Are we still evolving? Part 2

The perennial question I’m asked in public lectures is this:  Are we still evolving? (“We,” of course, means “humans.”)  A while back I highlighted a paper by Byars et al. that measured selection on various traits in a human population, showing that features like weight, blood pressure, and age of menopause were indeed under selection.  This shows that we’re undergoing selection, but not necessarily whether we’re evolving—for whether selection causes evolution depends on other factors like the amount of genetic variation that’s available to respond to selection, and whether there are correlated traits that may be selected in other directions.

But if you want a more comprehensive answer to the question of selection, check out the penultimate issue of Nature Reviews Genetics, where you’ll find a review paper by the same group of researchers summarizing all available studies in Homo sapiens. It’s a very good analysis, discussing the problems encountered in human studies, the methods used to estimate selection, and, of course, the data.

Those data come mostly from long-term studies in which individuals were followed over long spans of time.  Various traits of interest were measured (height, age at menopause,  blood pressure and so on) and were correlated with “fitness”, which the authors took as completed family size.  Differential fitness, which incorporates both survival and reproductive output, is the index of natural selection. For obvious reasons, this is more accurately measured in women than in men, so most measures of selection come from females.

The authors find three consistent results among the studies:

• Both women and men are under selection for earlier age at first birth in all populations.  The authors see this as a result of lowered juvenile mortality resulting from improvements in medical care, nutrition, and sanitation.  If it costs you to reproduce early, but the benefits are increased because your kids no longer die so often, then selection will favor your reproducing at an earlier age.
• “Women are under selection for later age at last birth in a pre-industrial population [Finland, 17th-19th century] and later age at menopause in two post-industrial populations [USA and Australia, 20th century]. The authors don’t explain the basis of this finding, but I suppose it’s because women now live longer, giving a selective advantage to females who can produce more offspring at ages that they wouldn’t have attained earlier.  Combined, these first two observations show the “temporal window of reproductive opportunity” is broadening in humans:  we reproduce both earlier and later than populations a few centuries ago.
• “Women are under selection for increased height in one pre-industrial population [Gambia, 20th century] and for decreased height in three post-industrial populations [Great Britain and USA, 20th century].” The authors suggest that, for the same reasons that it’s adaptive to reproduce earlier in industrial populations, it’s also adaptive to mature at a smaller size and divert your effort to reproduction.  To explain the Gambian data, they suggest that this trade-off doesn’t exist in populations where infant mortality of shorter and younger mothers is higher.  This is, of course, special pleading, but that’s just speculation. The data are what is important here.

I don’t have the expertise to judge the authors’ evolutionary speculations, but these three conclusions represent the best answer to the question of “are we under selection?”.  Again, whether that selection produces evolutionary change depends on other factors that weren’t measured.  It will take a few centuries (if ever!) to determine if this selection really has caused evolutionary change.

Finally, the authors estimate selection on some behavioral traits, but take these estimates less seriously.  That’s because there are likely to be substantial cultural components to inheritance here, but also—though the authors don’t say this—because traits like IQ, income, and education are political minefields.  For what it’s worth, the authors conclude that “contemporary humans are under directional selection for less education, less income in women, and more income and wealth and higher rank in men, and decreased intelligence in both sexes.”  Reality shows and Tea Party members instantiate the last process.

Why is this important?  Given that these indices of selection, if real, aren’t all that strong, and will take centuries to produce any substantial evolutionary change, why does this matter?  I’m not sure, actually.  It does provide a solid answer to the question I’m asked most frequently, but remember that cultural changes can produce much more rapid and pronounced changes in human populations than can genetic evolution itself.

Since 1900, for example, the average height of Japanese has increased by more than 11 cm.—about 4.3 inches!  That’s only a couple of generations, and these changes are certainly due not to genetic change but to improved nutrition and medical care.  (Note that they’re also in the opposite direction to selection acting in other “post-industrial” populations.)  Even if selection is acting on us, the evolutionary change it causes is likely to be overwhelmed by changes due not to genes but to culture. And that’s something I always add when asked about whether we’re evolving now.

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Stearns, S. C., S. G. Byars, D. R. Govindaraju and D. Ewbank. 2010.  Measuring selection in contemporary human populations.  Nature Reviews Genetics 11:611-622.