A number of people have argued that there can’t be real evolutionary change in less than some huge amount of time – 50k years, 100k years, basically whatever number they need to win an argument. That’s all bullshit. On the other hand, some people seem to think that you can get a lot of change in one generation without particularly trying. That, too, is bullshit.
When someone suggests that evolutionary change is the reason that behavior X or trait Y has changed noticeably (in a particular population) since the 1950s – they’re wrong. Roland Fryer has argued that the rigors of the Middle Passage selected for salt sensitivity in African-Americans. But he’s for-sure wrong. First, there is no evidence that a higher-than-average tendency to hold on to salt would make much difference in someone’s chance of survival on a slave ship. Lots of the mortality had nothing to do with salt retention: why would it protect you against smallpox or pneumonia? Second, even if there was, it wouldn’t have a huge effect, partly because mortality on slave ships wasn’t that high – around 15% in the 17th century, not that much higher than among immigrants from Europe in that period (~10%). If they lost the bottom 15% of the people (in ways related to salt retention) during the Middle Passage, a cutoff of about one std below average, the increase in salt retention would be about a tenth of a standard deviation, assuming a narrow-sense heritability of 50%. You’d never notice the difference.
Along the same lines, Charles Murray, in an essay in Commentary, explains higher-than-average IQs among the Jewish people as a whole (not just among the Ashkenazim) as caused in part by the exile to Babylon. Nebuchadnezzzar “carried into exile all Jerusalem: all the officers and fighting men, and all the craftsmen and artisans… only the poorest people of the land were left” (2 King 24:10). Murray suggests that this group was an elite, and that later the Jews were largely descended from returned exiles. First, non-Ashkenazi Jews do not even have higher-than-average IQs. There is not much reason to think that they ever did. Certainly no one in Classical times ever had that impression.
Second, if you take along the army and all the potters and shoemakers, they’ll way outnumber anyone you could call an intellectual elite – it’s not a strong selective event.. Third, we have no reason to believe that most of the ancestry of the Jews in Israel in, say, Roman times was primarily from returned exiles. It certainly can’t be true today: about half of Ashkenazi ancestry is actually European, probably Italian as much as anything else. Sephardic Jews are also around half-European, although those were probably different Europeans.
In using an ancient and poorly-documented possibly-selective one-generation event as an explanation of a general phenomenon which does not even exist, Murray makes Fryer look like a genius. By comparison only, because Fryer’s argument has zero chance.
How much could IQ have changed, since the 1950s, due to selection? 2 points would be a stretch. Now replacement of one population by another can happen in that time, certainly locally, and then you may be comparing populations that have been subject to different selective pressures for as much as a hundred thousand years. They can be as different as all get out. I know of actual live people who are convinced that the schools in Brooklyn have gone to hell, since the black and Hispanic kids of today seem to be doing so much worse academically than the Jewish kids of yesteryear. Really. They think that.
Selection changing IQ at half a point a generation can happen – is happening - and can create big, interesting differences in a thousand years. But we must remind everyone that 1000 is considerably bigger than 25: it is right to worry about what happens if this goes on, but it is numerically incorrect to pretend that all that much has actually happened so far within populations, since the birth of effective medicine, the welfare state, feminism, or whatever trend you’re worried about.