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Most red cell polymorphisms that have been considered due to malaria selection are found in high frequencies in populations with endemic malaria, as would be expected from the genetic theory of adaptation. On the other hand, the Duffy negative blood group allele, which leads to an almost complete resistance to vivax malaria, is found in highest frequencies where there is no vivax malaria. This seeming paradox leads to the question of whether vivax malaria has been eliminated from West Africa by the genetic adaptation of the populations to this parasite, or whether the prior-existing high frequencies of the Duffy negative allele have prevented vivax malaria from becoming endemic in West Africa. The temperate climate adaptations of the vivax parasite and its probable primate malaria ancestor point to the latter possibility. Thus, fortuitous genetic variation in human populations may contribute to their differential susceptibility to new infectious diseases and result in “group selection.