Question

Why has natural selection not eliminated sickle cell anaemia?

Answer 1

Natural selection has not eliminated sickle cell anaemia because of the protective advantage conferred by the heterozygous state against malaria. Individuals who are homozygous for the sickle cell gene suffer from severe anaemia and often die early, but heterozygous individuals have red blood cells that are partially sickled. These cells inhibit the malaria parasite’s ability to survive, giving heterozygotes a higher chance of survival in malaria-endemic regions. Consequently, natural selection maintains this sickle cell allele in the population because it provides a survival advantage against malaria despite its harmful effects in the homozygous condition. This is why the allele persists in populations where malaria is common.