In genetics the Wahlund effect is a reduction of heterozygosity
in a population.It is caused by a change in the subpopulation
structure.If two or more subpopulations have different allele
frequencies then the overall heterozygosity gets reduced even if
they satisfies Hardy Weinberg equilibrium.
FOR EXAMPLE;
A Wahlund effect typically occurs for organisms for which
population subdivision is unknown. Suppose there is a population of
worms whose subdivision is unknown with allele frequencies of A and
a given by p and q respectively.If this population is split into
two equally-sized subpopulations and that all the A alleles are in
subpopulation and all the a alleles are in subpopulation to each
other.Then, there will be no heterozygotes even though the
subpopulations are in a Hardy Weinberg equilibrium.