Genetic drift

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As genetic drift (genetic drift, the low German word drift is related to the German drive , also Alleldrift or Sewall-Wright effect called) is referred to in the population genetics a random change in the allele frequency within the gene pool of a population . Gene drift is an evolutionary factor . The gene shift represents a quantitative expansion , in which entire segments of genes are exchanged together. This often results in particularly pronounced functional and qualitative changes.

Gene drift and gene shift represent a kind of complement to natural selection . Natural selection has no random influence on the change in the gene frequency of a population, but is directly linked to the survival and reproductive success of individuals, i.e. their adaptation to their environment. The genetic drift or shift, on the other hand, has no such causes, but is purely random ( stochastic ).

Since a random change in the gene frequency is statistically more significant in smaller populations, the gene drift and gene shift represent an important factor in the evolution of founder populations and thus speciation . It is based on the fact that a cut off random population living in a certain area only possesses a small section of the possible allele frequencies, which are also in a different relationship to one another than in the overall population. The evolutionary further development of this population depends on these shifted gene frequencies.

The bottleneck effect is a special type of genetic drift in which the size of the population is greatly reduced by a random event, thereby reducing the variability that occurs in the population . The allele frequencies afterwards usually differ from those of the original population; the reduced genetic diversity makes future adaptive changes more difficult.

Genetic drift can also occur in larger panmictic populations after being divided into smaller subpopulations. The prerequisites are random changes in genes and the passing on of the changed genes.

Expansion of meaning: The spread of such changes in larger populations is also referred to as genetic drift. Today, gene drift is also used to describe the penetration of genes into other areas, whether intentionally or accidentally.

Gene frequencies

From the perspective of population genetics , gene drift is a probability effect. The genes that are passed on to the next generation are not a complete copy of the genes of the successful members of the parent generation. They are a random ( stochastic ) selection, a sample with random fluctuations. The allele frequency in the parent population differs from that in the child population due to random statistical fluctuations . The smaller the population, the more noticeable genetic drift is. This has statistical reasons. Example: When throwing coins, heads or tails appear on average with the same probability. However, if there are only a few throws, it is unlikely that heads and tails will appear with the exact same frequency. The greater the number of litters, the closer one comes to a ratio of 50:50. Therefore, the fluctuations in the gene frequencies are greater in small populations than in populations with many individuals ( effective population size ).

If the frequency of an allele increases or decreases sharply in the successive generations, then an allele in the population can disappear entirely or it becomes the only allele in the population (fixation). The genetic diversity is reduced, the gene pool is depleted.

Gene drift and gene shift against natural selection

Gene drift or gene shift and natural selection are evolution factors and work at the same time. They change the composition of the gene pool . The frequency of alleles (gene variations) and thus the predominant phenotypic characteristics in a population change over time. In the case of gene drift and gene shift, the change in the frequency of the alleles is independent of whether they are beneficial or disadvantageous for the phenotype . Genetic drift is to be considered random and independent of genetic fitness . In contrast to this, those phenotypic traits and thus those alleles which increase genetic fitness are preferred in natural selection. In large populations in which the genetic drift is small, even with low selection pressure, natural selection will have the greater amount of altering gene frequencies. In small populations, on the other hand, the larger statistical fluctuations due to genetic drift will superimpose the changes due to selection.

Gene drift and gene shift in populations

Gene shift and gene drift can have profound and often bizarre effects on the evolutionary history of a population. This can even lead to the extinction of a population. When a population shrinks to a small size and then grows again (this is believed to have happened during human evolutionary history), then genetic drift can lead to sudden and dramatic changes in gene frequency, regardless of natural selection. On such occasions, many beneficial adaptations can be lost ( genetic bottleneck ). Gene defects associated with a genetic bottleneck are occasionally partially compensated for by purging .

The founder effect occurs in a similar way in migratory populations, for example, in which only a few individuals with a rare allele composition form the starting point of a new population. Here the gene frequencies can contradict the previous natural selection. The founder effect is sometimes blamed for the increased occurrence of hereditary diseases .

Gene shift in virology

Some viruses are particularly unstable genetically, e.g. B. HIV as well as the causative agents of influenza and “flu-like” colds. Influenza virus gene shifts are considered to be the starting point of a pandemic like the Spanish flu of 1918 and the Asian flu of 1957.

See also

literature

  • Andrew Cockburn: Evolutionary Ecology . Gustav Fischer, Stuttgart - Jena - New York 1995, ISBN 3-437-30775-4 .
  • Douglas J. Futuyma: Evolutionary Biology . Birkhäuser, Basel - Boston - Berlin 1990, ISBN 3-7643-2200-4 .
  • Wilhelm Nultsch: General botany . 11th edition. Georg Thieme Verlag, Stuttgart - New York 2001.

Web links

Wiktionary: Gendrift  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ Charlotte Avers: Process and Pattern in Evolution . Oxford University Press, 1989.