Recent selection for height
As noted by Steve Hsu and Razib Khan, a new paper in Nature Genetics reports evidence of recent selection on existing variation in height in European populations. The paper’s authors summarise as follows:
In summary, we have provided an empirical example of widespread weak selection on standing variation. We observed genetic differences using multiple populations from across Europe, thereby showing that the adult height differences across populations of European descent are not due entirely to environmental differences but rather are, at least partly, genetic differences arising from selection. Height differences across populations of non-European ancestries may also be genetic in origin, but potential nongenetic factors, such as differences in timing of secular trends, mean that this inference would need to be directly tested with genetic data in additional populations. By aggregating evidence of directionally consistent intra-European frequency differences over many individual height-increasing alleles, none of which has a clear signal of selection on its own, we observed a combined signature of widespread weak selection. However, we were not able to determine whether this differential weak selection (either positive or negative) favored increased height in Northern Europe, decreased height in Southern Europe or both.
Although it is not clear what the nature of the selection was, this paper provides yet another example of human evolution since the origin of agriculture. Add it to growing list.
This finding reflects some of the material in Cochran and Harpending’s book The 10,000 Year Explosion: How Civilization Accelerated Human Evolution, where they argued that selection on variation in existing functional genes, as opposed to new mutations, would have played a major role in the initial response to agriculture. Selection could have been particularly strong in the case of balanced polymorphisms, which occur where a population maintains two different alleles of a gene. In such a case, balanced polymorphisms can respond quickly to the environmental change as the alleles are already at a substantial prevalence in the population. In comparison, a new mutation present in a single person may take thousands of years (if it spreads at all) to reach the higher frequencies where the rate of spread will be largest.
However, Cochran and Harpending also suggested that as population increased, the importance of new mutations increased:
Human genetic variation was limited in the days before agriculture, in part because populations were small, and it was often not useful, since many of the changes that were favored among agriculturalists would actually have been deleterious among their hunter-gatherer ancestors. This means that some of the alleles with the right effects in farmers would have been extremely rare or nonexistent in their hunter-gatherer ancestors.
Therefore, new mutations must have played a major role in the evolutionary response to agriculture—and as luck would have it, there was a vast increase in the supply of those mutations just around this time because of the population increase associated with agriculture. … Increased population size increased the supply of beneficial mutations just as buying many lottery tickets increases your chance of winning the prize.
By the beginnings of recorded history some 5,000 years ago, new adaptive mutations were coming into existence at a tremendous rate, roughly 100 times more rapidly than in the Pleistocene.
Regardless, it is positive to see the Nature Genetics paper as a sign of increasing research on selection of standing variation across large numbers of loci as a complement to the search for novel mutations.