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Natural selection can maintain balanced polymorphisms. A balanced polymorphism is one in which two or more phenotypes are maintained in fairly stable proportions over many generations. Natural selection preserves balanced polymorphisms when heterozygotes have higher relative fitness, when different alleles are favored in different environments, and when the rarity of a phenotype provides a selective advantage. A balanced polymorphism can be maintained by heterozygote advantage, when heterozygotes have higher relative fitness than either homozygote. One example of heterozygote advantage is maintenance of the HbS allele. When heterozygous HbA/HbS individuals contact malaria, their infected red blood cells assume the same sickle shape as those of homozygous HbS/HbS individuals. The sickled cells lose potassium, killing the parasites, which limits their spread within the infected individual. Heterozygous individuals often survive malaria because the parasites do not multiply quickly inside them; their immune system can effectively fight the infection; and they retain a large population of uninfected red blood cells. Homozygous HbA/HbA individuals are also subject to malarial infection, but because their infected cells do not sickle, the parasites multiply rapidly, causing a severe infection with a high mortality rate. Therefore, HbA/HbS heterozygotes have greater resistance to malaria and are more likely to survive severe malarial infection.