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Genetic drift

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noun, plural: genetic drifts

The drifting of the frequency of a gene variant (allele) relative to that of the other gene variants (alleles) in a population over time as a result of a chance or random event


The genetic drift pertains to the process of change in the frequency of an allele (gene variant) in a population over time. The change is caused by chance or random events (e.g. a disastrous event in a habitat) rather than by natural selection. The effect of genetic drift in large populations is usually negligible whereas in small populations, it predominates. In a small population, genetic drift results in some alleles becoming more common while others becoming less over time; thus, it results in a seemingly drifting apart of the frequencies of genetic variants in a population.

An example where the effect of genetic drift is magnified is the so-called bottleneck effect. For instance, an island inhabited by a large population of a particular snake species is hit by a calamitous event resulting in a substantial decrease in their population. With only few surviving snakes, the gene pool is affected and the allelic frequency would eventually change. Over time, the genetic composition of the re-establishing population of these snakes would be different from that of the initial population. There is a possibility that in the new snake population a dominant allele declines in terms of frequency (or is eventually lost) while the recessive allele is relatively becoming more common over time.


  • allelic drift
  • Sewall Wright effect

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