Genetic drift

History of discovery and terminology

The phenomenon of the random change in the gene pool was discovered and described almost simultaneously by several independent groups of researchers. In Russia, a huge contribution was made by N.P. Dubinin and D.D. Romashov, who proposed the term "genetic-automatic processes". Parallel to them, A.S. Serebrovsky described the same phenomenon under the name of "stochastic processes".

In the USA, Sewall Wright studied the problem, and it was he who introduced the classic term "genetic drift" into international practice. In England, similar concepts were developed by the geneticist Ronald Fisher. Today, all these terms are used in population genetics to describe random fluctuations in allele frequencies.

Sewall Wright's concept

The concept formulated in 1931 is also known in science as the "Wright effect". Its central idea is that the gene pool of a small population (deme) is formed as an absolutely random and small sample from the gene pool of the entire source population.

The main causes triggering genetic drift are the mass mortality of a significant portion of individuals or their sudden migration outside their habitual range. The most crucial feature of the Wright effect is that any completely random event in evolution is completely unrepeatable and untestable in the natural environment. A direct consequence of drift is the loss of part of the gene pool, an increase in homozygosity, and a general decline in variability.

Laboratory evidence

Despite the fact that the existence of genetic drift in natural populations has not yet been definitively proven (there are only various estimates of its contribution to evolution), the process has been confirmed experimentally under laboratory conditions.

Sewall Wright conducted a large-scale experiment with the fruit fly Drosophila:

• 108 micropopulations were established, with exactly 8 pairs of flies placed in each test tube.
• The initial ratio of alleles (wild-type and mutant) was set at 0.5.
• The experiment was maintained for 17 generations.

The results of the experiment clearly demonstrated the random nature of gene survival. In 98 test tubes, exclusively the wild allele survived. In 3 test tubes, complete fixation of the mutant allele occurred. And only in 10 micropopulations did both allele variants survive.

Genetic drift in human populations

In human history, the most indicative period for the action of genetic drift is the hunter-gatherer stage. At that era, the population structure consisted of small isolated or semi-isolated groups numbering from 200 to 500 adult individuals. Interbreeding occurred strictly within these closed collectives.

In such isolates, sharp, unpredictable fluctuations in allele frequencies are observed, up to their complete loss due to random events. Classic examples:

1. Polar Eskimos: an abnormally high frequency of the $I^0$ allele has been recorded, reaching 84%.
2. Eskimo tribe in Greenland (Thule region): prolonged complete isolation of a group of 271 people led to the frequency of the $I^A$ allele becoming fixed at 9%.

Genetic drift also includes inbreeding (consanguineous marriage). The genetic mechanism of this phenomenon lies in a sharp increase in homozygosity. The phenotypic consequences of inbreeding are most often extremely unfavorable, as they lead to the manifestation of hidden recessive pathologies.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• The founder effect and population bottleneck effect within the framework of Wright's concept
• Natural mechanisms compensating for the reduction in biodiversity during drift
• The specifics of gene pool loss and the disappearance of rare alleles in demes
• Interspecific hybridization and impaired gene transmission (using the mule from a horse and a donkey as an example)

Other topics in «Elementary evolutionary factors»

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