As Peter Ralph and Graham Coop have shown, the multiple origins in Africa of sickle cell mutations and of mutations that allow people to digest cow’s milk imply that the rate of migration among these populations—even in parts of sub-Saharan Africa less than a couple of thousand kilometers from each other—has been extraordinarily low since the need for these mutations arose. As a result, the most efficient way for evolutionary forces to spread beneficial mutations has often been to invent mutations anew rather than to import them from other populations.
