In population genetics, the Wahlund effect refers to reduction of heterozygosity (that is when an organism has two different alleles at a locus) in a population caused by subpopulation structure. Namely, if two or more subpopulations have different allele frequencies then the overall heterozygosity is reduced, even if the subpopulations themselves are in a Hardy-Weinberg equilibrium. The underlying causes of this population subdivision could be geographic barriers to gene flow followed by genetic drift in the subpopulations.
Contents
The Wahlund effect was first documented by the Swedish geneticist Sten Wahlund in 1928.
Simplest example
Suppose there is a population
Case of two alleles and two subpopulations
To make a slight generalization of the above example, let
Let the allele frequency in each population be different, i.e.
Suppose each population is in an internal Hardy–Weinberg equilibrium, so that the genotype frequencies AA, Aa and aa are p2, 2pq, and q2 respectively for each population.
Then the heterozygosity (
which is always smaller than
Generalization
The Wahlund effect may be generalized to different subpopulations of different sizes. The heterozygosity of the total population is then given by the mean of the heterozygosities of the subpopulations, weighted by the subpopulation size.
F-statistics
The reduction in heterozygosity can be measured using F-statistics.