October 08, 2008

Some comments on Steve Jones and human evolution

Leading geneticist Steve Jones says human evolution is over
Human evolution is grinding to a halt because of a shortage of older fathers in the West, according to a leading genetics expert.

Fathers over the age of 35 are more likely to pass on mutations, according to Professor Steve Jones, of University College London.
It is certainly true that older fathers pass more mutations to their offspring. One would, of course, have to account for the fact that many of these mutations are deleterious, and hence are no-starts in terms of evolution. One would also need to consider that the human population today is much larger than any time in human history, and hence a smaller percentage of old fathers is perfectly consistent with a lot more alleles entering the gene pool, an effect recently discussed.

More from the Times article:
Another factor is the weakening of natural selection. “In ancient times half our children would have died by the age of 20. Now, in the Western world, 98 per cent of them are surviving to 21.”
It is true that as human technology progresses, old selection pressures are relaxed. A lot of things that would have killed or otherwise incapacitated our ancestors can be treated effectively today.

But, technological progress brings with it its own selection pressures. Consider, for example, that a few centuries ago, being illiterate did not seriously affect a person's reproductive potential. Or, very few people had to work at night, because there just wasn't enough light for much human activity to occur.

So, while technology frees us from some environmental influences, it also puts us into novel environments for which we are not well-adapted.

Decreasing randomness is another contributing factor. “Humans are 10,000 times more common than we should be, according to the rules of the animal kingdom, and we have agriculture to thank for that. Without farming, the world population would probably have reached half a million by now – about the size of the population of Glasgow.

Small populations which are isolated can evolve at random as genes are accidentally lost. World-wide, all populations are becoming connected and the opportunity for random change is dwindling. History is made in bed, but nowadays the beds are getting closer together. We are mixing into a glo-bal mass, and the future is brown.”

Genetic drift is one of the factors affecting evolution, and this has certainly been relaxed in large modern populations. But, what does this mean?

In a small population, an allele becomes fixed sooner than in a large one. If we think in terms of genetic drift alone, allele frequencies will shift faster in a small population.

However, drift isn't the only factor at play. As stated above, more novel alleles are created in a larger population. Moreover, there is room for more genetic variability in a large population (compare a small village with a large city), and, correspondingly, a broader range of phenotypes. This is exactly what selection needs to proceed, variability in genotypes and phenotypes. The more variability in the population, the faster selection will proceed.

1 comment:

Kosmo said...

Human evolution is over? What the...??? A geneticist is saying this? Seriously? Is he joking?

Frankly, I'd wonder if this Jones guy has read any recent linkage disequilibrium studies. Has he not read John Hawks? What about Hawks' study showing that human evolution is actually moving faster now that at any point in the past?

We have taken ourselves out of nature and are functioning under a kind of relaxed selection (which may be the point that Jones is trying to make) but it has relaxed in certain vectors only; and that, in itself, will result in a change in gene frequencies over time, which is pretty much the definition of evolution. But even that simple surface argument aside, the reality is that genes are being discovered all the time which show evidence of MASSIVE selective pressure in humans. The 7-repeat allele of the DRD4 gene, for example. And ASPM. Certain immunity haplotypes are being selected for, and others are being selected against. We are in the midst of numerous selective sweeps.


In a nut shell, it may seem like evolution has stalled in humans, but that is not the case. Even if we’re not being eaten by lions anymore, even if we don’t have to run to catch our dinners, the reality is that some people will still leave behind more descendants than others, and to the extent which that differential is impacted by a person’s genetic make-up, evolution will continue.