Charkraborty et al 1975

Effect of Bottlenecks on heterozygosity (Chakraborty et al. 1975)

    A population bottleneck occurs when a large population undergoes a drastic decrease in size. The expected effect is a loss of variation, measured as decrease in heterozygosity and loss of rare alleles. For example, if a population is reduced to N = 2 individuals (one male and one female), there will be a maximum of 4 alleles per locus, This sample will almost certainly not represent the allele frequencies in the pre-bottleneck population, and most or all of the rare alleles in that population will not make it through the bottleneck. In some cases, a rare allele that persists through chance may generate a population with novel genetic characteristics.

    Chakraborty et al. (1975) modeled bottleneck effects on heterozygosity for four combinations of size (N0 = 2 or 10) and population growth (r = 0.1 or 1.0, where r is the reproductive rate, such that r=1 means the population is doubling each generation). An initial Hobs = 0.14 is assumed. In the mildest case, a bottleneck of
N0 = 10 followed by a doubling in population size each generation (r = 1.0) has minimal impact on Hobs: the initial sample of 2N = 20 alleles / locus is a reasonable sample of the pre-bottleneck variation, and if N doubles from 10 to 20, 40, 80, ..., 1,280 in the first eight generations, very few alleles will be lost subsequently. The same is true for a more severe bottleneck (N0 = 2): although rare alleles are lost, if N doubles to 1,024 over ten generations, recovery is rapid. Only an extreme bottleneck (N0 = 2) with slow growth subsequently (r = 0.1) has a severe impact on heterozygosity. Every generation in which N remains < 10 is in effect another bottleneck. Variation in all cases is recovered by new mutation: Note that the time scale is logarithmic: the increase in heterozygosity over time is actually a straight line as genetic diversity is restored by mutation.

Text material © 2021 by Steven M. Carr