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Humans still evolving (No. Really?)

From the NYTimes report: [Requires free registration. Or use BugMeNot for Firefox]

“Providing the strongest evidence yet that humans are still evolving, researchers have detected some 700 regions of the human genome where genes appear to have been reshaped by natural selection, a principal force of evolution, within the last 5,000 to 15,000 years.

The genes that show this evolutionary change include some responsible for the senses of taste and smell, digestion, bone structure, skin color and brain function.

Many of these instances of selection may reflect the pressures that came to bear as people abandoned their hunting and gathering way of life for settlement and agriculture, a transition well under way in Europe and East Asia some 5,000 years ago.”

In other news: many trees have green leaves.

I’m sorry, but to me this seems like a “Duh!” moment. Of course humans are still evolving. All Pritchard and company have done is find indications in the DNA that this is so. (These are indications, by the way, and not proof. They are good indications and deserve to be believed, but I expect you will hear people arguing that this isn’t proof.)

The interesting questions are: what kind of selection is happening? Some of it is natural selection, i.e. it has nothing to do with what people admire in mates. Lactose tolerance is probably one of these. People who could benefit from milk survived to reproduce and so their genes for lactose tolerance survived in that population.

Others might be mate selection rather than natural selection. Pritchard and company talk about hair texture genes. It’s hard to see what survival advantage different hair types would bring in a northern climate. (In the tropics, where it can provide an essential insulator, the situation is different.) Hair texture in temperate climates could well be due to the whims of fashion. The important thing to remember here is that sexual selection is notorious for leading to extreme forms that can lead to lower survival rates and even extinction of the whole species. Consider the Irish elk, which developed huge antlers for the purpose of showing off. When the environment changed, and their diet was no longer rich enough to support formation of what was practically an extra skeleton, they ran into big trouble. Sexual selection (and fashion, its young cousin) is an iffy thing.

The other factor is genetic drift. Counterintuitively, random processes can lead to a preponderance of one set of traits. Because of the way reproduction works, once the traits are preponderant, they tend to become even more so. In the end, the minority trait can disappear by random processes alone, without any selection, natural or sexual. Loss of wisdom teeth follows that trajectory. (This is not one of the characteristics the researchers mention.)

The researchers do mention some genes that affect brain development, whose precise function is unknown. The conclusion in the popular press will probably be that these genes must make some people smarter, faster, and better, if not cheaper. Those qualities are so complex, it is highly unlikely that one or a few genes would have much effect on any of them. The brain genes caught in the act of evolving may well do boring things like determine the distribution of glial cells, or affect the blood-brain barrier in subtle ways. They might well be changing by random genetic drift and have no particular significance.

Take all this stuff with many grains of salt. (It tastes better then, too.)

Technorati tags: human evolution, Pritchard