In areas where malaria is a common disease, it causes high levels of mortality, and being heterozygous at the sickle-cell gene has high fitness. Another example that has been discovered more recently is cystic fibrosis. This could explain why in populations in Europe and of European descent suffer from high levels of cystic fibrosis. Both sickle-cell anemia and cystic fibrosis are much more common, as genetic diseases, in the populations where they occur, than are the recessive genetic diseases discussed above. This is predicted because heterosis results in much higher frequencies of the harmful alleles than does the situation in which the dominant has highest fitness.
Genetic diseases can also evolve through genetic drift.
Various genetic diseases and abnormalities turn out to be more common in small, relatively isolated populations than they are in large populations. This is most likely because they have increased, by chance, through drift. Another phenomenon that we studied when we were considering microevolution was local adaptation. Click here to review the lecture notes on local adaptation. Remember that local adaptation is most likely if there is a little gene flow, at least one individual per generation, to maintain some genetic variation, but not so much gene flow that there are no differences among populations.