One of the main mechanisms of evolution, along with mutations, migration processes and gene transformations, is natural selection. Types of natural selection involve changes in the genotype that increase the organism's chances of survival and procreation. Evolution is often seen as a consequence of this process, which may arise from differences in species survival, fertility, development rates, mating success, or any other aspect of life.
Natural balance
Gene frequencies remain constant from generation to generation, provided that there are no disturbing factors that upset the natural equilibrium. These include mutations, migration (or gene flow), random genetic drift, and natural selection. A mutation is a spontaneous change in the frequency of genes in a population characterized by a low rate of development. In this case, an individual moves from one population to another and then changes. Random is a change that is transmitted from one generation to the next in a completely random manner.
All these factors change gene frequencies without taking into account the increase or decrease in the probability of survival of the organism and reproduction in its own way. natural environment. All of them are random processes. And natural selection, types of natural selection, are moderate disorganizing consequences of these processes, since they multiply the frequency beneficial mutations for many generations and eliminate harmful components.
What is natural selection?
Natural selection promotes the conservation of those groups of organisms that are better adapted to the physical and biological conditions of their habitat. He
can act on any heritable phenotypic trait and, through selective pressure, can influence any aspect of the environment, including sexual selection and competition with members of the same or other species.
However, this does not mean that this process is always directed and effective in adaptive evolution. Natural selection, types of natural selection in general, often results in the elimination of less fit options.
Variation exists within an entire population of organisms. This occurs in part because random mutations occur in the genome of one organism, and its offspring can inherit such mutations. Throughout life, genomes interact with the environment. Consequently, the population evolves.
The concept of natural selection
Natural selection is one of the cornerstones of modern biology. It acts on a phenotype whose genetic basis provides a reproductive advantage for greater prevalence in the population. Over time, this process can lead to the emergence of new species. In other words, this is an important (though not the only) evolutionary process within a population.
The concept itself was formulated and published in 1858 by Charles Darwin and Alfredo Russell Wallace in a joint presentation of papers regarding
The term has been described as analogous i.e. it is the process by which animals and plants with certain traits are considered desirable for breeding and reproduction. The concept of "natural selection" was originally developed in the absence of a theory of heredity. At the time Darwin wrote his works, science had yet to develop the combination of traditional Darwinian evolution with subsequent discoveries in the field of classical and molecular genetics is called the modern evolutionary synthesis. 3 types of natural selection remain the main explanation for adaptive evolution.
How does natural selection work?
Natural selection is the mechanism by which an animal organism adapts and evolves. At its core, individual organisms, which turn out to be best adapted to the environment, survive and reproduce most successfully, producing fertile offspring. After numerous breeding cycles, such species are dominant. Thus, nature filters out poorly adapted individuals for the benefit of the entire population.
It is a relatively simple mechanism that causes members of a given population to change over time. In fact, it can be broken down into five main stages: variation, inheritance, selection, time and adaptation.
Darwin on natural selection
According to Darwin's teachings, natural selection consists of four components:
- Variations. Organisms within a population exhibit individual differences in appearance and behavior. These changes may include body size, hair color, facial markings, vocal characteristics, or the number of offspring produced. On the other hand, some personality traits are not associated with differences between individuals, such as the number of eyes in vertebrates.
- Inheritance. Some traits are passed on sequentially from parents to offspring. Such traits are heritable, while others are strongly influenced by environmental conditions and are weakly inherited.
- High populations. The majority of animals annually produce offspring in much greater quantities than is necessary for an equal distribution of resources between them. This leads to interspecific competition and premature mortality.
- Differential survival and reproduction. All types of natural selection in populations leave behind those animals that know how to fight for local resources.
Natural selection: types of natural selection
Darwin's theory of evolution radically changed the direction of future scientific thought. At its center is natural selection, a process that occurs over successive generations and is defined as the differential reproduction of genotypes. Any change in the environment (for example, a change in the color of a tree trunk) can lead to adaptation at the local level. Exist the following types natural selection (table No. 1):
Stabilizing selection
Often, the frequency of DNA mutations is statistically higher in some species than in others. This type of natural selection tends to eliminate any extremes in the phenotypes of the most environmentally fit individuals in a population. Due to this, diversity within one species decreases. However, this does not mean that all individuals are exactly the same.
Stabilizing natural selection and its types can be briefly described as averaging or stabilization, in which the population becomes more homogeneous. Polygenic traits are primarily affected. This means that the phenotype is controlled by several genes and there is wide range possible outcomes. Over time, some of the genes are turned off or masked by others, depending on favorable adaptation.
Many human characteristics are the result of such selection. A person's birth weight is not only a polygenic trait, it is also controlled by environmental factors. Newborns with an average birth weight are more likely to survive than those who are too small or too large.
Directed natural selection
This phenomenon is usually observed in conditions that have changed over time, for example weather, climate or food supply can lead to directional selection. Human participation can also speed up this process. Hunters most often kill large specimens for meat or other large ornamental or useful parts. Consequently, the population will tend to skew towards smaller individuals.
The more predators kill and eat slow individuals in the population, the more there will be a bias towards luckier and faster members of the population. Types of natural selection (table with examples No. 1) can be more clearly demonstrated using examples from living nature.
Charles Darwin studied directional selection while he was in the Galapagos Islands. The beak length of native finches has varied over time due to available food sources. In the absence of insects, finches survived with large and long beaks, which helped them eat seeds. Over time, insects became more numerous, and with the help of directed selection, bird beaks gradually acquired smaller sizes.
Features of diversification (disruptive) selection
Disruptive selection is a type of natural selection that opposes the averaging of species characteristics within a population. This process is the rarest, if we describe the types of natural selection briefly. Diversification selection can lead to the speciation of two or more various forms in places sudden changes environment. Like directed selection, this process can also be slowed down by destructive influences human factor and environmental pollution.
One of the best-studied examples of disruptive selection is the case of butterflies in London. In rural areas, almost all individuals were light in color. However, these same butterflies were very dark in color in industrial areas. There were also specimens with medium intensity colors. This is due to the fact that dark butterflies have learned to survive and escape predators in industrial areas in urban environments. Light-colored moths in industrial areas were easily detected and eaten by predators. The opposite picture was observed in rural areas. Butterflies of medium color intensity were easily visible in both locations and therefore very few remained.
Thus, the meaning of disruptive selection is to move the phenotype towards an extreme that is necessary for the survival of the species.
Natural selection and evolution
The main idea of the theory of evolution is that all species diversity gradually developed from simple shapes life that appeared more than three billion years ago (for comparison, the age of the Earth is approximately 4.5 billion years). Types of natural selection with examples from the first bacteria to the first modern people played a significant role in this evolutionary development.
Organisms that have been poorly adapted to their environment are less likely to survive and produce offspring. This means their genes are less likely to be passed on to the next generation. The path to genetic diversity must not be lost, nor must the ability at the cellular level to respond to changing environmental conditions.
Evolution is a story of winners, and natural selection is an impartial judge, deciding who lives and who dies. Examples of natural selection are everywhere: all the diversity of living beings on our planet is a product of this process, and humans are no exception. However, one can argue about man, because he has long been accustomed to interfering in a businesslike manner in those areas that were previously the sacred secrets of nature
How does natural selection work?
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 living conditions 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 only difference between these two species is who acts as a judge when choosing certain properties of organisms - a person or the environment. 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 scientific world there is no 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 are capable of surviving and reproducing 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. In nature, the evolutionary process, as a rule, proceeds slowly, and the following stages can be distinguished:
Principles of classification of natural selection
Based on the direction of action, positive and negative (cutting) types of natural selection are distinguished.
Positive
Its action is aimed at consolidating and developing useful traits and helps to increase the number of individuals possessing these traits in the population. Thus, within specific species, positive selection works to increase their viability, and on the scale of the entire biosphere - to gradually increase the complexity of the structure of living organisms, which is well illustrated by the entire history of the evolutionary process. For example, transformation of gills that took millions of years in some species of ancient fish, the middle ear of amphibians accompanied the process of “coming to land” of living organisms under conditions of strong ebb and flow.
Negative
In contrast to positive selection, cutting selection forces those individuals that carry harmful traits that can significantly reduce the viability of the species under existing environmental conditions to drop out of the population. This mechanism acts like a filter that does not allow the most harmful alleles to pass through and prevents their further development.
For example, when with the development thumb On the hand, the ancestors of Homo sapiens learned to form 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), giving way to living space to individuals with stronger skulls.
A very common classification is, 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 eliminates mutations with one average value of a trait, replacing them with mutations with a different average value of the same trait. As a result, for example, it is possible to 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 the ancestors of humans. Other life forms, on the contrary, have significantly decreased in size. Thus, ancient dragonflies in conditions high content oxygen in the atmosphere was gigantic compared to modern sizes. The same goes for other insects..
Stabilizing
In contrast to the driving force, it strives to preserve existing characteristics and manifests itself in cases of long-term preservation of environmental conditions. Examples include species that have come down to us from ancient times almost unchanged: crocodiles, many types of jellyfish, giant sequoias. There are also species that exist, practically unchanged, for millions of years: this is the ancient ginkgo plant, a direct descendant of the first lizards of the hatteria, coelacanth (a lobe-finned fish, which many scientists consider an “intermediate link” between fish and amphibians).
Stabilizing and driving selections act in conjunction and are two sides of the same process. The driver strives to preserve mutations that are most advantageous in changing environmental conditions, and when these conditions stabilize, the process will end with the creation the best way adapted form. Here comes the turn of stabilizing selection– it preserves these time-tested genotypes and does not allow those deviating from them to reproduce general norm 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 the species. Under conditions of environmental heterogeneity, a process occurs that is opposite to stabilizing selection: traits with a wider reaction rate receive 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 the 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 characterized by polymorphism -
coexistence within one species of two or even several forms based on some characteristic. This phenomenon may be caused for various reasons, both natural and anthropogenic origin. For example, droughts unfavorable for fungi, falling in mid-summer, determined the development of their spring and autumn species, and haymaking, which also occurred at this time in other areas, led to the fact that within some types of grass, the seeds of some individuals ripen early, while others - late, that is before and after haymaking.
Sexual
Sexual selection stands apart in this series of logically based processes. 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. . At the same time, they often develop those signs, which negatively affect their viability. A classic example is the peacock with its luxurious tail, which has no practical use; moreover, it makes it visible to predators and can interfere with movement. Its only function is to attract a female, and it successfully fulfills this function. There are two hypotheses explaining the mechanism of female choice:
- The “good genes” hypothesis - a female chooses a father for future offspring based on his ability to survive even with such secondary sexual characteristics that make existence difficult;
- Attractive Sons Hypothesis - The female strives to produce successful male offspring that retain the father's genes.
Sexual selection has great value for evolution, after all the main objective for individuals of any species - not to survive, but to leave offspring. Many species of insects or fish die immediately as soon as they complete this mission - without this there would be no life on the planet.
The considered instrument of evolution can be characterized as an endless process of movement towards an unattainable ideal, because the environment is almost always one step or two ahead of its inhabitants: what was achieved yesterday is changing today in order to become obsolete tomorrow.
Natural selection is the main, leading, guiding factor of evolution, which underlies the theory of Charles Darwin. All other factors of evolution are random; only natural selection has a direction (towards the adaptation of organisms to environmental conditions).
Definition: selective survival and reproduction of the fittest organisms.
Creative role: By selecting useful traits, natural selection creates new ones.
Efficiency: The more different mutations there are in a population (the higher the heterozygosity of the population), the greater the efficiency of natural selection, the faster evolution proceeds.
Shapes:
- Stabilizing - acts under constant conditions, selects average manifestations of the trait, preserves the characteristics of the species (coelacanth fish)
- Driving - acts in changing conditions, selects extreme manifestations of a trait (deviations), leads to changes in traits (birch moth)
- Sexual - competition for a sexual partner.
- Tearing - selects two extreme forms.
Consequences of natural selection:
- Evolution (change, complication of organisms)
- Emergence of new species (increase in the number [diversity] of species)
- Adaptation of organisms to environmental conditions. All fitness is relative, i.e. adapts the body to only one specific condition.
Choose the one that suits you best correct option. The basis of natural selection is
1) mutation process
2) speciation
3) biological progress
4) relative fitness
Answer
Choose one, the most correct option. What are the consequences of stabilizing selection?
1) preservation of old species
2) change in reaction norm
3) the emergence of new species
4) preservation of individuals with altered characteristics
Answer
Choose one, the most correct option. In the process of evolution, a creative role plays
1) natural selection
2) artificial selection
3) modification variability
4) mutational variability
Answer
Choose three options. What features characterize driving selection?
1) operates under relatively constant living conditions
2) eliminates individuals with an average trait value
3) promotes the reproduction of individuals with an altered genotype
4) preserves individuals with deviations from the average values of the trait
5) preserves individuals with an established norm of reaction of the trait
6) promotes the appearance of mutations in the population
Answer
Select three traits that characterize the driving form of natural selection
1) ensures the emergence of a new species
2) manifests itself in changing environmental conditions
3) the adaptability of individuals to the original environment improves
4) individuals with deviations from the norm are discarded
5) the number of individuals with the average value of the trait increases
6) individuals with new characteristics are preserved
Answer
Choose one, the most correct option. The starting material for natural selection is
1) struggle for existence
2) mutational variability
3) change in the habitat of organisms
4) adaptability of organisms to their environment
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 survival conditions
4) adaptability of populations to their environment
Answer
Choose three options. The stabilizing form of natural selection manifests itself in
1) constant environmental conditions
2) change average norm reactions
3) preservation of adapted individuals in their original habitat
4) culling of individuals with deviations from the norm
5) preservation of individuals with mutations
6) preservation of individuals with new phenotypes
Answer
Choose one, the most correct option. The efficiency of natural selection decreases when
1) the occurrence of recessive mutations
2) an increase in homozygous individuals in the population
3) change in the reaction norm of the trait
4) increasing the number of species in the ecosystem
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. Pests become resistant to pesticides over time as a result of
1) high fertility
2) modification variability
3) preservation of mutations by natural selection
4) artificial selection
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 true? A) The emergence of resistance to pesticides in insect pests of agricultural plants is an example of a stabilizing form of natural selection. B) Driving selection contributes to an increase in the number of individuals of a species with an average value of the trait
1) only A is correct
2) only B is correct
3) both judgments are correct
4) both judgments are wrong
Answer
Establish a correspondence between the results of the action of natural selection and its forms: 1) stabilizing, 2) driving, 3) disruptive (tearing). Write the numbers 1, 2 and 3 in the correct order.
A) Development of antibiotic resistance in bacteria
B) The existence of fast and slow growing predatory fish in the same lake
C) Similar structure of the visual organs in chordates
D) The appearance of flippers in waterfowl mammals
E) Selection of newborn mammals with average weight
E) Preservation of phenotypes with extreme deviations within one population
Answer
1. Establish a correspondence between the characteristics of natural selection and its form: 1) driving, 2) stabilizing. Write numbers 1 and 2 in the correct order.
A) preserves the average value of the characteristic
B) promotes adaptation to changed environmental conditions
C) retains individuals with a trait that deviates from its average value
D) helps to increase the diversity of organisms
D) contributes to the preservation of species characteristics
Answer
2. Compare the characteristics and forms of natural selection: 1) Driving, 2) Stabilizing. Write numbers 1 and 2 in the correct order.
A) acts against individuals with extreme values of traits
B) leads to a narrowing of the reaction norm
B) usually operates under constant conditions
D) occurs during the development of new habitats
D) changes the average values of a trait in the population
E) can lead to the emergence of new species
Answer
3. Establish a correspondence between the forms of natural selection and their characteristics: 1) driving, 2) stabilizing. Write numbers 1 and 2 in the order corresponding to the letters.
A) acts in changing environmental conditions
B) operates under constant environmental conditions
C) aimed at preserving the previously established average value of the characteristic
D) leads to a shift in the average value of a trait in the population
D) under its influence, both strengthening and weakening of the characteristic can occur
Answer
4. Establish a correspondence between the characteristics and forms of natural selection: 1) stabilizing, 2) driving. Write numbers 1 and 2 in the order corresponding to the letters.
A) forms adaptations to new environmental conditions
B) leads to the formation of new species
C) maintains the average norm of the trait
D) rejects individuals with deviations from the average norm of characteristics
D) increases the heterozygosity of the population
Answer
Establish a correspondence between the examples and the forms of natural selection that these examples illustrate: 1) driving, 2) stabilizing. Write numbers 1 and 2 in the order corresponding to the letters.
A) an increase in the number of dark butterflies in industrial areas compared to light ones
B) the emergence of resistance to pesticides in insect pests
C) the preservation to this day of the reptile tuateria, which lives in New Zealand
D) a decrease in the size of the cephalothorax in crabs living in muddy water
E) in mammals, the mortality rate of newborns with an average birth weight is lower than with very low or very high birth weights
E) the death of winged ancestors and the preservation of insects with reduced wings on islands with strong winds
Answer
Establish a correspondence between the forms of struggle for existence and examples illustrating them: 1) intraspecific, 2) interspecific. Write numbers 1 and 2 in the order corresponding to the letters.
A) fish eat plankton
B) seagulls kill chicks when there are a large number of them
B) mating of wood grouse
D) big-nosed monkeys try to outshout each other, inflating their huge noses
D) the chaga mushroom settles on a birch tree
E) the main prey of the marten is squirrel
Answer
Analyze the table “Forms of Natural Selection.” For each letter, select the corresponding concept, characteristic and example from the list provided.
1) sexual
2) driving
3) group
4) preservation of organisms with two extreme deviations from the average value of the trait
5) the emergence of a new feature
6) formation of bacterial resistance to antibiotics
7) preservation of a relict species of the plant Ginkgo biloba 8) increase in the number of heterozygous organisms
Answer
© D.V. Pozdnyakov, 2009-2019
NATURAL SELECTION, the process of selective survival and differential reproduction of organisms, the main driving factor in their evolution. Ideas about the existence of natural selection have been expressed since the beginning of the 19th century by various English naturalists (including A. Wallace). But only Charles Darwin (1842, 1859) assessed it as the main factor in evolution. According to Darwin, natural selection is the result of the struggle for existence; even minor heritable differences between individuals of the same species can provide advantages in this struggle, which is due to the tendency of organisms to high intensity of reproduction (in geometric progression) and the impossibility of preserving all offspring due to limited natural resources. The death of the overwhelming number of individuals in each generation inevitably leads to natural selection - “survival of the fittest” to given conditions. As a result of the accumulation of beneficial changes over many generations, new adaptations are formed and, ultimately, new species arise. Darwin based his discussions about the action of natural selection primarily on generalizing the experience of domestication of animals and plants by analogy with artificial selection, emphasizing, however, that, unlike human selection, natural selection is determined by the interaction of organisms with environmental conditions and does not have a specific goal.
Systematic research into natural selection, expansion and improvement of methods for studying it began at the end of the 19th century. The use of biometric methods made it possible to establish statistically significant differences between surviving and dead organisms when environmental conditions changed. Thanks to the developments of R. Fisher, J. Haldane, S. Wright and S. S. Chetverikov, who carried out the synthesis of classical Darwinism and genetics, it became possible to begin the experimental study of the genetic foundations of natural selection. The examined natural populations turned out to be literally saturated with mutations, many of which became useful when conditions of existence changed or when combined with other mutations. It was found that the mutation process and free crossing (panmixia) provide genetic heterogeneity of populations and the uniqueness of individuals with different chances of survival; this determines the high intensity and efficiency of natural selection. In addition, it became obvious that natural selection deals not with individual traits, but with entire organisms, and that the genetic essence of natural selection lies in the non-random (differentiated) preservation of certain genotypes in a population, selectively transmitted to subsequent generations. Natural selection is probabilistic in nature, acts on the basis of the mutation process and the existing gene pool, affects the frequency of distribution of genes and their combinations, and helps to reduce negative action mutations and the formation of defense mechanisms against their harmful effects, thereby determining the pace and direction of evolution. Under the control of natural selection are not only various traits, but also the factors of evolution themselves, for example, the intensity and nature of mutability, the apparatus of heredity (hence the concept of “evolution of evolution”). In the absence of natural selection, a decrease or loss of fitness of organisms occurs due to the accumulation of undesirable mutations, which manifests itself in an increase in genetic load, including in modern human populations.
There are more than 30 forms of natural selection; none of them exists in a pure form, but rather characterizes the tendency of selection in a specific ecological situation. Thus, driving selection contributes to the preservation of a certain deviation from the previous norm and leads to the development of new adaptations through a directed restructuring of the entire gene pool of populations, as well as the genotypes and phenotypes of individuals. It can lead to the dominance of one (or several) pre-existing forms over others. A classic example of its action was the predominance in industrial areas of dark-colored forms of the birch moth butterfly, invisible to birds on tree trunks contaminated with soot (until the mid-19th century, only a light form was found, imitating lichen spots on light birch trunks). The rapid adaptation to poisons of various species of insects and rodents and the emergence of resistance of microorganisms to antibiotics indicate that the pressure of driving selection in natural populations is sufficient to ensure a rapid adaptive response to sudden changes in the environment. As a rule, selection for one trait entails a whole series of transformations. For example, long-term selection for the protein or oil content in corn grains is accompanied by changes in the shape of the grains, the size of the cobs, their location above the soil level, etc.
The result of driving selection in the phylogeny of large taxa is orthoselection, an example of which is the directed evolution of the limb of the horse’s ancestors established by V. O. Kovalevsky (from five-toed to one-toed), which lasted for millions of years and ensured an increase in the speed and economy of running.
Disruptive, or disruptive, selection favors the preservation of extreme deviations and leads to an increase in polymorphism. It manifests itself in cases where none of the intraspecific forms with different genotypes receives an absolute advantage in the struggle for existence due to the diversity of conditions simultaneously occurring in the same territory; in this case, individuals with average or intermediate character traits are eliminated first of all. At the beginning of the 20th century, the Russian botanist N.V. Tsinger showed that the large rattle (Alectoroleophus major), which blooms and bears fruit in unmown meadows throughout the summer, forms two races in mowed meadows: the early spring race, which manages to bear seeds before mowing begins, and late autumn - low plants that are not damaged when mowing, and then quickly bloom and have time to produce seeds before the onset of frost. Another example of polymorphism is the difference in the color of shells in the land snail (Capacea nemoralis), which is food for birds: in dense beech forests, where litter of red-brown litter remains throughout the year, individuals with brown and pink colors are common; in meadows with yellow litter, yellow-colored snails predominate. In mixed deciduous forests, where the nature of the background changes with the onset of a new season, snails with brown and pink colors dominate in early spring, and yellow ones in summer. Darwin's finches (Geospizinae) on the Galapagos Islands ( classic example adaptive radiation) is the end result of long-term disruptive selection, which led to the formation of dozens of closely related species.
If these forms of natural selection lead to changes in both the phenotypic and genetic structure of populations, then stabilizing selection, first described by I. I. Shmalgausen (1938), preserves the average value of traits (norm) in the population and does not allow the genomes of individuals that deviate most from the population to pass into the next generation. this norm. It is aimed at maintaining and increasing stability in a population of an average, previously established phenotype. It is known, for example, that during snow storms Birds that, in many respects (length of wing, beak, body weight, etc.) approach the average norm, survive, and individuals that deviate from this norm die. The size and shape of flowers in plants pollinated by insects are more stable than in plants pollinated by the wind, which is due to the conjugate evolution of plants and their pollinators, the “culling” of forms that deviate from the norm (for example, a bumblebee cannot penetrate a too narrow corolla of a flower, and the butterfly's proboscis does not touch the stamens that are too short in plants with a long corolla). Thanks to stabilizing selection, with an externally unchanged phenotype, significant genetic changes can occur, ensuring the independence of the development of adaptations from fluctuating environmental conditions. One of the results of the action of stabilizing selection can be considered the “biochemical universality” of life on Earth.
Destabilizing selection (the name was proposed by D.K. Belyaev, 1970) leads to a sharp disruption of ontogenesis regulatory systems, the opening of the mobilization reserve and an increase in phenotypic variability with intensive selection in any particular direction. For example, selection to reduce the aggressiveness of predatory animals in captivity through the restructuring of the neurohumoral system leads to destabilization of the reproduction cycle, shifts in the timing of molting, changes in the position of the tail, ears, coloring, etc.
Genes have been discovered that can be lethal or reduce the viability of organisms in homozygous state, and in heterozygous, on the contrary, increase environmental plasticity and other indicators. In this case, we can talk about the so-called balanced selection, which ensures the maintenance of genetic diversity with a certain ratio of allele frequencies. An example of its action is the increase in resistance in patients with sickle cell anemia (heterozygous for the hemoglobin S gene) to infection with various strains of malarial plasmodium (see Hemoglobins).
An important step in overcoming the desire to explain all the characteristics of organisms by the action of natural selection was the concept of neutral evolution, according to which some of the changes are at the level of proteins and nucleic acids occurs by fixing adaptively neutral or almost neutral mutations. It is possible to select species that appear in peripheral populations “suddenly” from a geochronological point of view. Even earlier, it was proven that catastrophic selection, in which a small number of individuals and even a single organism survive during periods of sudden environmental changes, can become the basis for the formation of a new species due to chromosomal rearrangement and a change in the ecological niche. Thus, the formation of the xerophytic, endemic species Clarkia lingulata in the Sierra Nevada Mountains in California is explained by severe drought, which caused massive plant death, which became catastrophic in peripheral populations.
Natural selection that affects the secondary sexual characteristics of individuals is called sexual (for example, the bright nuptial coloration of males in many species of fish and birds, inviting calls, specific odors, highly developed tools for tournament combat in mammals). These traits are useful because they increase the possibility of their carriers participating in the reproduction of offspring. In sexual selection greatest activity exhibited by males, which is beneficial for the species as a whole, because females remain safer during the breeding season.
There is also group selection, which promotes the preservation of traits useful to a family, flock, or colony. Its special case in colonial insects is the selection of relatives, in which sterile castes (workers, soldiers, etc.) ensure (often at the cost of their own lives) the survival of fertile individuals (queens) and larvae and thereby the preservation of the entire colony. The altruistic behavior of parents, pretending to be wounded in order to lure the predator away from their children, threatens the death of the imitator, but in general increases the chances of survival of his offspring.
Although ideas about the leading role of natural selection in evolution have been confirmed in many experiments, they are still subject to criticism based on the idea that organisms cannot be formed as a result of a random combination of mutations. This ignores the fact that each act of natural selection is carried out on the basis of the previous results of its own action, which, in turn, predetermine the forms, intensity and directions of natural selection, and therefore the paths and patterns of evolution.
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The idea of comparing artificial and natural selection is that in nature the selection of the most “successful”, “best” organisms also occurs, but in the role of an “evaluator” of the usefulness of properties in in this case It is not the person who acts, but the environment. In addition, the material for both natural and artificial selection is small hereditary changes that accumulate from generation to generation.
Mechanism of natural selection
In the process of natural selection, mutations are fixed that increase the adaptability of organisms to their environment. Natural selection is often called a "self-evident" mechanism because it follows from such simple facts as:
- Organisms produce more offspring than can survive;
- There is heritable variation in the population of these organisms;
- Organisms with different genetic traits have different survival rates and ability to reproduce.
The central concept of the concept of natural selection is the fitness of organisms. Fitness is defined as the ability of an organism to survive and reproduce in its existing environment. This determines the size of his genetic contribution to the next generation. However, the main thing in determining fitness is not the total number of descendants, but the number of descendants with a given genotype (relative fitness). For example, if the offspring of a successful and rapidly reproducing organism are weak and do not reproduce well, then the genetic contribution and therefore the fitness of that organism will be low.
Natural selection for traits that can vary over some range of values (such as the size of an organism) can be divided into three types:
- Directional selection- changes in the average value of a trait over time, for example an increase in body size;
- Disruptive selection- selection for extreme values of a trait and against average values, for example, large and small body sizes;
- Stabilizing selection- selection against extreme values of a trait, which leads to a decrease in the variance of the trait.
A special case of natural selection is sexual selection, the substrate of which is any trait that increases the success of mating by increasing the attractiveness of the individual to potential partners. Traits that have evolved through sexual selection are especially noticeable in the males of some animal species. Characteristics such as large horns, 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 bright colors. pronounced signs.
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, for example, group selection, can lead to cooperation (see Evolution#Cooperation).
Forms of natural selection
There are different classifications of selection forms. A classification based on the nature of the influence of forms of selection on the variability of a trait in a population is widely used.
Driving selection
Driving selection- a form of natural selection that operates when directed changing conditions external environment. Described by Darwin and Wallace. In this case, individuals with traits that deviate in a certain direction from the average value receive advantages. In this case, other variations of the trait (its deviations in the opposite direction from the average value) are subject to negative selection. As a result, in a population from generation to generation there is a shift in the average value of the trait in a certain direction. In this case, the pressure of driving selection must correspond to the adaptive capabilities of the population and the rate of mutational changes (otherwise, environmental pressure can lead to extinction).
An example of the action of driving selection is “industrial melanism” in insects. “Industrial melanism” is a sharp increase in the proportion of melanistic (dark-colored) individuals in those populations of insects (for example, butterflies) that live in industrial areas. Due to industrial impact, the tree trunks darkened significantly, and light-colored lichens also died, which is why light-colored butterflies became better visible to birds, and dark-colored ones became less visible. In the 20th century, in a number of areas, the proportion of dark-colored butterflies in some well-studied moth populations in England reached 95%, while for the first time a dark-colored butterfly ( morpha carbonaria) was captured in 1848.
Driving selection occurs when the environment changes or adapts to new conditions when the range expands. It preserves hereditary changes in a certain direction, moving the reaction rate accordingly. For example, during the development of soil as a habitat, various unrelated groups of animals developed limbs that turned into burrowing limbs.
Stabilizing selection
Stabilizing selection- a form of natural selection in which its action is directed against individuals with extreme deviations from the average norm, in favor of individuals with an average expression of the trait. The concept of stabilizing selection was introduced into science and analyzed by I. I. Shmalgauzen.
Many examples of the action of stabilizing selection in nature have been described. For example, at first glance it seems that the greatest contribution to the gene pool of the next generation should be made by individuals with maximum fertility. However, observations of natural populations of birds and mammals show that this is not the case. The more chicks or cubs in the nest, the more difficult it is to feed them, the smaller and weaker each of them is. As a result, individuals with average fertility are the most fit.
Selection toward the mean has been found for a variety of traits. In mammals, very low-weight and very high-weight newborns are more likely to die at birth or in the first weeks of life than average-weight newborns. Taking into account the size of the wings of sparrows that died after a storm in the 50s near Leningrad showed that most of them had wings that were too small or too large. And in this case, the average individuals turned out to be the most adapted.
Disruptive selection
Disruptive selection- a form of natural selection in which conditions favor two or more extreme variants (directions) of variability, but do not favor the intermediate, average state of a trait. As a result, several new forms may appear from one original one. Darwin described the action of disruptive selection, believing that it underlies divergence, although he could not provide evidence for its existence in nature. Disruptive selection contributes to the emergence and maintenance of population polymorphism, and in some cases can cause speciation.
One of the possible situations in nature in which disruptive selection comes into play is when a polymorphic population occupies a heterogeneous habitat. Wherein different shapes adapt to various ecological niches or subniches.
An example of disruptive selection is the formation of two races in the greater rattle in hay meadows. IN normal conditions The flowering and seed ripening periods of this plant cover the entire summer. But in hay meadows, seeds are produced mainly by those plants that manage to bloom and ripen either before the mowing period, or bloom at the end of summer, after mowing. As a result, two races of rattle are formed - early and late flowering.
Disruptive selection was carried out artificially in experiments with Drosophila. The selection was carried out according to the number of bristles; only individuals with a small and large number of bristles were retained. As a result, from about the 30th generation, the two lines diverged very much, despite the fact that the flies continued to interbreed with each other, exchanging genes. In a number of other experiments (with plants), intensive crossing prevented the effective action of disruptive selection.
Sexual selection
Sexual selection- This is natural selection for reproductive success. The survival of organisms is an important, but not the only component of natural selection. To others an important component is attractive to individuals of the opposite sex. Darwin called this phenomenon sexual selection. “This form of selection is determined not by the struggle for existence in the relations of organic beings among themselves or with external conditions, but by the competition between individuals of one sex, usually males, for the possession of individuals of the other sex.” Traits that reduce the viability of their hosts can emerge and spread if the advantages they provide for reproductive success are significantly greater than their disadvantages for survival.
Two hypotheses about the mechanisms of sexual selection are common.
- According to the “good genes” hypothesis, the female “reasons” as follows: “If a given male, despite his bright plumage and long tail, managed not to die in the clutches of a predator and survive to sexual maturity, then he has good genes that allowed him to do this . Therefore, he should be chosen as the father of his children: he will pass on his good genes to them.” By choosing colorful males, females are choosing good genes for their offspring.
- According to the “attractive sons” hypothesis, the logic of female choice is somewhat different. If brightly colored males, for whatever reason, are attractive to females, it is worth choosing a brightly colored father for his future sons, because his sons will inherit the brightly colored genes and will be attractive to females in the next generation. Thus, a positive feedback occurs, which leads to the fact that from generation to generation the brightness of the plumage of males becomes more and more intense. The process continues to grow until it reaches the limit of viability.
When choosing males, females do not think about the reasons for their behavior. When an animal feels thirsty, it does not reason that it should drink water in order to restore water-salt balance in the body - it goes to water because it feels thirsty. In the same way, females, when choosing bright males, follow their instincts - they like bright tails. Those for whom instinct suggested different behavior did not leave offspring. The logic of the struggle for existence and natural selection is the logic of a blind and automatic process, which, acting constantly from generation to generation, has shaped what amazing variety forms, colors and instincts that we observe in the world of living nature.
Selection methods: positive and negative selection
There are two forms of artificial selection: Positive And Cut-off (negative) selection.
Positive selection increases the number of individuals in a population that have useful signs, increasing the viability of the species as a whole.
Eliminating selection eliminates from a population the vast majority of individuals that carry traits that sharply reduce viability under given environmental conditions. Using selection selection, highly deleterious alleles are removed from the population. Individuals with chromosomal rearrangements and a set of chromosomes that sharply disrupt normal work genetic apparatus.
The role of natural selection in evolution
In the example of the worker ant we have an insect extremely different from its parents, yet absolutely sterile and, therefore, unable to transmit from generation to generation acquired modifications of structure or instincts. A good question to ask is how reconcilable is this case with the theory of natural selection?
- Origin of Species (1859)
Darwin assumed that selection could apply not only to an individual organism, but also to a family. He also said that perhaps, to one degree or another, this could explain people's behavior. He was right, but it was only with the advent of genetics that it became possible to provide a more expanded view of the concept. The first sketch of the “theory of kin selection” was made by the English biologist William Hamilton in 1963, who was the first to propose considering natural selection not only at the level of an individual or an entire family, but also at the level of the gene.
see also
Notes
- , 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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