On average every 25 years or so, London saw about a tenth or more of its population simply wiped out

It is odd what elements get thrown together in a wide reading pattern.

Tyler Cowen endorses London: A Social and Cultural History, 1550-1750, by Robert O. Buchholz and Joseph P. Ward in his brief note and mentions that:

…between 1563 and 1665, on average every 25 years or so, London saw about a tenth or more of its population simply wiped out.

A little earlier I was reading the article Why is the IQ of Ashkenazi Jews so High? - 20 Possible Explanations by Hank Pellissier. At least six of the explanations explicitly involve the IQ consequences of population winnowing (Babylonian Eugenics, Mandatory Schools for Males, Clever Clerics Propogate, Breeding for Brains, Squeezed Into Brilliance, and Winnowed by Persecution). For example,

Mandatory Schools For Males - In 64 A.D., the high priest Joshua ben Gamla issued and implemented an ordinance mandating schools for all boys, beginning at age 6. Within 100 years, Jews had established universal male literacy and numeracy, the first ethnicity in history to achieve this.

The progressive, demanding edict created a huge demographic shift. The high, oft-times prohibitive cost of educating children in the subsistence farming economy of the 2nd to 6th centuries prompted numerous Jews to voluntarily convert to Christianity, leading to a decline in Jewish population from 4.5 million to 1.2 million.

I am leary of Just-So explanations but that doesn't mean that some of them might not be right.

Then there is the argument by Gregory Clark in Farewell to Alms, (as summarized by Paul Seabright):

Gregory Clark argues that the Industrial Revolution was the gradual but inevitable result of a kind of natural selection during the harsh struggle for existence in the pre-industrial era, in which economically successful families were also more reproductively successful. They transmitted to their descendants, culturally and perhaps genetically, such productive attitudes as foresight, thrift, and devotion to hard work.

Finally, this sentence in Steve Olson's Mapping Human History (which I am enjoying), leapt out as part of the emerging pattern.

Human evolution has not been a straightforward slog from lower to higher. It's been a maze of dead ends, unexpected detours, and sudden changes of direction. Many of the fossils that we have assumed belong to our ancestors probably represent failed evolutionary experiments, lineages of different kinds of humans that did not survive. In the end, we are the product of a relentless winnowing process, a trial by extinction.

I am skeptical of the exact role that IQ plays in success; probably necessary but not sufficient - sufficiency being some combination of values, behaviors and decision-making.

These disparate articles/books are a good reminder, however, of just how brutal, extensive and frequent exogenous events were in winnowing the half of the bell curve that makes the top half possible. In the 80 generations since the Romans, Britain (as an example) has probably suffered more than a dozen exogenous events (plague, famine, invasion, etc.) resulting each time in a reduction in population of 5-30%. I think in most of those events, the impacts were asymmetric with the wealthy and smart always having a greater probability of surviving. That is a lot of culling of the herd and I have to acknowledge that it probably did have some effect.