A new study on how humans are evolving

I’ve often said that the most frequent question I’m asked in public lectures about evolution is this: “In what direction are humans evolving?” I’ve addressed it in several posts, including here, here, and here. The answers aren’t exciting, and are usually limited to just one population, since such studies involve following a cohort of humans for a long time so one can see what traits are correlated with higher (or lower, or intermediate) reproductive success.  Here’s a summary of the changes we think are occurring by natural selection:

Humans in Massachusetts:

• Total cholesterol: going down.  Projected to drop 3.6% in ten generations
• Weight:  going up a tad, projected to increase 1.4% in ten generations
• Height:  we’re getting shorter projecting a drop of 1.3% (2.1 cm) in ten generations.
• Systolic blood pressure:  Going down, as predicted. Projected to drop 1.9% in ten generations
• Age at menopause:  Going up; projected to rise 1.6% (0.8 years) in ten generations.
• Age at first reproduction: Going down. Projected to drop 1.7% (from 26.18 to 25.74 years).

Humans in several countries (see below), but no worldwide study:

• 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.

Divergent selection: local adaptations in some human populations but not others. Traits involved, which are largely those over historical time rather than the traits above—thought to evolve in “real time” (i.e. now)—are:

• Adaptation to marine diet
• Lactose tolerance
• Malaria resistance
• Cholera resistance
• Cold climate
• Adaptation to arsenic-rich environment
• Adaptation to high altitude
• Light pigmentation
• Short stature

A new paper in Proc. Nat. Acad. Sci. by Jaleal Sanjak and colleagues (reference below; only the abstract is free, though judicious inquiry might yield the paper) adds further data from a longitudinal dataset from the UK. The data come from the UK Biobank, which has 500,000 individuals not only measured for various traits and the individuals’ “long term reproductive success” relative to other people (LTRS; an index of “fitness” based on number of kids relative to the average), but who were also genotyped for various bits of DNA that allow the genetic correlations between traits to be analyzed. What that means is that apparent selection on one trait, say height, might not reflect a reproductive advantage of being shorter or taller, but only the fact that the trait’s expression involves some of the same genes also involved in the real trait under selection, say body mass.  To gauge this effect, the researchers examined examined the genetic data from 157,807 females and 115,902 males in the sample to look at the genetic correlation between traits, assuming that that would involve the presence of the same genetic variants associated with correlated traits.

Further, what you want to look for, if a trait is experiencing selection, is that the same genetic variants associated with the trait are also associated with LTRS. If they’re not, then we can’t say that variation in the trait has some connection with variation in “fitness”.

This is an improvement over previous studies in which traits are, by and large, studied individually, with genetic correlations not taken into account.

There were lots of correlations, but many were ruled out by showing that “selection” on a trait might really have been selection on a genetically correlated trait. So here’s a short list of the traits that, in this UK study, were being independently selected:

• Age of first birth (or first reproduction) in females. It’s getting younger. This isn’t a new result, but is from UK data, and it seems that this may be a ubiquitous phenomenon.
• Selection favors shorter females. (It appeared to favor taller males, too, but male height wasn’t genetically correlated with LTRS).
• Selection favors higher body mass index (BMI) in males. (The results showed an insignificant trend in females in the same direction). BMI is the mass of the body (usually expressed in kilograms) divided by the square of the height (usually expressed in meters). It’s often used to judge whether people are skinny, “normal” or overweight.

Those are the major results, and most of them, save selection for higher BMI in males (and perhaps a weaker effect in females), are consonant with previous results. As to why selection is doing this, we can only speculate. Clearly, those women who reproduce earlier will have higher fitness, so that’s not problematic. But why shorter women or chunkier males leave more offspring is something I can’t comment on.

There were also some contradictions with previous results: this study, for instance, found no correlation between LTRS and age of menopause, while earlier studies showed that selection was favoring later menopause. This is one of the issues with extrapolating results from one population or study to others.

Finally, it’s worth looking at the authors’ list of potential problems with their results, which is pretty clear to the non-specialist:

There are a few other important caveats and limitations to our present analyses. All of our results are conditional on the suite of phenotypes that we have measured; there is a real possibility that there are unmeasured phenotypes that drive or confound some of our results. This issue is related to the phenomenon of apparent selection and should always be kept in mind when studying phenotypic selection. In addition, the genetic correlations are estimated using common SNP [single nucleotide polymorphism] markers (minor allele frequency > 0.01), which may be a source of bias because the genetic variants with deleterious effects on fitness are likely to be rare and thus absent from our analyses. However, this should simply reduce the power of our analyses. Further, there is evidence that the population of the UKB may not be perfectly representative of the whole population of the United Kingdom.  The potential ascertainment bias (heathy participant bias) in the UKB is important to consider and may have a quantitative effect on our estimates, but the bias is not likely to be large enough to disrupt the conclusions of our work in a qualitative way.

The upshot? Not anything astounding, but a thorough analysis that confirms to some extent previous findings and gives us more confidence in these authors’ results than do earlier studies. If there’s one finding that I have faith in, it’s that in nearly all studies done, selection is on women to reproduce earlier. (That’s a shorthand way of saying that “women who reproduce earlier leave more offspring.”) Given that early reproduction doesn’t have some other effect that in net reduces LTRS, it’s pretty clear why selection would do that. There may of course be a limit (young girls aren’t equipped to give birth), but it appears that in the future the average age of first reproduction will be lower than it is now.

But that isn’t exciting to most people who, when they ask me THE QUESTION, want to know if we’re getting smarter or handsomer or stronger. Well, we just don’t have the evidence.

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Sanjak, J. S. et al. 2017. 2017. Evidence of directional and stabilizing selection in contemporary humans. Proc. Nat. Acad. Sci. USA 115:151-156. published ahead of print December 18, 2017,