Sewall Wright Effect

ByDr. Girish Chandra

Genetic Drift

(Sewall Wright effect)

               The American scientist Sewall Wright (1930) gave the principles of gene frequency fluctuations in small populations in his essay, Effect of population size. 

           According to genetic drift, in small, non-randomly mating populations gene frequencies are found to fluctuate purely by chance. Smaller the population, larger will be the fluctuation in gene frequency.

            Genetic drift works on the principle of tossing of a coin. If a coin is tossed, then chances of getting heads and tail would be equal only if it is tossed for a large number of times, so that the standard error is low. But if the coin is tossed only few times, then standard error will go up and you may get head or tail any number of time.

            Standard Error = square root of Pxq/n, where P=frequency of dominant gene (or head of a coin), q=frequency of recessive gene (or tail of a coin) and n=number of individuals in a population (or number of times the coin is tossed).

            For example, let us imagine a small population of black hamsters; say only one pair, one MM and the other mutant Mm. If they can produce only 2 offspring, then the chance that the first offspring will be MM is 0.5 and  that the second offspring will also be MM is also 0.5. The chance that both offspring will be MM is reduced to 0.5×0.5=0.25. In such a case the mutation m will be lost forever. Similarly, by chance both offspring can be Mm, in which case the mutation m will have a chance to express in the next generation. Thus the population may drift towards losing or fixing a mutation purely by chance. Gene frequency will continue to fluctuate until one allele is lost and the other fixed.

            Extinction of certain species, which are left with small populations, is known to be due to genetic drift, when lethal mutations are fixed, e.g. passenger pigeon and cheetah became extinct due to genetic drift that fixed lethal mutations. Carnivores usually have small populations and are affected by genetic drift. Human tribes that marry within their own communities also face genetic drift and accumulate lethal mutations.


Founders effect & Bottleneck effect: When a small population migrates to a new area, the frequency of genes is determined by the genetic drift (Mayr, 1963; Sheppard, 1960). For example, American Indians have no ‘B’ group in their blood. However, in Asia, which is the ancestral home of American Indians, ‘B’ group is widespread. The ancestral population of mongoloids that migrated across Bering strait to North America might have been very small and must be having all kinds of blood groups, but due to genetic drift, ‘O’ group has been fixed and ‘B’ group eliminated purely by chance.

           Bottleneck effect is a phenomenon found in animals that follow seasonal cycle of dormancy and activity, such as insects, amphibians and a large number of invertebrates. During breeding season their populations are very large but as the adverse climate arrives majority of individuals are killed and very few manage to find protective shelters and undergo diapause to tide over adverse period. This small population then produces next generation by way of genetic drift.


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