Genetic drift reflects the test. Genetic drift: the main patterns of this process. Drift in natural populations

drift independently. Therefore, the results of drift turn out to be different in different populations - in some, one set of alleles is fixed, in others, another. Thus, genetic drift leads, on the one hand, to a decrease in genetic diversity within populations, and, on the other hand, to an increase in differences between populations, to their divergence in a number of traits. This divergence, in turn, can serve as the basis for speciation.

During the evolution of populations, genetic drift interacts with other evolutionary factors, most notably natural selection. The ratio of the contributions of these two factors depends both on the intensity of selection and on the number of populations. With a high intensity of selection and a high number of populations, the influence of random processes on the dynamics of gene frequencies in populations becomes negligible. On the contrary, in small populations with small differences in fitness between genotypes, genetic drift becomes crucial. In such situations, the less adaptive allele may become fixed in the population, while the more adaptive one may be lost.

As we already know, the most common consequence of genetic drift is the impoverishment of genetic diversity within populations due to the fixation of some alleles and the loss of others. The mutation process, on the contrary, leads to the enrichment of genetic diversity within populations. An allele lost as a result of drift can arise again and again due to mutation.

Since genetic drift is an undirected process, while reducing diversity within populations, it increases differences between local populations. This is counteracted by migration. If an allele is fixed in one population A, and in the other A, then the migration of individuals between these populations leads to the fact that allelic diversity reappears within both populations.

Rice. 3. N is the number of individuals in the population. It can be seen that at 25 individuals after the 40th generation, one allele disappears, at 250, the ratio of alleles changes, and at 2500, it remains close to the original .

bottle neck effect played, apparently, a very significant role in the evolution of human populations. Ancestors modern people spread throughout the world for tens of thousands of years. Along the way, many populations completely died out. Even those that survived often found themselves on the brink of extinction. Their numbers dropped to a critical level. During the passage through the "bottleneck" of the population, the allele frequencies changed differently in different populations. Certain alleles were completely lost in some populations and fixed in others. After the restoration of the populations, their altered genetic structure was reproduced from generation to generation. These processes, apparently, determined the mosaic distribution of some alleles that we observe today in local human populations. Below is the distribution of the allele IN according to the blood group system AB0 in people. Significant differences between modern populations from each other may reflect the consequences of genetic drift, which occurred in prehistoric times at the moments when ancestral populations passed through the "bottleneck" of numbers.

Genetic-automatic processes, or genetic drift, lead to a smoothing of variability within a group and the appearance of random, non-selective differences between isolates. This is what was revealed by observations of the characteristics of the phenotypes of small groups of the population in conditions, for example, of geographic isolation. Thus, among the inhabitants of the Pamirs, Rh-negative individuals are 2-3 times less common than in Europe. In most villages, such people make up 3-5% of the population. In some isolated villages, however, they number up to 15%, i.e. about the same as in the European population.

Human blood contains haptoglobins, which bind free hemoglobin after the destruction of red blood cells, thereby preventing its excretion from the body. The synthesis of Hp1-1 haptoglobin is controlled by the Hp1 gene. The frequency of this gene in representatives of two neighboring tribes in the North of South America is 0.205 and 0.895, differing by more than 4 times.

An example of the action of genetic drift in human populations is progenitor effect. It occurs when several families break with the parent population and create a new one in another territory. This population usually supports high level marital isolation. This contributes to the random fixation of some alleles in its gene pool and the loss of others. As a result, the frequency of a very rare allele can become significant.

Thus, the members of the Amish sect in Lancaster County, Pennsylvania, numbering approximately 8,000 by the middle of the nineteenth century, were almost all descended from three married couples who immigrated to America in 1770. In this isolate, 55 cases of a special form of dwarfism with polydactylism, which is inherited according to autosomal recessive type. This anomaly has not been reported among the Amish of Ohio and Indiana. There are hardly 50 such cases described in the world medical literature. Obviously, among the members of the first three families that founded the population, there was a carrier of the corresponding recessive mutant allele - the "ancestor" of the corresponding phenotype.

In the XVIII century. 27 families immigrated from Germany to the United States and founded the Dunker sect in Pennsylvania. Over the 200-year period of existence in conditions of strong marital isolation, the gene pool of the Dunker population has changed in comparison with the gene pool of the population of the Rhineland of Germany, from which they originated. At the same time, the degree of differences in time increased. In persons aged 55 years and above, the allele frequencies of the MN blood group system are closer to those typical for the population of the Rhineland than in persons aged 28–55 years. In the age group of 3–27 years, the shift reaches even greater values ​​(Table 1).

The increase among the Dunkers of persons with blood type M and the decrease in those with blood type N cannot be explained by the action of selection, since the direction of change does not coincide with that of the population of Pennsylvania as a whole. In favor of genetic drift is also the fact that in the gene pool of American Dunkers, the concentration of alleles that control the development of obviously biologically neutral traits, for example, hairiness of the middle phalanx of the fingers, the ability to set aside thumb brushes (Fig. 4).

Table 1. Progressive change in the concentration of alleles of the MN blood group system in the Dunker population

For much of human history, genetic drift has affected the gene pools of human populations. Thus, many features of narrow-local types within the Arctic, Baikal, Central Asian, Ural population groups of Siberia are, apparently, the result of genetic-automatic processes in the conditions of isolation of small collectives. These processes, however, were not decisive in human evolution.

Rice. 4. Distribution of neutral traits in Pennsylvania isolatedunkers: A- hair growth on the middle phalanx of the fingers, b— ability to extend the thumb

The consequences of genetic drift of interest to medicine lie in uneven distribution across populations globe some hereditary diseases. Thus, the isolation and drift of genes apparently explains the relatively high frequency of cerebromacular degeneration 1 in Quebec and Newfoundland, childhood cestinosis in France, alkaptonuria in the Czech Republic, one of the types of porphyria among the Caucasoid population in South America, adrenogenital syndrome in Eskimos. These same factors could be responsible for the low incidence of phenylketonuria in Finns and Ashkenazi Jews.

A change in the genetic composition of a population due to genetic-automatic processes leads to homozygotization of individuals. In this case, the phenotypic consequences are more often unfavorable. Homozygotization is the transfer of heterozygotes to homozygotes during closely related crossing. Ch. Darwin describes a phenomenon that can be quite explained by genetic drift. “Rabbits running wild on the island of Porto Santo, near about. Madeira", deserve more full description*. However, it should be remembered that the formation of favorable combinations of alleles is also possible. As an example, consider the genealogies of Tutankhamen (Fig. 5) and Cleopatra VII (Fig. 6), in which closely related marriages have been the rule for many generations.

Tutankhamen died at the age of 18. Analysis of his image in childhood and the captions for this image suggest that he suffered from a genetic disease - celiac disease, which manifests itself in a change in the intestinal mucosa, excluding the absorption of gluten.

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1 cerebromacular degeneration, Tay-Sachs disease. It belongs to the group of hereditary lipid diseases of the brain. Based on the age of onset of the disease, clinical manifestations, fundus pattern and biochemical data, five forms of amaurotic idiocy are distinguished: congenital, early childhood, late childhood, juvenile and late. Some of these forms also differ in the nature of inheritance. A characteristic sign of the disease is diffuse degeneration of ganglion cells in all departments nervous system. The process of disintegration of ganglion cells and the transformation of many of them into a granular mass - Schaffer's degeneration - is a pathognomonic sign of amaurotic idiocy. There is also a breakdown of myelin fibers, especially in the visual and pyramidal tracts, degenerative changes in glia. congenital form is a rare disease. The child already at birth has micro- or hydrocephalus, paralysis, convulsions. Death comes quickly. The content of Gm3 ganglioside was increased in the brain tissue.

Tutankhamun was born from the marriage of Amenophis III and Sintamone, who was the daughter of Amenophis III. So the pharaoh's mother was his stepsister. Mummies of two, apparently stillborn, children from marriage with Ankesenamun, his niece, were found in Tutankhamen's tomb.

The pharaoh's first wife was either his sister or daughter. Tutankhamen's brother Amenophis IV allegedly suffered from Frohlich's disease and died at the age of 25-26. His children from marriages with Nefertiti and Ankesenamun (his daughter) were barren. On the other hand, Cleopatra VII, known for her intelligence and beauty, was born in the marriage of the son of Ptolemy X and his sister preceded by consanguineous marriages for at least six generations.

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*This is interesting

In 1418 or 1419, Gonzales Zarco happened to have a pregnant rabbit on a ship who gave birth during the voyage. All cubs were released to the island. The rabbits have shrunk nearly three inches in length and nearly doubled in body weight. The color of the Porto Santo rabbit differs significantly from that of the common rabbit. They are unusually wild and agile. According to their habits, they are more nocturnal animals. They produce 4 to 6 babies per litter. It was not possible to mate with females of other breeds. "An example of the impact of genetic drift can be cats on Ascension Island. More than 100 years ago, rats appeared on the island. They bred in such numbers that the English commandant decided to get rid of them with the help of cats. At his request, they brought But they fled to the remote corners of the island and began to destroy not rats, but poultry and wild guinea fowls.

Another commandant brought in dogs to get rid of the cats. The dogs did not take root - they injured their paws on the sharp edges of the slag. Cats eventually became ferocious and bloodthirsty. Over the course of a century, they grew almost dog fangs for themselves and began to guard the houses of the islanders, follow on the heels of the owner and rush at strangers.

Rice. 5. Pedigree of the pharaoh of the XVIII dynasty Tutankhamun

Rice. 6. Pedigree of Cleopatra VII

Conclusion and Conclusions:

Traditionally, waves of abundance (life, population) - periodic and aperiodic changes in the number of individuals inherent in all species as a result of the influence of abiotic and biotic factors affecting the population, are considered the "supplier" of elementary evolutionary material.

The best evidence for the importance of genetic drift in microevolution

is the nature of random local differentiation in a series of permanently or periodically isolated small colonies. Differentiation of this type has been repeatedly found in various groups of animals and plants, the populations of which represent a system of colonies. This differentiation, if it does not prove, then at least strongly inclines the opinion that genetic drift plays a role. important role in population systems of this type.

References:

1. Ginter E.K. Medical genetics: Textbook. - M.: Medicine, 2003. - 448 p.: silt

2. Green N., Stout W., Taylor D "Biology" in 3 volumes Moscow "Mir" 2000

3. Guttman B., Griffiths E., Suzuki D., Kulis T. Genetics. M.: FAIR - PRESS, 2004., 448 p.

4. Zhimulev I.F. Genetics. Publishing house of the Siberian University., 2007. - 480 p.: ill.

5. Kurchanov, N.A. Human genetics with the basics of general genetics. / ON THE. Kurchanov. - St. Petersburg: SpecLit, 2006. - 174 p.

6. Mamontov S.G. Biology - M., 2004

7. Shevchenko V.A., Topornina N.A., Stvolinskaya N.S. Human Genetics: Textbook for students. Higher textbook establishments. - M.: VLADOS, 2002. - 240 p.9.

8. Yarygin V.N., V.I. Vasilyeva, I.N. Volkov, V.V. Sinelytsikova Biology. In 2 books: Textbook for medical. specialist. Universities M.: Higher. school., 2003.- 432p.: ill.

Periodic or aperiodic fluctuations in the number of individuals in a population are characteristic of all living organisms without exception. The reasons for such fluctuations can be various abiotic and biotic environmental factors. The action of population waves, or waves of life, involves the indiscriminate, random destruction of individuals., due to which a rare genotype (allele) before the population fluctuation can become common and be picked up by natural selection. If in the future the population is restored due to these individuals, then this will lead to a random change in the frequencies of genes in the gene pool of this population. Population waves are the supplier of evolutionary material.

Classification of population waves

1. Periodic fluctuations in the number of short-lived organisms characteristic of most insects, annual plants, most fungi and microorganisms. Basically, these changes are caused by seasonal fluctuations in numbers.

2. Non-periodic population fluctuations depending on a complex combination of different factors. First of all, they depend on relationships in food chains that are favorable for a given species (population): a decrease in predators, an increase in food resources. Typically, such fluctuations affect several species of both animals and plants in biogeocenoses, which can lead to radical restructuring of the entire biogeocenosis.

3. Species outbreaks in new areas where their natural enemies are absent.

4. Sharp non-periodic population fluctuations associated with natural disasters (as a result of drought or fires). Influence population waves, especially noticeable in populations of very small size (usually when the number of breeding individuals is not more than 500). It is under these conditions that population waves can, as it were, expose rare mutations to natural selection or eliminate already fairly common variants.

Gene drift - these are fluctuations in gene frequencies over a number of generations, caused by random causes, such as a small number of populations. Genetic drift is a completely random process and belongs to a special class of phenomena called sampling errors. General rule is that the value sampling errors is inversely related to sample sizes. In relation to living organisms, this means that the smaller the number of interbreeding individuals in a population, the more changes due to gene drift will undergo allele frequencies.

A random increase in the frequency of any one mutation is usually due to preferential reproduction in isolated populations. This phenomenon is called "progenitor effect" . It occurs when several families create a new population in a new territory. It maintains a high degree of marital isolation, which contributes to the fixation of some alleles and the elimination of others. The consequences of the "effect" are the uneven distribution of hereditary diseases of human populations on earth.

Random changes in allele frequencies, similar to those due to the "ancestor effect", also occur if a population undergoes a sharp reduction in the evolutionary process.

Gene drift leads to:

1) change genetic structure populations: increased homozygosity of the gene pool;

2) a decrease in the genetic variability of populations;

3) population divergence

DRIFT GENES - this is a change in the frequency of genes and genotypes of a population that occurs due to the action of random factors. These phenomena occur independently of each other. These phenomena were discovered by the English scientist Fisher and the American Wright. Domestic geneticists Dubinin and Romashov - introduced the concept genetic-atomic process. This is the process that results from genetic drift fluctuations in the frequency of the allele may occur, or this allele may become fixed in the population or disappear from the gene pool of the population.

This phenomenon has been studied in some detail by Wright. He showed that Genetic drift is closely related to 4 factors:

1. Population size

2. Mutation pressure

3. Gene flow

4. Selective value of a given allele

The larger the population, the less efficient genetic drift is. In large populations, selection is effective.

The higher the mutation pressure, the more frequent the mutations, the less effective the drift of genes.

Gene flow is the exchange of genes between neighboring populations. The higher the flow of genes, the higher the exchange of migrants, the less efficient the drift of genes.

The higher the selective value of an allele, the less efficient genetic drift is.

The effectiveness of genetic drift as a factor in evolution is more pronounced when the population consists of small isolated positions, between these colonies, there is a very small exchange of migrants.

When the population has a high number, then periodically this population sharply reduces its number and death. A high number of individuals and a newly emerging population is formed due to a small number of surviving individuals, i.e. bottleneck effect (manifestation as the "founder principle"). (Mlter).

For example, in some territory there is an extensive maternal population, genetically diverse. Several individuals of it accidentally turned out to be isolated from the maternal population. Those animals that are isolated, they do not represent representative sample, i.e. are not carriers of all the genes that the maternal population possesses. The gene pool of these individuals (new individuals), isolated, is random and depleted.

If the conditions in the isolated territory are favorable, then closely related crossing will occur between individuals and homozygotes for individual traits will occur. This newly formed daughter population will differ from the original parent population. Its gene pool will be determined genetically, especially in those individuals that founded this population.

Genetic drift, as a factor in evolution, is of high importance at different stages of the emergence of a population, when the population size is not large.

An example of genetic drift. Among American entrepreneurs, there are often people with Morfan's syndrome. They can be easily identified by appearance(tall, cutting, short torso, physically strong). Body features are the result of genetic drift. The passengers of the ship arriving in America were alone and the spread of these qualities was due to people from the polar (northern) Eskimo tribe in northern Greenland. 270 people have been isolated for generations. As a result, there were changes in the frequency of alleles that determine the blood group.

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 the population". Let us consider in more detail what this process is and what are its consequences.

General information

Genetic drift, examples of which will be given in the article below, is a certain change that is recorded from generation to generation. It is believed that this phenomenon has its own mechanisms. Some researchers are concerned that in the gene pool of many, if not all, nations, the amount of anomalous genes that are emerging is currently increasing quite rapidly. They determine hereditary pathology, form the prerequisites for the development of many other diseases. It is also believed that pathomorphosis (changes in signs) of various diseases, including mental illnesses, determines precisely the drift of genes. The phenomenon about which in question is happening at a rapid pace. As a result, a number of mental disorders take unknown forms, become unrecognizable when compared with their description in classical publications. At the same time, significant changes are noted directly in the very structure of psychiatric morbidity. So, genetic drift erases some forms of schizophrenia that have occurred before. Instead of them, such pathologies appear that can hardly be determined by modern classifiers.

Wright's theory

Random genetic drift has been studied using mathematical models. Using this principle, Wright deduced a theory. He believed that the decisive role of genetic drift in constant conditions seen in small groups. They become homozygous and the variability decreases. Wright also believed that as a result of changes in groups, negative hereditary traits could form. As a result, the entire population may die without contributing to the development of the species. At the same time, selection plays an important 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 are rather slow. In this regard, the evolution of large populations does not differ. high speed. In groups of intermediate size, increased variability is noted. At the same time, the formation of new beneficial genes occurs by chance, which, in turn, accelerates evolution.

Wright's findings

When one allele is lost from a population, it can appear due to a certain mutation. But if the species is divided into several groups, one of which lacks one element, the other lacks another, then the gene can migrate from where it is to where it is not. Thus, variability will be preserved. Given this, Wright concluded that the fastest development will occur in those species that are divided into numerous populations of different sizes. At the same time, some migration is also possible between them. Wright agreed that natural selection plays a very significant role. However, along with this result of evolution is the drift of genes. It defines ongoing changes within a view. In addition, Wright believed that the set hallmarks that arose through drift were indifferent, and in some cases even harmful to the viability of organisms.

Researcher controversy

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

1. In the event that selection occupies 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 the surrounding conditions.
2. If genetic drift is more significant over a long period, then directed changes will not be due to natural environment. At the same time, unfavorable signs, even if they occur in small quantities, can spread quite widely in the group.

However, it should be noted that the process of changes itself, as well as the cause of genetic drift, are not sufficiently studied today. In this regard, there is no single and specific opinion about this phenomenon in science.

Gene drift is a factor in evolution

Due to changes, a change in allele frequencies is noted. This will continue 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, one set of elements is fixed in some, and another is fixed in others. Genetic drift, therefore, on the one hand, leads to a decrease in diversity. However, at the same time, it also causes differences between groups, divergences in some respects. This, in turn, can act as the 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 factor is the size of the group. So, if the intensity and number are high, random processes have negligible influence on the dynamics of genetic frequencies. At the same time, in small groups with insignificant differences in fitness, the influence of changes is incomparably greater. In such cases, fixation of the less adaptive allele is possible, while the more adaptive one is lost.

Consequences of change

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 process of mutation, in turn, on the contrary, contributes to the enrichment of genetic diversity within populations. Due to mutation, the lost allele can occur again and again. Due to the fact that genetic drift is a directed process, the difference between local groups increases simultaneously with the decrease in intrapopulation diversity. Migration counteracts this phenomenon. So, if the allele "A" is fixed in one population, and "a" in the other, then diversity again appears within these groups.

End 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 will be. As some calculations show, the possibility of fixation is equal to the allele frequency 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 - the carrier of a new mutation. With a population of one hundred thousand, each group of descendants with a probability close to one will have a mutant allele. However, its frequency in the population, as well as the possibility of its fixation, 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 a mutation. But after its occurrence, the consolidation passes quickly. From this we can draw the following conclusion: the chance of fixing neutral alleles depends only on the frequency of mutational occurrence. At the same time, the population size does not affect this process.

molecular clock

Due to the fact that the frequency of occurrence of neutral mutations in different types approximately the same, the fixing speed 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, not depending on each other.

Polling and Zukurkendl's research

These two American scientists found that the number of differences in the amino acid sequence in cytochrome and hemoglobin in certain mammalian species is the higher, the earlier their discrepancy occurred. evolutionary paths. 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 course of the molecular clock is carried out 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 in them is different. To do this, an estimate is made of the number of substitutions that have accumulated in a particular gene in taxa. Their divergence times have been 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 there was a divergence (divergence) between different taxa. This is possible even if their common ancestor has not yet been identified in the fossil record.

Relative to the previous generation.

Encyclopedic YouTube

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Flu shift and drift

The sequence of processes characteristic of speciation

Evolution. Directing and non-directing factors of evolution,

Subtitles

Let's imagine that these are 2 communities, the community of orange and purple and they are separate from each other. And your goal is to infiltrate these communities and find out what is the most common type of influenza virus circulating among these people. So you do this, and the first thing you find is something very interesting. Namely, it turns out that in the orange community, only influenza A virus is noted. You did not forget that we have 3 types of viruses, and here, apparently, only type A is observed to affect people in this group. Let's, I'll write it down here, type A. And if you look at the purple community, you'll see the opposite. You will see that here people also get flu, but the causative agent is always type B. So these people are affected by influenza type B. And influenza type B also has 8 pieces of RNA. Let's write it in purple right here, type B. So, this is the first thing you should learn on your first day on the job. And now there are many different type A subtypes that affect the orange community, and I've only depicted the dominant strain here. And in fact, there may be many types of A circulating in the orange community, but this is the dominant strain. And you know, the same is true for the purple community. It also has a few Type B strains circulating. However, the dominant strain in it is the one I've drawn for 4. And now I'll clear a little space and let's explain to you what we're going to do. Over the next year, over the next 12 months, we will be watching these two communities. And what is required of you is to note, in general, what is happening in the community with the dominant strain. So, what is important for us is not all strains, but the dominant strain. And we want to know how genetically different strains can compare and what will happen on the first day of our work? So when I say genetic changes, I'm really comparing it to what we had on the first day of our work - comparison to the original strain. And within 12 months you accumulate information about what changes took place during your work. So let's say you started here and live near the purple community. And of course, initially we do not notice any changes. You analyze a type B strain and conclude that it also lacks changes. However, some time passes. Let's say it's been a while and you're back and looking around the purple community. And you ask what type of strain B is most common in them today. And they report that he's in in general terms the same as it was before, and it has not changed significantly, but there have been two point mutations. And in the dominant strain, a couple of point mutations occurred, and therefore it became a little different from the original. And you say, "Well, of course, there have been some genetic changes." The dominant strain has changed somewhat. And then you go and visit them after a while and they thank you for the return visit. And there have been some other changes since your last visit. And you say, "How interesting." This requires a slightly deeper analysis. And now it's a virus, type B virus, it looks a little different from how it looked when you started. And you keep watching this process, and you know there's a mutation here, and another one here. So, mutations sort of accumulate. And you end up with a dotted line - something like this - where the following mutations take place all the way through to the end of the year. And when the end of the year comes, and you analyze the dynamics of your virus, you can say that several mutations have occurred. It is somewhat different from what it was in the beginning. And I will mark these small mutations with yellow X's. And what do we call this process? We'll call it genetic drift. This is genetic drift. This is a normal process that occurs in many types of viruses and bacteria. In fact, all viruses and bacteria make mistakes when they replicate, and you can see some degree of genetic drift over time. And now the most interesting. You go to an orange community, an orange country if you like, and you say you want to do the same thing with influenza type A. And at the beginning of the observation period, there is no difference. However, you come back a little later and you notice that there have been some changes here, a few mutations, just like the ones we talked about above. And you say it's good that there seems to be a little change. And then you find out that, as you know, another mutation happened when you returned from another trip. And you say, "Okay, it looks like there's been some more changes," and then something really interesting happens. You find, upon returning from your third journey, that the entire segment has completely disappeared and been replaced by another. And you find a big new piece of RNA. And how do you imagine the chain of genetic changes? The differences are really significant, aren't they? And you agree that now about 1/8 of everything has changed, and it will look something like this. And that's a huge leap. And you say, "Okay, now there's been a significant genetic change." And then you come back from the trip again, and you find that there's been a little mutation in this green RNA, and maybe another one over here. And again, you noted small changes. And you find another mutation here, and maybe even here. And you keep rebuilding the chain of events - you take your job very seriously - you keep drawing up the diagram. And then it turns out that another significant shift has taken place. Let's say that this section has become different from this one. And so, again, you've had a huge leap. Something like that. And finally, at the end of the year, it continues as you have discovered a few more mutations. So let's say that these additional mutations happened here and here. Here is how it began to look. Do you agree with me? The genetic changes over time for the orange population, type A, do look somewhat different. And it contains elements that I have labeled as genetic drift and shift. And to be more precise, this part is a variant of a large shift. Here, a whole fragment of RNA, as it were, was integrated into a dominant virus. Here are 2 shifts that could have happened this year. And these areas - let's I circle them with a different color, say, here - this one and this one, really look more like what we talked about above. It's a kind of stable change, stable mutation over time. And this is what we usually refer to as "genetic drift." So, with the influenza type A virus, marked in orange, you can see that there is some drift and shift going on. And with the influenza type B virus, only genetic drift occurs. And what's happening at the moment is the most frightening information about influenza type A virus, and that means that whatever giant shifts you see, you have 2 giant drifts, 2 here, if these shifts happened, then the whole community hasn't encountered this new type A influenza virus yet. It's not ready for it. The immune system of the inhabitants of the community does not know what to do with it. And as a result, a lot of people get sick. And what we call a pandemic is happening. There have been several similar pandemics in the past. And each time, as a rule, they were due to a major genetic shift. And as a result, many people, as I said, get sick, end up in the hospital and may even die. Subtitles by the Amara.org community

Gene drift by example

The mechanism of genetic drift can be demonstrated with a small example. Imagine a very large colony of bacteria isolated in a drop of solution. Bacteria are genetically identical except for one gene with two alleles A And B. allele A present in one half of the bacteria, the allele B- at the other. So the allele frequency A And B equals 1/2. A And B- neutral alleles, they do not affect the survival or reproduction of bacteria. Thus, all bacteria in the colony have the same chance of survival and reproduction.

Then the droplet size is reduced in such a way that there is enough food for only 4 bacteria. All others die without reproduction. Among the four survivors, 16 combinations for alleles are possible A And B:

(A-A-A-A), (B-A-A-A), (A-B-A-A), (B-B-A-A),
(A-A-B-A), (B-A-B-A), (A-B-B-A), (B-B-B-A),
(A-A-A-B), (B-A-A-B), (A-B-A-B), (B-B-A-B),
(A-A-B-B), (B-A-B-B), (A-B-B-B), (B-B-B-B).

The probability of each of the combinations

where 1/2 (probability of allele A or B for each surviving bacterium) is multiplied 4 times ( overall size resulting population of surviving bacteria)

If you group the variants by the number of alleles, you get the following table:

As can be seen from the table, in six out of 16 variants, the colony will have the same number of alleles A And B. The probability of such an event is 6/16. The probability of all other options, where the number of alleles A And B unequally somewhat higher and is 10/16.

Genetic drift occurs when allele frequencies in a population change due to random events. In this example, the bacterial population was reduced to 4 survivors (bottleneck effect). At first, the colony had the same allele frequencies A And B, but the chances that the frequencies will change (the colony will undergo genetic drift) are higher than the chances of maintaining the original allele frequency. There is also a high probability (2/16) that one allele will be completely lost as a result of genetic drift.

Experimental proof by S. Wright

S. Wright experimentally proved that in small populations the frequency of the mutant allele changes rapidly and randomly. His experience was simple: he planted two females and two males of Drosophila flies heterozygous for gene A (their genotype can be written Aa) in test tubes with food. In these artificially created populations, the concentration of normal (A) and mutational (a) alleles was 50%. After several generations, it turned out that in some populations all individuals became homozygous for the mutant allele (a), in other populations it was completely lost, and, finally, some of the populations contained both the normal and the mutant allele. It is important to emphasize that, despite the decrease in the viability of mutant individuals and, therefore, contrary to natural selection, in some populations the mutant allele completely replaced the normal one. This is the result of a random process - genetic drift.

Literature

• Vorontsov N.N., Sukhorukova L.N. Evolution organic world. - M.: Nauka, 1996. - S. 93-96. - ISBN 5-02-006043-7.
• Green N., Stout W., Taylor D. Biology. In 3 volumes. Volume 2. - M.: Mir, 1996. - S. 287-288. -