genetic drift

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genetic drift:

see geneticsgenetics,
scientific study of the mechanism of heredity. While Gregor Mendel first presented his findings on the statistical laws governing the transmission of certain traits from generation to generation in 1856, it was not until the discovery and detailed study of the
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Genetic Drift


(in Russian, spontaneous genetic processes), the random processes that determine the changes in frequency of various alleles in a population.

In large, freely interbreeding populations free of selection and mutation pressure, the allelic ratio should persist in all generations, independent of absolute original frequency. In real populations of limited numbers, however, gene frequency does not remain constant both because of mutation and selection pressure and because of random deviations. Genetic drift has been analyzed in detail by the Soviet geneticists N. P. Dubinin (1931) and N. P. Dubinin and D. D. Romashov (1932), the English geneticist R. Fisher (1931), and the American geneticist S. Wright (1931). Random fluctuations in allele frequency in a population are due to the fact that the distribution of alleles between gametes and the combining of gametes in a zygote are random processes. Genetic drift does not have a systematic effect because allele frequency may increase or decrease in various generations. A purely random stabilization (homozygosity) or elimination of alleles may occur in small populations or in populations that are breaking up (because of isolation mechanisms) into individual subgroups; new, stabilized gene combinations appear fairly quickly as a result. Genetic drift is most evident during the formation of new isolated populations. For example, the Mennonite sect in Lancaster, Pennsylvania, which numbers about 8,000 persons, has a significant percentage of polydac-tylous dwarfs (13 percent of the Mennonites are heterozygous for the gene which, in the homozygous state, is reponsi-ble for this kind of dwarfism). The reason for this large percentage is that members of the sect marry only among themselves; their isolation is conducive to the appearance of homozygous individuals. Genetic drift cannot bring about the stabilization or elimination of alleles in large populations because the effect of these processes is compensated by various factors in the following generations or in the different subdivisions of the population. The theory of genetic drift has accounted for the genetic consequences of isolation, the fate of recessive mutations at low concentrations, and the evolution of populations with respect to neutral characters. It explains many of the racial differences in man, which have arisen without selection. The term “genetic drift,” coined by S. Wright, is now widely used along with the synonymous “spontaneous genetic processes.” Emphasizing the role of its statistically random patterns, the Soviet geneticist S. S. Chetverikov suggested that the phenomenon be called “stochastic genetic processes.”


Dubinin, N. P. Evoliutsiia populiatsii i radiatsiia. Moscow, 1966. Pages 421-33.


genetic drift

[jə¦ned·ik ′drift]
The random fluctuation of gene frequencies from generation to generation that occurs in small populations.
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High proportion of GD loss due to the random genetic drift was observed.
Like 'selection', which is not the sole alternative to 'random genetic drift alone', it is rather a member of a whole family of hypotheses which together constitute the alternative.
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With constant mutational effects equal to s = 0.05, as used in this study, such a population would be highly vulnerable to deleterious mutation accumulation because all mutant alleles would be subject to random genetic drift while also having substantial effects on individual fitness.
In the case of the less constrained pleiotropic system, part of the loci are now only submitted to random genetic drift and thus ought to be affected by the size of the population independently of the product Nih.
Under the assumption that the allele frequencies in the lines represent those in the parental populations, this indicates that not only random genetic drift but also selection was responsible for the divergence of the two populations.
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In contrast, Mayr emphasized the role of random genetic drift in producing reproductive isolation and morphological innovations.
Problems associated with finite population sizes, such as the fixation of undesirable alleles due to random genetic drift, have been demonstrated theoretically and empirically to affect response to selection and ultimately, the limit to selection response (Robertson, 1960; Frankham et al., 1968; Baker and Curnow, 1969; Enfield, 1980).
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