In large populations which contain sub-populations there are fewer homozygotes than in the average for the set of subdivided populations. This is a general, and mathematically automatic, result. The increased frequency of homozygotes in subdivided populations is called the Wahlund effect.
The Wahlund effect has a number of important consequences:
• We have to know about the structure of a population when applying the Hardy-Weinberg principle to it, otherwise there may seem to be more homozygotes than expected from the Hardy-Weinberg principle. We might then suspect that selection, or some other factor, was favoring homozygotes. In fact both sub-populations are in perfectly good Hardy-Weinberg equilibrium and the deviation is due to the unwitting pooling of the separate populations.
• A second consequence of the Wahlund effect is that when a number of previously subdivided populations merge together, the frequency of homozygotes will decrease. In humans, this can lead to a decrease in the incidence of rare recessive genetic diseases when a previously isolated population comes into contact with a larger population. The recessive disease is only expressed in the homozygous condition, and when the two populations start to interbreed, the frequency of those homozygotes goes down.
Why is the frequency of homozygotes higher in populations which contain subdivisions?