In order for the allele frequency to increase, certain factors must act - genetic drift, migration and natural selection.
Genetic drift is the random non-directional growth of an allele when exposed to multiple events. This process is associated with the fact that not all individuals in the population take part in reproduction.
Sewall Wright called gene drift in the narrow sense of the word a random change in the frequency of alleles during a change of generations in small isolated populations. In small populations, the role of individuals is great. The accidental death of one individual can lead to a significant change in the allele pool. The smaller the population, the more likely it is to fluctuate - a random change in allele frequencies. In ultra-small populations, for completely random reasons, a mutant allele can take the place of a normal allele, i.e. going on random commit mutant allele.
In domestic biology, a random change in the allele frequency in ultra-small populations was for some time called genetic-automatic (N.P. Dubinin) or stochastic processes (A.S. Serebrovsky). These processes were discovered and studied independently of S. Wright.
Gene drift has been proven in the lab. For example, in one of S. Wright's experiments with Drosophila, 108 micropopulations were established - 8 pairs of flies in a test tube. The initial frequencies of the normal and mutant alleles were 0.5. During 17 generations, 8 pairs of flies were randomly left in each micropopulation. At the end of the experiment, it turned out that only the normal allele was preserved in most test tubes, both alleles were preserved in 10 test tubes, and the mutant allele was fixed in 3 test tubes.
Genetic drift can be considered as one of the factors in the evolution of populations. Due to drift, allele frequencies can randomly change in local populations until they reach an equilibrium point - the loss of one allele and the fixation of another. In different populations, genes "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 factors of evolution, primarily with 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.
Causes of Genetic Drift
Population waves and gene drift
An example is the situation with cheetahs - representatives of cats. Scientists have found that the genetic structure of all modern cheetah populations is very similar. At the same time, genetic variability within each of the populations is extremely low. These features of the genetic structure of cheetah populations can be explained if we assume that relatively recently (a couple of hundred years ago) this species passed through a very narrow neck of abundance, and all modern cheetahs are descendants of several (according to American researchers, 7) individuals.
Fig 1. Bottleneck effect
bottle neck effect played, apparently, a very significant role in the evolution of human populations. The ancestors of modern people settled all over 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.
founder effect. Animals and plants, as a rule, penetrate into territories new to the species (to islands, to new continents) in relatively small groups. The frequencies of certain alleles in such groups may differ significantly from the frequencies of these alleles in the original populations. Settlement in a new territory is followed by an increase in the number of colonists. Numerous populations that arise reproduce the genetic structure of their founders. This phenomenon was called by the American zoologist Ernst Mayr, one of the founders of the synthetic theory of evolution. founder effect.
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Fig. 2. The frequency of allele B according to the AB0 blood group system in human populations
The founder effect apparently played a leading role in the formation of the genetic structure of animal and plant species inhabiting volcanic and coral islands. All of these species are descended from very small groups of founders who were lucky enough to reach the islands. It is clear that these founders were very small samples from parental populations, and the allele frequencies in these samples could be very different. Let us recall our hypothetical example with foxes, which, drifting on ice floes, ended up on uninhabited islands. In each of the daughter populations, the allele frequencies differed sharply from each other and from the parent population. It is the founder effect that explains the amazing diversity of oceanic fauna and flora and the abundance of endemic species on the islands. The founder effect has also played an important role in the evolution of human populations. Note that the allele IN completely absent from the American Indians and from the Aborigines of Australia. These continents were inhabited by small groups of people. Due to purely random reasons, among the founders of these populations there could not be a single carrier of the allele IN. Naturally, this allele is also absent in derived populations.
Long term isolation
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.
The members of the Amish sect in Lancaster County, Pennsylvania, numbering approximately 8,000 by the mid-nineteenth century, were almost all descended from three married couples who immigrated to America in 1770. This isolate contained 55 cases of a special form of dwarfism with polydactylism, which is inherited in an autosomal recessive manner. 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).
blood groups MN in the Dunker population
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. The genetic drift is also supported by the fact that the concentration of alleles in the gene pool of American Dunkers that control the development of obviously biologically neutral traits, for example, hairiness of the middle phalanx of the fingers, the ability to put the thumb aside, has increased (Fig. 3).
Rice. 3. Distribution of neutral traits in the Pennsylvania Dunker isolate:
A-hair growth on the middle phalanx of the fingers,b-ability to extend the thumb
3. The Importance of Genetic Drift
The consequences of genetic drift can be different.
First, the genetic homogeneity of the population may increase, i.e. her homozygosity. In addition, populations that initially have a similar genetic composition and live in similar conditions may, as a result of the drift of various genes, lose their original similarity.
Secondly, due to genetic drift, contrary to natural selection, an allele that reduces the viability of individuals can be retained in the population.
Thirdly, due to population waves, a rapid and sharp increase in the concentrations of rare alleles can occur.
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.
The consequences of genetic drift, which are of interest to medicine, are the uneven distribution of certain hereditary diseases among the population groups of the globe. Thus, the isolation and drift of genes apparently explains the relatively high frequency of cerebromacular degeneration 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. However, it should be remembered that the formation of favorable combinations of alleles is also possible. As an example, consider the genealogies of Tutankhamun (Fig. 12.6) and Cleopatra VII (Fig. 4), in which closely related marriages were the rule for many generations.
Tutankhamen died at the age of 18. An analysis of his image as a child 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 that excludes the absorption of gluten. Tutankhamun was born from the marriage of Amenophis III and Sintamone, who was the daughter of Amenophis III. Thus, the pharaoh's mother was his half-sister. 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 Ankesenamon (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 own sister, which was preceded by consanguineous marriages for at least six generations.
Rice. Fig. 4. Pedigree of the pharaoh of the XVIII dynasty Tutankhamun Fig. 5. Pedigree of Cleopatra VII
GENE DRIFT
This concept is sometimes called the "Sewell-Wright effect" after the two population geneticists who proposed it. After Mendel proved that genes are the 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 the change in the frequency of alleles in small populations is 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 on the basis of a theoretical model. Genetic drift is one of the factors that disrupt the Hardy-Weinberg equilibrium.
Large populations with random interbreeding are greatly affected 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 by natural selection. In small populations, other processes are going on and they are influenced by other factors. For example, in small populations, the probability of a random change in the frequency of genes is high. 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 accidental disappearance or appearance of an allele of a gene in the 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 crosses 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 is understood as random changes in gene frequencies caused by a small population size and infrequent interbreeding. Genetic drift is observed among small populations, for example, in island settlers, koalas or giant pandas.
See also the articles “The Bottleneck Effect”, “Hardy-Weinberg Equilibrium”, “Mendelism”, “Natural Selection”.
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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 the allele, the less efficient the drift of genes.
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 their appearance (tall, sharp, 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.
Caused by random statistical causes.
One of the mechanisms of genetic drift is as follows. In the process of reproduction in the population, a large number of germ cells - gametes are formed. 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 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) multiplied 4 times (total size of the 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 has been 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 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
Nikolai Petrovich Dubinin The area of scientific interests of N. P. Dubinin was general and evolutionary genetics, as well as the application of genetics in agriculture. evolutionary genetics Together with A. S. Serebrovsky, he showed the fragmentation of the gene, as well as the phenomenon of gene complementarity.A. S. Serebrovsky of the gene of complementarity Published a number of important scientific works on the structure and functions of chromosomes, showed the presence of a genetic load of lethal and sublethal mutations in populations. Chromosomes of the genetic load of mutations. He also worked in the field of space genetics, on the problems of radiation genetics.
Genetic drift as a factor in evolution Due to drift, allele frequencies can randomly change in local populations until they reach a point of equilibrium - the loss of one allele and the fixation of another. In different populations, genes "drift" independently. 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.
Genetic Drift as a Factor of Evolution At 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. 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 the allele A is fixed in one population, and the allele a is fixed in the other, then the migration of individuals between these populations leads to the fact that allelic diversity reappears within both populations.
Population Waves and Genetic Drift Populations rarely remain constant over time. Booms are followed by recessions. S.S. Chetverikov was one of the first to draw attention to periodic fluctuations in the number of natural populations, population waves play a very important role in the evolution of populations.
Sergei Sergeevich Chetverikov () an outstanding Russian biologist, evolutionary geneticist, who took the first steps towards the synthesis of Mendelian genetics and the evolutionary theory of Charles Darwin. He organized an experimental study of hereditary properties in natural animal populations before other scientists. These studies allowed him to become the founder of modern evolutionary genetics geneticist evolutionist
Population Waves and Genetic Drift During periods of a sharp decline in numbers, the role of genetic drift greatly increases. At such moments, it can become a decisive factor in evolution. During a recession, the frequency of certain alleles can change dramatically and unpredictably. There may be a loss of certain alleles and a sharp impoverishment of the genetic diversity of populations. Then, when the population begins to grow, the population will from generation to generation reproduce the genetic structure that was established at the time of passage through the “bottleneck” of the population.
Bottleneck effect in real populations Example: Situation with feline cheetahs. Scientists have found that the genetic structure of all modern cheetah populations is very similar. At the same time, genetic variability within each of the populations is extremely low. These features of the genetic structure of cheetah populations can be explained by assuming that relatively recently this species passed through a very narrow neck of abundance, and all modern cheetahs are descendants of several (according to American researchers, 7) individuals.
A modern example of the bottleneck effect is the saiga population. Saiga antelopes have declined by 95% from about 1 million in 1990 to less than in 2004, mainly due to poaching for traditional Chinese medicine Saiga saiga 1990 2004
Year American bison population to individuals individuals individuals
Founder effect Animals and plants, as a rule, enter territories new to a species in relatively small groups. Allele frequencies in such groups may differ significantly from the frequencies of these alleles in the original populations. Settlement in a new territory is followed by an increase in the number of colonists. Numerous populations that arise reproduce the genetic structure of their founders. The American zoologist Ernst Mayr, one of the founders of the synthetic theory of evolution, called this phenomenon the founder effect.
It is clear that the founders were very small samples of parental populations and the allele frequencies in these samples could be very different. It is the founder effect that explains the amazing diversity of oceanic fauna and flora and the abundance of endemic species on the islands. The founder effect has also played an important role in the evolution of human populations. Note that the B allele (according to the AB0 blood group system) is completely absent in American Indians and Australian Aborigines. These continents were inhabited by small groups of people. Due to purely random reasons, there could not have been a single carrier of the B allele among the founders of these populations. Naturally, this allele is also absent in derived populations.
Genetic Drift and the Molecular Clock of Evolution The end result of genetic drift is the complete elimination of one allele from a 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 genetic drift. Calculations show that the probability of fixing a neutral allele is equal to its frequency in the population.
Regularity Large populations "wait" for a short time for the mutational emergence of a new allele, but fix it for a long time. Small populations “wait” for a very long time for a mutation to occur, but after it has arisen, it can be quickly fixed. This leads to a seemingly paradoxical conclusion: the probability of fixation of neutral alleles depends only on the frequency of their mutational occurrence and does not depend on the size of populations.
Regularity The more time has passed since the moment of separation of two species from a common ancestral species, the more neutral mutational substitutions distinguish these species. On this principle, the method of "molecular clock evolution" is built - determining the time elapsed from the moment when the ancestors of different systematic groups began to evolve independently of each other.
Regularity American researchers E. Tsukurkendl and L. Polling discovered for the first time that the number of differences in the sequence of amino acids in hemoglobin and cytochrome c in different species of mammals is the greater, the earlier their evolutionary paths diverged.
