Drift: Introduction


Genetic drift is the random change in allele frequencies by the chance success of some alleles relative to others. Genetic drift is more important in small populations, where chance plays stronger role. If the population size is small enough relative to the strength of selection, genetic drift can sometimes cause slightly deleterious alleles to rise in frequency or beneficial alleles to be lost from a population. Drift can lead to the fixation or loss of alleles, and therefore drift can contribute to the loss of genetic variation. As a consequence, genetic drift in small populations is a source of concern for the future evolutionary potential for some endangered species.

Key Concepts:

  • Alleles may increase or decrease in frequency by chance.

  • The effects of chance on allele frequency change are most pronounced in small populations.

  • Genetic drift tends to lead to fixation or loss of alleles over time, and therefore contributes to the loss of genetic variation.

  • If the population size is small enough relative to the strength of selection, genetic drift can cause the fixation of deleterious alleles or loss of beneficial alleles.

  • Genetic drift can cause genetic divergence between species or populations. Most genetic differences between species are probably due to genetic drift.

  • Genetic drift is nonadaptive and nondirectional.

Keywords: small populations; evolutionary change; fixation; founder events; effective population size; neutral theory

Figure 1.

The effects of genetic drift on 107 laboratory populations of the fruit fly, D. melanogaster. The populations were maintained at a size of 16 individuals, giving a total of 32 possible different alleles (two per individual). The two horizontal axes respectively give the number of generations since the populations were all started (initial allele frequency of 0.5) and the number of mutant brown eye alleles in the population. The vertical axis reports how many of the populations possess that many alleles at a given time period. Looking across all 107 populations shows that genetic drift generates variation among the populations that increases with time, eventually ending in fixation of one or the other of the two alleles. Data from Buri (); figure from Hartl and Clark ().

Figure 2.

The effects of time and population size on populations undergoing genetic drift. Each line shows the allele frequency from generation to generation of a separate population of size either (a) 10 or (b) 100. Each population was started at the same initial allele frequency (0.4). Note that the populations diverge from one another over time as genetic drift takes them in different directions. Also note that the rate of divergence is much less in the larger population. Given enough time, however, even these larger populations will diverge as much as the smaller populations. Thus, population size affects the rate of drift but not necessarily its eventual outcome.



Buri P (1956) Gene frequency in small populations of mutant Drosophila. Evolution 10: 367–402.

Hartl DL and Clark AG (2006) Principles of Population Genetics. Sunderland: Sinauer.

Further Reading

Charlesworth B and Charlesworth D (2010) Elements of Evolutionary Genetics. Greenwood Village: Roberts&Co.

Kimura M (1983) The Neutral Theory of Molecular Evolution. Cambridge: Cambridge University Press.

Lynch M (1996) A quantitative‐genetic perspective on conservation issues. In: Avise JC and Hamrick JL (eds) Conservation Genetics: Case Histories from Nature, pp. 471–501. New York: Chapman & Hall.

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Whitlock, Michael C, and Phillips, Patrick C(Apr 2014) Drift: Introduction. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001698.pub2]