Allelic histories: positive selection on a HIV-resistance allele

The CCR5-Δ32 allele crucially determines the course of HIV infection and appears to be highly protective against the disease. Population genetic studies suggest that the allele has been under positive selection in Europe in the past. In a recent paper, Alison Galvani and Montgomery Slatkin collate the available evidence and use a mathematical model to strongly suggest that smallpox could have exerted sufficient selection pressure to explain the distribution of the allele across Europe. This is a beautiful example of the power of mathematical models in evolutionary genetics.