the key to explaining both the perfections and imperfections of modern structures should be sought in the past. Scientists believe that all modern vertebrates are descended from animals like the lancelet. In the lancelet, light-sensitive neurons are located at the anterior end of the neural tube. In front of them are nerve and pigment cells that cover the photoreceptors from light entering from the front. The lancelet receives light signals coming from the sides of its transparent body. It can be assumed that the common ancestor of the vertebrate eye was arranged in a similar way. Then this flat structure began to transform into an eye cup. The anterior part of the neural tube protruded inward, and the neurons that were in front of the receptor cells appeared on top of them. The development of the eye in the embryos of modern vertebrates in a certain sense reproduces the sequence of events that took place in the distant past.
Evolution does not create new constructions "from scratch", it changes (often unrecognizably changes) old constructions, so that each stage of these changes is adaptive. Any change should increase the fitness of its carriers, or at least not reduce it. This feature of evolution leads to the steady improvement of various structures. It is also the cause of the imperfection of many adaptations, strange inconsistencies in the structure of living organisms.
It should be remembered, however, that all adaptations, no matter how perfect they may be, are relative. It is clear that the development of the ability to fly is not very well combined with the ability to run fast. Therefore, the birds that have the best ability to fly are poor runners. On the contrary, ostriches, which are not able to fly, run very well. Adaptation to certain conditions can be useless or even harmful when new conditions appear. However, living conditions change constantly and sometimes very dramatically. In these cases, previously accumulated adaptations can hinder the formation of new ones, which can lead to the extinction of large groups of organisms, as happened more than 60-70 million years ago with the once very numerous and diverse dinosaurs.
