The one question I’m inevitably asked after lecturing on evolution to a general audience is this: “Are humans still evolving?” What they really want to know, of course, is whether we’re getting smarter, taller, handsomer, and so on. Well, with respect to those traits I always say, “I have no idea,” but humans are still evolving, albeit in ways that don’t excite most people. I’ve posted about this twice (see here and here), and in recent times we have evidence for H. sapiens evolving to produce, in women, earlier age of first birth, later age of last birth and (also in women) increases in height in some places and decreases in others. Studies in the U.S., which haven’t been conducted elsewhere, show the recent evolution of reduced cholesterol levels and lower blood pressure, and an increased age of menopause.
The U.S-specific data brings up the question of whether even if the entire human species is not evolving in a given direction, individual populations or groups might be undergoing “divergent evolution” in response to their specific environments. And for that the answer is clearer: “Yes.” We’ve known this for some time. Tibetan and South American populations have evolved adaptations to low-oxygen conditions, pastoral populations (those keeping sheep, goats and cows for milk) have evolved persistent lactose tolerance (a trait normally turned off after weaning), and some African populations have evolved small stature, a trait thought to be useful in tropical environments. The smaller-stature trait is a bit questionable as an adaptation, but in all three cases mentioned above, researchers have pinpointed the genes responsible for the adaptive changes.
There are other cases of human populations adapting divergently to their unique environments, with genes for that adaptation tentatively identified. In a short “news and views” article in the latest Science (free access, link below), geneticist Sarah Tishkoff summarizes both the genetic data and trait data, producing the nice figure below showing where that evolution has occurred, which traits were involved, and which genes are implication in those adaptations. (Be aware that adaptations, even in these cases, are likely to involve more genes than are listed, as genes of small effect are hard to detect):
Tishkoff wrote her piece to highlight a paper by Matteo Gumagalli et al. (many collaborators!), which I’ve referenced below (link not free). That paper used population-genetic analysis of Greenland Inuits to discover regions of two chromosomes that seem to have experienced strong selection in the recent past. (We know this because we can see that those regions have increased in frequency compared to similar regions in other places.) Those regions also happen to contain genes involved in fatty acid metabolism; and the variants of the genes that have increased in frequency in Inuits are also associated with small stature and lower weight. (The small and stocky bodies of northern populations may also be an adaptation to reduce heat loss—Bergmann’s Rule, as observed in many animal species—or it could simply be a byproduct of a metabolism altered by selection to reflect a novel, fish-rich diet.)
So if you’re asked “are humans still evolving?”, the answer is this: “Well, there’s some evidence that our species is evolving as a unit for traits like age of reproduction, but there is much more evidence that different parts of our species are evolving in different directions, adapting to environments that are geographically different or changing over time.” Of course our culture has slowed some forms of selection, as doctors and dentists help people survive who would have been ruthlessly culled on the savanna. (I am one of these misfits.) But so long as there are genetic variants associated with different numbers of offspring, our species will continue to evolve.
Tishkoff, S. 2015. Strength in small numbers. Science 349:1282-1283.
Fumagalli, M. et al. 2015. Greenlandic Inuit show genetic signatures of diet and climate adaptation. Science 349:1343-1347.