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| Description: In this pattern of natural selection, genetic diversity is maintained in a population. This example shows balancing selection maintaining two alleles of the ?-globin gene, designated HA and HS, in human populations in Africa. This situation occurs through heterozygote advantage, because the heterozygous condition confers resistance to malaria. (a) The geographic prevalence of malaria in Africa. (b) The frequency of the HS allele of the ?-globin gene in the same area. In the homozygous condition, the HS allele causes the disease sickle-cell anemia. However, this sickle-cell allele is maintained in human populations as a balanced polymorphism, because in areas where malaria is prevalent, the heterozygote carrying one copy of the HS allele has a higher fitness than either of the corresponding homozygotes (HAHA and HSHS).
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