Genetic drift: the main patterns of this process. Genetic drift: the main patterns of this process What is the essence of the process of genetic drift

In theoretical biology, it is believed that the transmission and distribution of genes from parents to children will always remain constant and unchanged from generation to generation (Hardy-Weinberg law). However, in practice, everything happens far from the same as in theory. Sometimes it happens that due to some random (or even natural) events, the frequency of gene distribution from generation to generation can be disrupted, even deviate; this phenomenon is called genetic drift.

Examples of genetic drift

Let's take this example: there is a group of plants in some isolated mountain valley. The population of plants is 100 specimens and only 2% of them have a special variant of the gene, say, responsible for the color of flowers. In other words, only two plants have a unique gene. And if as a result of some random incident, say a hurricane, flood or avalanche, these two plants die, then the special gene (in academic terms, the allel) will be lost from the population. As a result, future generations of these plants will also change, and in general there will be genetic drift in the population, or as scientists also call it the “bottleneck effect.”

Causes of genetic drift

Usually the causes can be various catastrophic natural consequences, natural disasters, storms, hurricanes, volcanic eruptions, leading to the mass death of living beings, but recently destructive human activity has become a frequent cause of this phenomenon. For example, the cause of genetic drift in Africa was their mass shooting in the 20th century, both by white hunters (for fun) and poachers (the price of ivory was always high on the black market).

Genetic drift in evolution

If we look at genetic drift from the point of view of the theory of evolution, then we can say that the result of evolution is genetic drift, since in its process some genes will still be lost. Moreover, according to some scientists, even humans went through genetic drift. If this is so, then this happened approximately 100,000 years ago, and it is the “bottleneck effect”, that is, genetic drift, that explains the genetic similarity of modern people to each other. For comparison, gorillas living in the African jungle have much greater genetic diversity than all people living on Earth.

DRIFT OF GENEES

This concept is sometimes called the Sewell-Wright effect, after the two population geneticists who proposed it. After Mendel proved that genes are units of heredity, and Hardy and Weinberg demonstrated the mechanism of their behavior, biologists realized that the evolution of traits can occur not only through natural selection, but also by chance. Genetic drift depends on the fact that changes in allele frequency in small populations are due solely to chance. If the number of crosses is small, then the actual ratio of different alleles of a gene may differ greatly from that calculated based on the theoretical model. Genetic drift is one of the factors that disturbs the Hardy-Weinberg equilibrium.

Large populations with random mating are greatly influenced by natural selection. In these groups, individuals with adaptive traits are selected, while others are ruthlessly eliminated, and the population becomes more adapted to the environment through natural selection. In small populations, other processes occur and are influenced by other factors. For example, in small populations there is a high probability of random changes in gene frequency. Such changes are not caused by natural selection. The concept of genetic drift is very important for small populations because they have a small gene pool. This means that the random disappearance or appearance of a gene allele in offspring will lead to significant changes in the gene pool. In large populations, such fluctuations do not lead to noticeable results, since they are balanced by a large number of crossings and the influx of genes from other individuals. In small populations, random events can lead to a bottleneck effect.

According to the definition, genetic drift refers to random changes in gene frequencies caused by a small population size and infrequent crossing. Genetic drift occurs among small populations, such as island migrants, koalas, and giant pandas.

See also the articles “Bottleneck Effect”, “Hardy-Weinberg Equilibrium”, “Mendelism”, “Natural Selection”.

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GENE DRIFT, genetic drift (from the Dutch drijven - to drive, to swim), random fluctuations in the frequency of gene alleles in a number of generations of a population with a limited number. Genetic drift was established in 1931 simultaneously and independently by S. Wright, who proposed this term, and by Russian geneticists D. D. Romashov and N. P. Dubinin, who called such fluctuations “genetic-automatic processes.” The reason for genetic drift is the probabilistic nature of the fertilization process against the background of a limited number of descendants. The magnitude of the allele frequency fluctuations in each generation is inversely proportional to the number of individuals in the population and directly proportional to the product of the allele frequencies of the gene. Such parameters of genetic drift should theoretically lead to the preservation in the gene pool of only one of 2 or more alleles of a gene, and which of them will be preserved is a probabilistic event. Genetic drift, as a rule, reduces the level of genetic variability and in small populations leads to homozygosity of all individuals for one allele; The speed of this process is greater, the smaller the number of individuals in the population. The effect of genetic drift, modeled on a computer, has been confirmed both experimentally and under natural conditions in many species of organisms, including humans. For example, in the smallest population of Greenland Eskimos (about 400 people), the absolute majority of representatives have blood type 0 (I), that is, they are homozygous for the I0 allele, which almost “displaced” other alleles. In 2 populations of much larger numbers, all alleles of the gene (I0, IA and IB) and all blood groups of the ABO system are represented with significant frequency. Genetic drift in permanently small populations often leads to their extinction, which is the reason for the relatively short-term existence of demes. As a result of a decrease in the reserve of variability, such populations find themselves in an unfavorable situation when environmental conditions change. This is due not only to the low level of genetic variability, but also to the presence of unfavorable alleles that constantly arise as a result of mutations. A decrease in the variability of individual populations due to genetic drift can be partially compensated at the level of the species as a whole. Since different alleles are fixed in different populations, the gene pool of the species remains diverse even at a low level of heterozygosity in each population. In addition, in small populations alleles with low adaptive value can be fixed, which, however, when the environment changes, will determine adaptability to new conditions of existence and ensure the preservation of the species. In general, genetic drift is an elementary evolutionary factor that causes long-term and directed changes in the gene pool, although in itself it does not have an adaptive nature. Random changes in allele frequencies also occur during a sharp one-time decrease in population size (as a result of catastrophic events or migration of part of the population). This is not genetic drift and is referred to as the bottleneck effect or founder effect. In humans, such effects underlie the increased incidence of certain hereditary diseases in some populations and ethnic groups.

Lit.: Kaidanov L.Z. Population genetics. M., 1996.

Along with natural selection, there is another factor that can influence the increase in the content of the mutant gene. In some cases, it can even displace the normal allelomorph. This phenomenon is called “genetic drift in a population.” Let us consider in more detail what this process is and what its consequences are.

General information

Genetic drift, examples of which will be given later in the article, represents certain changes that are recorded from generation to generation. It is believed that this phenomenon has its own mechanisms. Some researchers are concerned that the number of abnormal genes appearing in the gene pool of many, if not all, nations is currently increasing quite rapidly. They determine hereditary pathology and form the prerequisites for the development of many other diseases. It is also believed that pathomorphosis (changes in symptoms) of various diseases, including mental illnesses, is caused by genetic drift. The phenomenon in question is happening at a rapid pace. As a result, a number of mental disorders take unknown forms and become unrecognizable when compared with their descriptions in classical publications. At the same time, significant changes are noted directly in the very structure of psychiatric morbidity. Thus, genetic drift erases some forms of schizophrenia that were previously encountered. Instead, pathologies appear that can hardly be determined using modern classifiers.

Wright's theory

Random genetic drift has been studied using mathematical models. Using this principle, Wright developed a theory. He believed that the decisive importance of genetic drift under constant conditions is observed in small groups. They become homozygous and variability decreases. Wright also believed that as a result of changes in groups, negative hereditary traits could form. As a result of this, the entire population may die without making a contribution to the development of the species. At the same time, selection plays a major role in many groups. In this regard, genetic variability within the population will again be insignificant. Gradually the group will adapt well to the surrounding conditions. However, subsequent evolutionary changes will depend on the occurrence of favorable mutations. These processes take place quite slowly. In this regard, the evolution of large populations is not very fast. Intermediate groups show increased variability. In this case, the formation of new beneficial genes occurs randomly, which, in turn, accelerates evolution.

Wright's conclusions

When one allele is lost from a population, it may appear due to a specific mutation. But if a species is divided into several groups, one of which is missing one element, the other is missing another, then the gene can migrate from where it is to where it is not. This will preserve variability. Taking this into account, Wright concluded that development would occur faster in those species that are divided into numerous populations of different sizes. At the same time, some migration is possible between them. Wright agreed that natural selection played a very significant role. However, along with this, the result of evolution is genetic drift. It defines lasting changes within a species. In addition, Wright believed that many distinctive features that arose through drift were indifferent, and in some cases even harmful to the viability of organisms.

Researchers' disputes

There were several opinions regarding Wright's theory. For example, Dobzhansky believed that it was pointless to raise the question of which factor is more significant - natural selection or genetic drift. He explained this by their interaction. Essentially, the following situations are likely:

If selection takes a dominant position in the development of certain species, either a directed change in gene frequencies or a stable state will be noted. The latter will be determined by environmental conditions.
If genetic drift is more significant over a long period, then directional changes will not be determined by the natural environment. At the same time, unfavorable signs, even if they occur in small quantities, can spread quite widely in the group.

It should be noted, however, that the process of change itself, as well as the cause of genetic drift, has not been sufficiently studied today. In this regard, there is no single and specific opinion about this phenomenon in science.

Genetic drift - a factor of evolution

Thanks to the changes, a change in allele frequencies is noted. This will happen until they reach a state of equilibrium. That is, genetic drift is the isolation of one element and the fixation of another. In different groups, such changes occur independently. In this regard, the results of genetic drift in different populations are different. Ultimately, in some, one set of elements is fixed, in others, another. Genetic drift, therefore, on the one hand, leads to a decrease in diversity. However, at the same time, it also determines differences between groups and divergences in some respects. This, in turn, may act as a basis for speciation.

Influence ratio

During development, genetic drift interacts with other factors. First of all, the relationship is established with natural selection. The ratio of the contributions of these factors depends on a number of circumstances. First of all, it is determined by the intensity of selection. The second circumstance is the size of the group. Thus, if the intensity and abundance are high, random processes have a negligible influence on the dynamics of genetic frequencies. Moreover, in small groups with insignificant differences in fitness, the impact of changes is incomparably greater. In such cases, it is possible to fix a less adaptive allele, while the more adaptive one will be lost.

Consequences of changes

One of the main results of genetic drift is the impoverishment of diversity within a group. This occurs due to the loss of some alleles and the fixation of others. The mutation process, in turn, on the contrary, contributes to the enrichment of genetic diversity within populations. Due to mutation, the lost allele can arise again and again. Due to the fact that genetic drift is a directed process, simultaneously with a decrease in intrapopulation diversity, the difference between local groups increases. Migration counteracts this phenomenon. So, if allele “A” is fixed in one population, and “a” is fixed in another, then diversity again appears within these groups.

Final result

The result of genetic drift will be the complete elimination of one allele and the consolidation of another. The more often an element occurs in a group, the higher the probability of its fixation. As some calculations show, the possibility of fixation is equal to the frequency of the allele in the population.

Mutations

They occur at an average frequency of 10-5 per gene per gamete per generation. All alleles that are found in groups once arose due to mutation. The smaller the population, the lower the probability that each generation will have at least one individual carrying a new mutation. With a population of one hundred thousand, each group of descendants will have a mutant allele with a probability close to one. However, its frequency in the population, as well as the possibility of its establishment, will be quite low. The probability that the same mutation will appear in the same generation in at least one individual with a population of 10 is negligible. However, if it does occur in a given population, then the frequency of the mutant allele (1 in 20 alleles), as well as the chances of its fixation, will be relatively high. In large populations, the emergence of a new element occurs relatively quickly. At the same time, its consolidation is slow. Small populations, on the contrary, wait a long time for mutations. But after its occurrence, consolidation occurs quickly. From this we can draw the following conclusion: the chance of fixing neutral alleles depends only on the frequency of mutation occurrence. However, the population size does not affect this process.

Molecular clock

Due to the fact that the frequencies of occurrence of neutral mutations in different species are approximately the same, the rate of fixation should also be approximately equal. It follows from this that the number of changes accumulated in one gene should be correlated with the time of independent evolution of these species. In other words, the longer the period since the separation of two species from one ancestor, the more they distinguish between mutational substitutions. This principle underlies the molecular evolutionary clock method. This determines the time that has passed since the moment when previous generations of various systematic groups began to develop independently, without depending on each other.

Polling and Tsukurkendle's research

These two American scientists found that the number of differences in the amino acid sequence in cytochrome and hemoglobin in certain mammal species is higher, the earlier the divergence of their evolutionary paths occurred. Subsequently, this pattern was confirmed by a large amount of experimental data. The material included dozens of different genes and several hundred species of animals, microorganisms and plants. It turned out that the molecular clock moves at a constant speed. This discovery, in fact, is confirmed by the theory under consideration. The clock is calibrated separately for each gene. This is due to the fact that the frequency of occurrence of neutral mutations is different. To do this, an assessment is made of the number of substitutions accumulated in a certain gene in taxa. Their divergence times are reliably established using paleontological data. Once the molecular clock has been calibrated, it can be used further. In particular, with their help it is easy to measure the time during which divergence (divergence) occurred between different taxa. This is possible even if their common ancestor has not yet been identified in the fossil record.

The frequency of genes in a population can vary under the influence of random factors.

The Hardy-Weinberg law states that in a theoretical ideal population, the distribution of genes will remain constant from generation to generation. Thus, in a plant population, the number of “grandchildren” with genes for tallness will be exactly the same as there were parents with this gene. But in real populations the situation is different. Due to random events, the frequency of distribution of genes varies slightly from generation to generation - this phenomenon is called genetic drift.

Let's give a simple example. Imagine a group of plants inhabiting an isolated mountain valley. The population consists of 100 adult plants, and only 2% of the plants in the population contain a special gene variant (for example, affecting the color of a flower), i.e., in the population we are considering, only two plants have this gene. It is quite possible that a small incident (such as a flood or a falling tree) will cause both plants to die, and then this particular gene variant (or, in scientific terminology, this allele) will simply disappear from the population. This means that future generations will no longer be the same as the one we are considering.

There are other examples of genetic drift. Consider a large breeding population with a strictly defined allele distribution. Let's imagine that for one reason or another, part of this population separates and begins to form its own community. The distribution of genes in a subpopulation may be uncharacteristic of the wider group, but from that moment onwards the subpopulation will exhibit precisely this uncharacteristic distribution. This phenomenon is called founder effect.

Genetic drift of a similar type can be observed in the example of a phenomenon with a memorable name bottleneck effect. If for some reason a population declines sharply—due to forces other than natural selection (for example, an unusual drought or a brief increase in the number of predators) that quickly appear and then disappear—the result will be the random elimination of large numbers of individuals. As with the founder effect, by the time a population flourishes again, it will contain genes characteristic of the random survivors, and not at all of the original population.

At the end of the 19th century, northern elephant seals were almost completely exterminated as a result of hunting. Today, the population of these animals (which has recovered in size) shows an unexpectedly small number of genetic variants. Anthropologists believe that the first modern humans experienced a bottleneck effect about 100,000 years ago, and explain the genetic similarity between people. Even members of the gorilla clans living in one African forest have more genetic variants than all human beings on the planet.