Concerning the notion that there may be important regional differences in genetic load: guys, at this point they’re not hypothetical. Last year they were, and I kept my suspicions to myself, but now they’ve been seen, been measured. It is still early days, so I wouldn’t yet bet my life on their reality, but the odds are high that this pattern is real. Explaining why these differences should not exist is not going to get you very far.
Having more genetic load can’t be good. This we can be sure of. On the other hand, we don’t know just how important it is. Theory says that it is likely very important, particularly in the most complex adaptive systems. Brain function is surely the most complex of all. Once we know how IQ varies with genetic load within some population, we will have a better handle on its importance. As yet we do not really have that info – just a couple of very preliminary measurements – one that suggests that more deletions are bad for your IQ, another that suggests that people who have very high IQs (or are very tall) have less rare variation than average. Note: most rare variation is likely to be bad for you, according to theory. The measurements we would like to see will come from high-accuracy whole-genome (or at least whole-exome) data, but we may not see that info for several months…
None of this implies that there are no other factors influencing the distribution of IQ, or its evolution. Strong selective pressures could keep IQ high in environment A, even if environment A had a somewhat higher mutation rate than environment B. But all else equal – including selection pressures – IQ ought to be lower in a population that has lived for a long time in the environment with the higher mutation rate. By a ‘long time’, I think we’re talking something like a few thousand years, judging from the fact that most mutations that do anything negative at all have a small effect, say 1%. Their effects predominanate, given time, but the characteristic time should be on the order of 100 generations. You might want to look up the meaning of the phrase ‘on the order of’ – this is a rough estimate.
Strong-enough selective pressures could keep any function working well in a high mutation environment (assuming strong truncation selection) , but this is easier for simpler functions. For example, the fraction of kids who are born deaf for genetic reasons is something like 1 in 1500 (maybe a bit lower): while the fraction of kids born with something genetically wrong with their brain, wrong enough to noticeably screw up fitness, must be at least 1%. Probably more than that, if you add up genetic retardation, genetic schizophrenia, etc.
Such differences are compatible with the population with the higher mutational load being better at some things than the population with lower mutation load. Look, people in the tropics are better at withstanding falciparum malaria than Eskimos with zero mutational load would be: Eskimos don’t have any of the genetic malaria defenses. They don’t have those adaptations. If I look at known factors that partially explain high athletic performance in people of African descent – narrower hips, more fast-twitch muscles, a higher frequency of the wild-type version of alpha-actinin-three. more active versions of the androgen receptor, possibly different versions of myostatin – they look like adaptations, rather than differences in the noise level.
Anyhow, they’d better be compatible since A. we’ve found higher mutational load in people of African descent and B. they win the track events.
In fact, if this pattern is real and has existed for a long time, it will change selective pressures. In a high-mutation environment, brain function probably doesn’t deliver as much fitness per cubic centimeter – but those brains cost just as much per cubic centimeter. In that situation, selection would likely favor smaller brains (than it would in a lower-mutation environment). So we would expect to see smaller brains in populations with lower average IQ – not enough to explain all of the lower IQ, but some smaller – and we do. Cranial capacity varies, and much of that variation is explained by the correlation with latitude. Of course there are other possible explanations for this correlation than temperature, and they may be the real explanation, or part of it.