Effect of Bottlenecks on genetic variation
(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 large
population is reduced to N = 2 individuals (one male and
one female), there will be a maximum of four alleles per locus.This
sample is necessarily different from 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, rare
alleles may persist through chance, 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. As N
doubles from 10 to 20, 40, 80, ..., 1,280 in the first eight
generations, very few alleles will be lost subsequently. Loss of Hexp
is somewhat greater with a more severe bottleneck (N0 = 2.,
r = 1.0) or for a modest bottleneck with slow subsequent
growth (N0
= 10, r = 0.1). 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 linear, as
genetic diversity is restored by mutation.
