Oct 07 2008

Are Humans Evolving?

According to geneticist Steve Jones human evolution is grinding to a halt. Jones says there are several factors that drive evolution: mutations, natural selection, and randomness. He further argues that the decrease in older fathers is leading to a decrease in mutations, since the sperm of older fathers contains many more mutations than younger ones.

This is an interesting argument, but I find several problems with it. First, I am curious as to what the data say about the average age of fathers. I would have thought that this was increasing, not decreasing, as people are having children later. Jones points to alpha males who would father hundreds of children even into their 70’s or 80’s. It is true that in primate species where males have harems, like baboons, alpha males can father a significant portion of the next generation, but this in not true for homo sapiens. Humans dominantly follow a pair-bonding model of mating, even though there is much cultural variation.

Also, at least in the article about his lecture (I have not heard the lecture) there is no mention of recombination. When male and female gametes come together their chromosomes mix randomly in a process called recombination. This can bring gene variants (alleles) together in combinations that have never previously existed. Typically, the contribution of recombination to genetic variation is underestimated and the contribution of mutations overestimated. Both are important.

By randomness Jones means small isolated populations finding themselves under differing selective pressures or undergoing genetic drift. This is true – the larger and more “outbred” a population is the more stable it is genetically over time.

In fact, according to the theory of punctuated equilibrium proposed by Gould and Eldridge, most species are stable most of the time. They are at equilibrium with their environment. This equilibrium is occasionally punctuated by rapid speciation events, usually occuring in small populations at the edge of the range of the species. According to this theory, the human population is at equilibrium and not rapidly evolving.

This, however, does not mean that there aren’t changes in gene frequencies over time, only that there is no rapid evolution to a new species. There is copious evidence that human gene frequencies continue to change over time. Genes associated with higher intelligence have been increasing in frequency over the last few centuries, for example.  This strongly suggests a directional change under selective pressure.

Finally, Jones brings up the fact that modern medicine saves many lives that would have otherwise been lost – selected out of the gene pool. While this is true, this does not equate to evolution stopping. Rather, it means that our population will tolerate more diversity. Selecting against unhealthy individuals tends to narrow variation, not increase it. Technology therefore allows a greater variety of humans to survive. This may, in fact, help increase evolution. For example, a brilliant person with asthma may have died young of the asthma, never passing on their genes for brilliance. With modern medicine they can survive to reproduce.  This may make human populations less “hardy”, but does not hinder evolution.

While Professor Jones’ ideas are interesting, and the changing average age of fathers will likely have some impact on population genetics – I think reports of the death of human evolution are premature.

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