French Canadian researchers have shown that natural selection has noticeably sped up reproduction among the inhabitants of Île aux Coudres, an island in the St. Lawrence River – in less than 150 years. Between 1799 and 1940, the age at which women had their first child dropped from 26 to 22, and analysis shows this is due to genetic change.
This has to be the case for French Canada generally. There were about 5,000 permanent settlers, including 1600 women: they account for about 90% of the ancestry of the 7 million Francophones in Quebec (along with a substantial number in New England). During that rapid expansion, genes that favored fast reproduction surely increased in frequency. Today the French of Quebec must differ significantly (in those genes that influence this trait) from people in France, which has had relatively slow population growth. Slower reproduction must be favored – lead to greater fitness – in a more Malthusian society. Obviously the opportunity for large family size are rare in such resource-limited situations, and costs incurred as a consequence of a high-fertility phenotype detract from fitness.
The same must be the case for old American types whose ancestors – Puritans, for example – arrived early and went through a number of high-fertility generations in colonial days. It’s likely the case for the Mormons, who are largely descended from New Englanders. I’ve heard of odd allele frequencies in CEU (involving FSH) that may relate to this.
Something similar must be true of the Boers as well.
I would guess that a similar process operated among the first Amerindians that managed to get past the ice in North America. America south of the glaciers would have been a piece of cake for anyone tough enough to make a living as a hunter in Beringia – lush beyond belief, animals with no experience of humans. Truly, the Happy Hunting Ground. Selection would have favored high reproduction rates. That process would have reversed when the population saturated. Of course that probably followed a real crash, when they wiped out the megafauna. You might see signs of that old high-reproduction phase in ancient DNA.
The Amish must have experienced simultaneous selection for high reproductive rate and an unusual personality type – individuals that prefer the Amish way of life to that of outsiders.
Amish in Lancaster County, Ashkenazi Jews as a merchant/scribe caste, Tibetans & altitude, pastoralists and lactase tolerance, hunter-gatherers -> peasants : every significant, lasting ecological change creates an altered human population, different in both body and mind.