As I’ve often said, the question I’m always asked after my public lectures on evolution is this: “Are humans still evolving?” And my answer is always the same: “Yes, but the evidence we have for evolution occurring right now involves traits that aren’t that interesting.” When people ask that question, what they really want to know is whether humans are getting better looking, more athletic, smarter—whether we’re turning into a race of superheroes. And all I can tell them is that, over the last 10,000 years, our species has evolved in some places to be more lactose-tolerant, in other places to be more resistant to malaria, and in still other places to adapt to the low-oxygen conditions of living at high altitude. But that’s still in the past (see a summary here).
As for evolution in the present day, we have “real time”, “horizontal” observations for things like selection in women for 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 a recent evolution of reduced cholesterol levels and lower blood pressure, and an increased age of menopause (see here and here). But those results don’t excite people much. In general, though, as long as there is variation in some genes that causes variation in reproductive success, humans will continue to evolve. That hard part is documenting which genes and which traits are associated with reproductive success, and that means laborious studies correlating people’s genes and traits with their reproductive output.
One such study, by Jonathan Beauchamp, was just published in the Proceedings of the National Academy of Sciences (free download, link and title below). It used genetic, phenotypic (“trait”), and reproductive-success data from about 12,000 U.S. males and females of European ancestry, born between 1931 and 1953, all examined in a Health and Retirement Study (HRS). Using people of that age ensured that most of them had completed their reproduction, so the number of children they had (child mortality wasn’t counted) served as an index of their reproductive success (RS). And, of course, traits conferring a greater RS are those that selection will favor in populations. The currency of natural selection is reproductive output!
The authors did two types of analyses of the data.
a. Trait analysis. The authors used data on body mass index (BMI), educational attainment (EA), fasting glucose concentration (GLU), height (HGT), schizophrenia (SCZ), plasma concentration of total cholesterol (TC), and age at menarche (AAM), the last trait studied, of course, only in women. They then correlated the values of each trait with the reproductive success of their bearers:
The results? As the table below shows (asterisks denote statistical significance at the p < 0.01 level), there was evidence of phenotypic “selection” for stouter males and females, selection again educational attainment in both males and females, and selection for shorter women (but not men).
Taken at face value, these data show that Americans in that year class are experiencing selection to get chubbier, to stay in school fewer years, and, in women, to get shorter. But of course there are problems, because they are looking at a correlation between a trait and the number of children produced by people with different trait values—and there’s no genetics here. Perhaps there are cultural or other reasons, for instance, for the correlation between staying in school for less time and having more children. One example would be if people leave school to have children, or put off having children while they’re in school. You’d get the appearance of selection on the trait, but there might not be any genes involved, so there would be no evolution. Because of these issues, the author did a study explicitly incorporating genes.
b. Genetic analysis. This involved looking at the DNA of every person at many sites (between 80,000 and 400,000 DNA positions, depending on the trait), and finding those combinations of gene positions best correlated with the values of the trait (I’m simplifying matters here, but it’s not important). These combinations of DNA positions were then considered to be the genetic sites that could be influenced by selection on that trait. Having done that, Beauchamp could then see if those combinations of genes were themselves correlated with reproductive success, and thus could be under selection. After having done the appropriate statistical corrections for multiple tests, Beauchamp found the following results:
So the negative correlation between genes associated with staying in school and the reproductive success of their carriers was still observed. In other words, both men and women were undergoing natural selection for fewer years of schooling. There was still a marginally significant (p < 0.1) association between age of menarche and reproductive success, in line with previous studies showing that American women are evolving to reach menopause later. There was no significant association between genetic constitution and reproductive output on the other traits.
Three questions remain:
- How strong is selection against educational attainment? Answer: not very strong. The estimate given by Beauchamp is that we’re staying in school about one week to six weeks less per generation. If you take 25 years per generation, it would take about 35 generations, or 875 years, for selection to reduce our education by about a year.
- So are we getting dumber? Answer: nope, because phenotypically we’re staying in school longer. That is, the cultural trends on Americans show that between 1876 and 1951 (75 years), Americans’ EA increased by 6.2 years—an increase of about two years of education per generation. That far outstrips the genetic change, so, even if the selection data are correct, we’re still going to keep increasing our educational attainment. What we’re seeing is the reproductive advantage of leaving school a bit earlier is being overcome by the cultural trend to stay in school longer.
- What other problems are there? Beauchamp does a good job in pointing out the caveats of his study. I won’t recount them in detail, but they include the possibility that having more children doesn’t mean that you’re going to have better children—that is, that those extra kids may not themselves reproduce as well as kids from smaller families. That could weaken or even reverse the direction of selection. And, of course, this correlation was seen in only people from one or two generations, and there’s no guarantee that it will continue over the long term—or even that we’d see the same kind of selection in other countries.
In the end, we have a suggestive result, but one that needs a lot more work before it’s accepted as definitive. That work would somehow have to look at the reproductive output of the children themselves sired by the measured individuals, and nobody is going to do that. Further, the genetic result is being overcome by cultural trends, so we don’t really have to worry that we’re breeding a generation of dropouts!
Beauchamp, J. P. 2016. Genetic evidence for natural selection in humans in the contemporary United States. Proceedings of the National Academy of Sciences 113:7774-7779.