[jmopp]

Sickle-cell trait is found in sub-Saharan Africans from tropical regions. It is rare in South Africa, most of which is malaria-free and thus has no benefit to the mutation.

> But you're supposed to believe that the same genetic variation that causes all of these physical differences between groups has no effect at all on behavioral differences.

Well, yes. None of those differences you mentioned are in the brain. Why should they lead to behavioural differences?

Also: culture is not nearly as conserved as genetics. Populations can change languages within a few generations, and most culture shifts come from wanting to assimilate into the culture of the perceived elites, rather than population replacement or large-level mixing. The exceptions (European settlement in the New World, the Bantu migration, and possibly some others that don't immediately come to mind) are notable because they are the exceptions to the norm.