So much for absence of natural selection in modern human populations

I have recently criticized British geneticist Steve Jones for his claim that modern human societies are no longer subject to natural selection. My arguments were based on basic principles of evolution and population genetics. Now a new study shows that Jones is wrong on the basis of the available empirical evidence and, ironically, that evidence comes from research on the British population!

The paper in question was published by Daniel Nettle and Thomas Pollet, of Newcastle University, in the prestigious journal American Naturalist (November 2008). Nettle and Pollet took advantage of a large database called the National Child Development Study, an ongoing longitudinal survey of all people born in the UK in a particular week of March 1958. Note that this is not a sample, with all the statistical uncertainties that follow, but the entire population of the nation for a given slice of time.

The authors set out to disentangle the effects of education and wealth on number of progeny for both men and women, because most previous studies -- which typically found a negative relationship between education and offspring number -- are biased by the inability to separate these two factors. The results are simply stunning. There is a strong selection coefficient relating men’s wealth and the offspring they produce, meaning that the wealthier men do in fact have more children. This is despite a negative effect on (and therefore selection against) education, again in men. In other words, natural selection in contemporary British society is favoring wealthy but under-educated men (though the negative effect of education disappears at very high levels of wealth).

The data are equally clear for women, but the pattern is completely different. Selection is again strong, but it favors low income, with education having a negative effect when income is low and a positive effect when it is high. That is to say, natural selection is favoring women who both forgo education and do not accumulate wealth -- although if you really want to be educated as a woman, you better be rich for your education to have a small but positive effect on the number of progeny you have.

There is bit of empirical consolation for Jones, however. Nettle and Pollet compared their data to estimates of selection coefficients to a variety of other samples, both historical and contemporary. They found that the strongest coefficient of selection are detected in highly polygynous populations (i.e., not in Western-style industrial societies). Nettle and Pollet suggest that this is because polygynous groups have a higher variance in the number of offspring, and it is a well known principle of evolutionary biology that increased phenotypic variance makes selection more effective. (Jones’ argument, by the way, was different, and had to do with the changing age of reproduction in Western society, not with polygyny or lack thereof.)

Two caveats, of course, need to be kept in mind. First, this is by all means not a suggestion that women should aim for low paying jobs and drop from school to hunt for rich husbands. To go from a factual statement about what is happening to a value judgment about what ought to happen would be to commit the naturalistic fallacy that David Hume has warned us against, and which crops up regularly on this blog. Needless to say (or is it?), the British government should not look at these results and embark on a program to keep women from achieving equal pay on the job, or to discourage girls from entering higher education, just so that natural selection can do its job.

The second caveat is more subtle and interesting. A classic evolutionary biologist would point out that there is a difference between selection and evolution: the latter happens only if the traits under selection (in this case education and income) are heritable from one generation to the next. We do not know the extent to which male and female traits affecting education and wealth are genetically heritable (and I’m not too fond of so-called twin studies for a variety of reasons). But we do know that they are culturally heritable. Cultural inheritance does affect evolution, and in fact does so at a much higher rate than genetic inheritance, because cultural changes are much more rapid than genetic ones. There is nothing in Darwin’s theory that specifies what kind of inheritance is necessary for evolution: any mechanism that reliably passes traits to one’s offspring is good enough. Moreover, cultural inheritance can have a hitchhiking effect on the genetic makeup of the human population: even if the entire response to selection on wealth and education is due to culturally inherited factors, the next generation will still carry on a likely non-random subset of genetic markers of the British population, which means that biological evolution in the stricter sense of changes in gene frequencies will still be happening. Again, pace Jones.