Gene drift: the main regularities of this process.

In addition to natural selection, there is another factor that can contribute to an increase in the concentration of a mutant gene in a population and even completely displace its normal allelomorph.

Genetic drift (genetic drift) is a change in the gene pool of a population from generation to generation. It is believed that these changes are determined not by natural selection, but by other mechanisms. There is a growing concern among researchers that the share of abnormal genes that determine the development of hereditary pathology and predispose to the development of many other diseases is rapidly increasing in the gene pool of a number, if not all nations. Genetic drift is also one of the most significant factors in the pathomorphosis of various, including psychiatric diseases. This pathomorphosis is carried out at such a rapid pace that some mental disorders become unrecognizable (according to their descriptions in the classical literature), the structure of psychiatric morbidity also changes significantly, in particular, some previously common forms of schizophrenia are washed out and those that have difficulty finding a place in modern classifiers appear.

Biologist S. Wright investigated this random process (genetic drift) using mathematical models and applied this principle to the study of evolutionary problems. Under constant conditions, genetic drift is critical in very small populations, hence the population becomes homozygous for many genes and genetic variability decreases. He also believed that as a result of drift in the population, traits of harmful hereditary traits may arise, as a result of which such a population may die and not contribute to the evolution of the species. On the other hand, in very large populations, selection is critical, so the genetic variation in the population will again be negligible. The population gradually adapts well to environmental conditions, but further evolutionary changes depend on the emergence of new favorable mutations. Such mutations occur slowly, so evolution in large populations is slow. In populations of intermediate size, genetic variability is increased, new beneficial combinations of genes are formed by chance, and evolution is faster than the other two populations described above.

It should also be remembered that when one allele is lost from the population, it can only reappear due to a certain mutation. But if a species is divided into a number of populations, in some of which one allele is lost, and in others another, then a gene lost from this population may appear in it due to migration from another population where there is given gene... This is how genetic variation will be preserved. Based on this, Wright suggested that the most rapid evolutionary changes will occur in species subdivided into numerous populations of various sizes, with some migration between populations possible.

Wright agreed that natural selection is one of the most important factors in evolution, but genetic drift, in his opinion, is also a significant factor in determining long-term evolutionary changes within a species, and that many of the traits that distinguish one species from another arose through gene drift and were indifferent or even harmful in their influence on the viability of organisms.

The theory of genetic drift has sparked controversy among biologists. For example, T. Dobzhansky believed that it makes no sense to pose the question of which factor plays a major role - genetic drift or natural selection. These factors interact with each other.

Two situations are possible:

1) If selection is dominant in the evolution of any species, then either a directed change in gene frequencies or a stable state determined by environmental conditions will be observed.

2) When, over a long period of time, drift is more important, then directional evolutionary changes will not be associated with natural conditions, and even insignificant unfavorable signs that have arisen can spread widely in the population.

In general, the genetic drift has not yet been studied well enough and definite; there is still no consensus on this factor in science.

Gene drift as a factor in evolution

We can consider gene drift as one of the factors of population evolution. Due to the drift, allele frequencies can randomly change in local populations until they reach an equilibrium point - the loss of one allele and fixation of another. In different populations, genes "drift" independently. Therefore, the results of the drift are different in different populations - in some, one set of alleles is fixed, in others, another. Thus, gene 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.
In the course of population evolution, gene drift interacts with other factors of evolution, primarily natural selection. The ratio of the contributions of these two factors depends on both the selection intensity and the population size. With a high selection intensity and a high population size, the effect 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, gene drift becomes decisive. In such situations, the less adaptive allele can be fixed in the population, while the more adaptive allele can be lost.

As we already know, the most frequent consequence gene drift is the depletion 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 gene drift is an undirected process, simultaneously with a decrease in diversity within populations, it increases the differences between local populations. This is counteracted by migration. If allele A is recorded in one population, and allele a in another, then the migration of individuals between these populations leads to the fact that allelic diversity reappears within both populations.

The end result of gene drift is the complete elimination of one allele from the population and the fixation (fixation) of another allele in it. The more often this or that allele occurs in the population, the higher the probability of its fixation due to gene drift. Calculations show that the probability of fixing a neutral allele is equal to its frequency in the population.

Every allele that we observe in populations at one time arose as a result of a mutation. Mutations occur at an average frequency of 10-5 per gene per gamete per generation. Therefore, the smaller the population, the less likelythat in each generation at least one individual in this population will be the carrier of a new mutation. In a population of 100,000 individuals, in each new generation, with a probability close to unity, there will be a new mutant allele, but its frequency in the population (1 per 200,000 alleles) and, therefore, the probability of its fixation will be very low. The probability that the same mutation in the same generation will occur in at least one individual in a population of 10 individuals is negligible, but if such a mutation still occurs in this population, then the frequency of the mutant allele (1 per 20 alleles) and the chances of fixing it will be relatively high.

Large populations do not “wait” for a mutational emergence of a new allele for a long time, but they fix it for a long time, and small populations “wait” for a very long time for a mutation to appear, but after it has arisen, it can be quickly fixed. This implies a seemingly paradoxical conclusion: the probability of fixing neutral alleles depends only on the frequency of their mutational occurrence and does not depend on the population size.

Since the frequencies of occurrence of neutral mutations are approximately the same in different species, the rate of fixation of these mutations should be approximately the same. It follows from this that the number of mutations accumulated in the same gene should be proportional to the time of independent evolution of these species. In other words, the more time has passed since the separation of two species from the common limiting species, the more neutral mutational substitutions distinguish these species. This principle is the basis for the method of the "molecular clock of evolution" - determining the time elapsed from the moment when the ancestors of different systematic groups began to evolve independently of each other.

American researchers E. Tsukurkendl and L. Polling first discovered that the number of differences in the amino acid sequence in hemoglobin and cytochrome c in different types mammals are the more, the earlier their evolutionary paths diverged. Subsequently, this pattern was confirmed on a huge experimental material, including dozens of different genes and hundreds of species of animals, plants and microorganisms. It turned out that the molecular clock runs, as follows from the theory of gene drift, at a constant speed. Calibration of the molecular clock is performed for each gene separately, since different genes can differ in the frequency of occurrence of neutral mutations. For this, the number of substitutions accumulated in a certain gene in representatives of taxa is estimated, the time of divergence of which is reliably established from paleontological data. Once the molecular clock has been calibrated, it can be used to measure divergence times between different taxa, even if their common ancestor has not yet been found in the fossil record.



6. Genetic drift.

In addition to natural selection, there is another factor that can contribute to an increase in the concentration of a mutant gene in a population and even completely displace its normal allelomorph.

Biologist S. Wright investigated this random process (genetic drift) using mathematical models and applied this principle to the study of evolutionary problems. Under constant conditions, genetic drift is critical in very small populations, hence the population becomes homozygous for many genes and genetic variability decreases. He also believed that as a result of drift in the population, traits of harmful hereditary traits may arise, as a result of which such a population may die and not contribute to the evolution of the species. On the other hand, in very large populations, selection is critical, so the genetic variation in the population will again be negligible. The population gradually adapts well to environmental conditions, but further evolutionary changes depend on the emergence of new favorable mutations. Such mutations occur slowly, so evolution in large populations is slow. In populations of intermediate size, genetic variability is increased, new beneficial combinations of genes are formed by chance, and evolution is faster than the other two populations described above.

It should also be remembered that when one allele is lost from the population, it can only reappear due to a certain mutation. But if a species is divided into a number of populations, in some of which one allele is lost, and in others another, then a gene lost from a given population may appear in it due to migration from another population where this gene is present. This is how genetic variation will be preserved. Based on this, Wright suggested that the most rapid evolutionary changes will occur in species subdivided into numerous populations of various sizes, with some migration between populations possible.

Wright agreed that natural selection is one of the most important factors in evolution, but genetic drift, in his opinion, is also a significant factor in determining long-term evolutionary changes within a species, and that many of the traits that distinguish one species from another arose through gene drift and were indifferent or even harmful in their influence on the viability of organisms.

The theory of genetic drift has sparked controversy among biologists. For example, T. Dobzhansky believed that it makes no sense to pose the question of which factor plays a major role - genetic drift or natural selection. These factors interact with each other. Two situations are possible:

1) If selection dominates in the evolution of any species, then in this case either a directed change in gene frequencies will be observed, or a stable state determined by environmental conditions.

2) When, over a long period of time, drift is more important, then directional evolutionary changes will not be associated with natural conditions and even insignificant unfavorable traits that have arisen can spread widely in the population.

In general, the genetic drift has not yet been studied well enough and definite; there is still no consensus on this factor in science.

7. Conclusion.

Research in genetics and ecology has identified a number of factors that control adaptation and speciation. The forces behind the evolution of families, orders and classes cannot be so easily identified.

The synthesis of genetics and evolution mainly consisted in the interaction of Mendelian theory of heredity and Darwin's theory, great in its scientific significance.

On the present stage development of genetics and evolution all greater importance acquires genetic engineering. Scientists managed to decipher the structure of the DNA molecule, which made it possible to create on the basis of known species new, with pre-programmed, not characteristic of this type of qualities. The biggest problem in practical use genetic engineering is the safety of products of the use of products of genetic engineering for the existence of Mankind. Along with this, there is the problem of cloning, i.e. production of organisms that are absolutely similar in their molecular structure, and also modified in accordance with the requirements of scientists. Cloning entails many moral and ethical problems, the main of which is human cloning.

8. List of literature.

1. Sheppard F. M. Natural selection and heredity. - M .: Education, 1970.

2. Kiseleva E. A. Book for reading on Darwinism. - M .: Education, 1970.

3. Puzanov II Jean Baptiste Lamarck.- M .: Education, 1959.

4. Reznik S. Revealed mystery of being. - M .: Knowledge, 1976.

5. Ruzavin G. I. Concepts modern natural science... –M .: Unity, 2000.

6. Fundamentals of Ecology. / Ed. Obukhova V. L. and Sapunova V. B. -

S.-Pb: Special literature, 1998.

9. Glossary of terms.

Alleligens located in the same place on the chromosome.

A species is a set of living organisms inhabiting a certain ecological niche, having a common structure and physiology and constituting an integral genetic system.

Gametes are female and male germ cells that ensure, during fusion, the development of a new individual and the transfer of hereditary traits from parents to offspring.

Genes are giant molecules that determine the nature of hereditary traits by their structure and interaction with other similar molecules.

DNA (briefly) is a carrier of certain genetic information, certain sections of which correspond to certain genes.

Locus is a specific area on a chromosome.

Chromosome- structural element the cell nucleus, which contains the hereditary information of the organism.

In practice, methods should be developed to establish the degree of risk, either in individual families or by screening all parents. This will change the purpose of medical genetics from retrospective counseling genetics to a forward-looking genetic prevention service. A new attitude towards the responsibility of parents for the reproduction of offspring may arise, which, together with ...

Knowledge about the course of the evolutionary process. Accordingly, attempts are being made to overcome the disagreement between theory and fact. As one of the most criticized general provisions synthetic theory of evolution, you can bring its approach to explaining secondary similarity, that is, the same morphological and functional traits that were not inherited, but arose independently in different lineages ...

Active factors in the biosphere. Therefore, the genetic and hygienic regulation of the content of such factors in environment is an essential component of the prevention of human morbidity. Human genetics at the stage of its formation was designated in our country in the spirit of the times - eugenics. Discussion of the possibilities of eugenics, coinciding in time with the start and rapid development of genetic ...

Mobility from high values intelligence. These are the centers of progress, where the concentration and development of the golden gene pool of the Homo sapiens population takes place. So, apparently, the evolution of mankind goes along three diverging directions. The populations of the third world countries evolve mainly due to natural selection, aimed at increasing their viability in unfavorable economic ...

Gene drift by example

The mechanism of gene drift can be demonstrated with a small example. Imagine a very large colony of bacteria isolated in a drop of solution. The bacteria are genetically identical except for one gene with two alleles A and B... Allele A present in one half of bacteria, allele B - at the other. Therefore, the allele frequency A and B is 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 we reduce the droplet size so that there is enough food for only 4 bacteria. All others die without reproduction. Among the four survivors, 16 allele combinations 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 (the probability of the allele A or B for each surviving bacteria) is multiplied 4 times ( overall size of the resulting population of surviving bacteria)

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

As you can see from the table, in six variants out of 16, 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 variants, where the number of alleles A and B differently slightly higher and amounts to 10/16.

Genetic drift occurs when allele frequencies change in a population 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 of the frequencies changing (the colony undergo gene drift) are higher than the chances of maintaining the original allele frequency. There is also a high probability (2/16) that as a result of gene drift, one allele will be completely lost.

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: in test tubes with food, he put two females and two males of Drosophila flies heterozygous for gene A (their genotype can be written as Aa). 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 normal and mutant alleles. 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 - gene drift.

Literature

• Vorontsov N.N., Sukhorukova L.N. The evolution of the 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. - ISBN 5-03-001602-3

see also

Notes

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• Body (geometry)
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See what "Genes drift" is in other dictionaries:

DRIFT OF GENES - genetic and autosomatic processes, a change in the frequency of genes in a population in a number of generations under the influence of random (stochastic) factors, leading, as a rule, to a decrease in inheritance, variability of populations. Naib, clearly manifests itself when ... ... Biological encyclopedic Dictionary

DRIFT OF GENES - see Genetic drift. Ecological encyclopedic dictionary. Chisinau: Main editorial office of the Moldavian Soviet Encyclopedia. I.I. Grandfather. 1989 ... Ecological Dictionary

gene drift - Changes in gene frequencies in a small population according to the principle of random sampling Biotechnology topics EN genetic drift ... Technical translator's guide

gene drift - gene drift. See genetic automatic process. (Source: "The English Russian Explanatory Dictionary of Genetic Terms." Arefiev VA, Lisovenko LA, Moscow: VNIRO Publishing House, 1995) ... Molecular biology and genetics. Dictionary.

gene drift - genų dreifas statusas T sritis augalininkystė apibrėžtis Atsitiktinis populiacijos genetinės sandaros pakitimas. atitikmenys: angl. genetic drift rus. genetic drift; gene drift ... Žemės ūkio augalų selekcijos ir sėklininkystės terminų žodynas

gene drift - see Genetic drift ... Large Medical Dictionary

Gene drift - processes that determine the change in the frequency of genes, or the frequency of mutant forms in Populations. The term was proposed by the American geneticist S. Wright (1931). The same as Genetico-automatic processes ... Great Soviet Encyclopedia

Gene drift - random (stochastic) changes in gene frequencies in a series of generations occurring in small populations as a result of an error in the selection of gametes during crossing ... Physical Anthropology. Illustrated explanatory dictionary.

Gene drift - - genetic automatic processes - the phenomenon of undirected changes in the frequencies of allelic variants of genes in a population, due to random statistical reasons ... Dictionary of Psychogenetics

Drifting - means a slow constant movement of something. Specifically: Ship drift: Displacement (drift) of the ship off the heading line due to the wind. The drift is characterized by the angle between the track line and the true heading line, to measure this value is used ... Wikipedia

Books

• Biology. 9 11 cl. Biological constructor 3. 0. Interact collection. models. FGOS (CDpc), Vabishchevich A. P. The collection contains 80 virtual experiments and tasks, supplied with detailed guidelines... The models are designed to support the teaching of the following sections of the general ...

The piece of DNA that contains a particular gene is called a locus. It may contain alternative options genetic information - alleles. Any population has a large number of these structures. In this case, the share of a particular allele in the total genome of a population is called the gene frequency.

For a certain mutation to lead to evolutionary changes in species, its frequency must be high enough, and the mutant allele must be fixed in all individuals of each generation. With a small amount of it, mutational changes are not able to affect the evolutionary history of organisms.

In order for the allele frequency to grow, certain factors must act - gene drift, migration, and

Gene drift is a random growth of an allele under the influence of several events, which are combined and have a stochastic nature. This process is associated with the fact that not all individuals in the population take part in reproduction. It is most typical for traits or diseases that are rare, but due to lack of selection, they can be stored in a genus or even a whole population of a small size for a long time. Such a pattern is often traced in a small one of which does not exceed 1000 individuals, since in this case migration is extremely small.

In order to better understand gene drift, you should know the following patterns. In cases where the allele frequency is 0, it does not change in subsequent generations. If it reaches 1, then they say that the gene is fixed in the population. Random gene drift is the result of the fixation process with the simultaneous loss of one allele. Most often, this pattern is traced when mutations and migrations do not cause permanent changes in the constituent loci.

Since the frequency of genes is non-directional, it decreases the diversity of species and also increases the differences between local populations. It should be noted that this is counteracted by migration, in which different groups of organisms exchange their alleles. It must also be said that gene drift has practically no effect on the frequency of individual genes in large populations, but it can become decisive. In this case, the number of alleles changes dramatically. Some genes can be lost irrevocably, which significantly impoverishes genetic diversity.

As an example, we can cite mass epidemics, after which the restoration of the population was carried out practically at the expense of several of its representatives. Moreover, all descendants had a genome identical to their ancestors. In the future, the expansion of allelic diversity was provided by the importation of sires or outbound mating, which contribute to the growth of differences at the gene level.

The extreme manifestation of gene drift can be called the emergence of a completely new population, which is formed from only a few individuals - the so-called founder effect.

It should be said that the patterns of genome rearrangement are studied by biotechnology. - This is the technique of this science, which allows you to transfer hereditary information. At the same time, gene transfer allows you to fight the interspecies barrier, as well as to give organisms the necessary properties.

Due to random statistical reasons.

One of the mechanisms of gene drift is as follows. In the process of reproduction in the population, big number sex cells - gametes. Most of these gametes do not form zygotes. Then a new generation in the population is formed from a sample of gametes that managed to form zygotes. In this case, a shift in allele frequencies relative to the previous generation is possible.

Gene drift by example

The mechanism of gene drift can be demonstrated with a small example. Imagine a very large colony of bacteria isolated in a drop of solution. The bacteria are genetically identical except for one gene with two alleles A and B... Allele A present in one half of bacteria, allele B - at the other. Therefore, the allele frequency A and B is 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 we reduce the droplet size so that there is enough food for only 4 bacteria. All others die without reproduction. Among the four survivors, 16 allele combinations 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

1 2 ⋅ 1 2 ⋅ 1 2 ⋅ 1 2 \u003d 1 16 (\\ displaystyle (\\ frac (1) (2)) \\ cdot (\\ frac (1) (2)) \\ cdot (\\ frac (1) (2) ) \\ cdot (\\ frac (1) (2)) \u003d (\\ frac (1) (16)))

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

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

As you can see from the table, in six variants out of 16, 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 variants, where the number of alleles A and B differently slightly higher and amounts to 10/16.

Genetic drift occurs when allele frequencies change in a population 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 of the frequencies changing (the colony undergo gene drift) are higher than the chances of maintaining the original allele frequency. There is also a high probability (2/16) that as a result of gene drift, one allele will be completely lost.

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: in test tubes with food, he put two females and two males of Drosophila flies heterozygous for gene A (their genotype can be written as Aa). 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 normal and mutant alleles. 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 - gene drift.