NS 02-03

The effect of a Genetic Bottleneck on genetic variability

    [Left] The initial population has a high degree of variability, as illustrated by the variety of colored balls in the box, each one an allele from a haploid individual. [Middle] If the population passes through a strong reduction in population numbers - a genetic bottleneck - both the variety of alleles and their relative proportions are subject to drastic change. (This is also called "sampling error"). In the example, the population is reduced to one red and one blue allele, the black, yellow, and green alleles having been lost by chance. [Right] On recovery of population numbers, there are nine red and seven blue alleles, which shows the effect of genetic drift (Fig. 2.2). Note that drift over several generations could easily have produced a much more biased representation of the original population, for example, 75% green and 25% blue.

    It is important to recognize the similarities and differences between genetic drift and bottlenecks. Drift occurs in all finite populations, and is most noticeable in the smallest, where fluctuations of allele frequencies between generations are most marked. A bottleneck may occur in one generation, by sharp reduction of population numbers as illustrated above, OR over several generations if the population remains small,
OR at the founding of a new population by a small random sample. The last is called Founder Effect, and is a speciation mechanism if the population sample is established in a new environment, subject to modified natural selection acting on a modified gene pool. Founder effect is also exaggerated if the founder population remains small over several generations.

Figure ©2013 by Sinauer; Text material © 2021 by Steven M. Carr