Apr 15, 2016

Bugs Versus Drift

via West Hunter

For a while now we have seen occasional articles about how people outside of sub-Saharan Africa may have more genetic load, generated by drift over a population history in which Eurasians generally had a smaller effective population size. This is related to those recent papers with similar but stronger conclusions about Eurasian archaic humans like Neanderthals and Denisovans.

As I mentioned, I kind of doubt that that Neanderthals and Denisovans were all that screwed up – partly because a more complete theory indicates that salvage mutations get easier as you drift away from the optimum and ameliorate the effects of low population size to a surprising degree, but even more because the Neanderthals obviously weren’t all that screwed up, didn’t have their fitness depressed by tens of percent, because they stood off anatomically modern humans for something like 70,000 years after first encounter. Results count. The mechanism makes a lot more sense for Flores hobbits because their population was much smaller, but even more because they actually were screwed up: it shows in their skeletons.

So, just how screwed up do Eurasians look, compared to Africans? I mean, if we’re going to be busy explaining a phenomenon, shouldn’t we bother to make a cursory check to see whether it even exists? I know that probably sounds radical…

Of course there’s no bloody sign of any such thing. Sub-Saharan Africans have shorter lifespans and lower IQs than most Eurasian populations. East Asia has lower genetic diversity than Europeans – so has had lower effective population size over the past few tens of thousands of years – yet those populations have higher average IQs and longer lifespans than Europeans.

On the other hand, it is also the case that strong selection for any particular trait tends to mess up other traits – logical, considering trade-offs. So strong selection for resistance to falciparum malaria has made lots of deleterious variants common in the tropical and subtropical parts of the Old World. We know many that are strongly, obviously bad for you – even lethal – but there are surely many others with milder (but still negative) effects. We know that selection for resistance to sleeping sickness has selected for APOL1 variants that greatly increase the risk of kidney failure, so that African-Americans develop end-stage renal disease (ESRD) about 3.5 times more often than people of European ancestry. Those APOL1 variants also exacerbate kidney disease caused by sickle cell, and there’s an AIDs-related kidney-wrecking syndrome (HIVAN: HIV-associated collapsing glomerulopathy) (while AIDs is of course much more common in blacks) that seems to require those African APOL1 variants – HIVAN is up to 50 times more common in blacks than whites. Those social constructs can sure seem real when they’re shoveling the dirt over your face.

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