A Theory about the Frequencies of the CCR5Δ32 Allele
The high frequency of Δ32 in certain populations suggests a strong selective pressure. Stephens and colleagues hypothesized that a selection occurred approximately 700 years ago, but the recent emergence of HIV means it cannot have been the selective agent. It is possible that the Δ32 allele gave resistance to bubonic plague, which caused fatal epidemics at the time of the predicted selection.

Diseases are known to act as selection agents for human alleles. (See the Human Evolution unit’s online text for the example of malaria as the selective agent for a defective hemoglobin allele, which causes sickle cell anemia when homozygous.)

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