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Monday, July 19, 2010

Recent human evolution



Nicholas Wade writes about recent human evolution in the NY Times.

NYTimes: ... Scientists from the Beijing Genomics Institute last month discovered another striking instance of human genetic change. Among Tibetans, they found, a set of genes evolved to cope with low oxygen levels as recently as 3,000 years ago. This, if confirmed, would be the most recent known instance of human evolution.

[I believe the common Tibetan variant is found in about 8% of Han Chinese, so it isn't necessarily that a new mutation swept the Tibetan population during the 3k years. Rather, modern Tibetans are likely to be descended from the people who were well adapted to living at high altitude. As in the case of height (below), extant variation can be acted on by selection faster than new mutations are likely to appear. Most people who don't believe in rapid human evolution don't understand that new mutations are not required. All that is required is a redistribution of allele frequencies. See here for more on the Tibetan results.]

Many have assumed that humans ceased to evolve in the distant past, perhaps when people first learned to protect themselves against cold, famine and other harsh agents of natural selection. But in the last few years, biologists peering into the human genome sequences now available from around the world have found increasing evidence of natural selection at work in the last few thousand years, leading many to assume that human evolution is still in progress.

“I don’t think there is any reason to suppose that the rate has slowed down or decreased,” says Mark Stoneking, a population geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

... Soft sweeps work on traits affected by many genes, like height. Suppose there are a hundred genes that affect height (about 50 are known already, and many more remain to be found). Each gene exists in a version that enhances height and a version that does not. The average person might inherit the height-enhancing version of 50 of these genes, say, and be of average height as a result.

Suppose this population migrates to a region, like the Upper Nile, where it is an advantage to be very tall. Natural selection need only make the height-enhancing versions of these 100 genes just a little more common in the population, and now the average person will be likely to inherit 55 of them, say, instead of 50, and be taller as a result. Since the height-enhancing versions of the genes already exist, natural selection can go to work right away and the population can adapt quickly to its new home.

In the case of height, no individual gene variant has yet been found which accounts for more than a fraction of a percent of total variance. But, it's possible that half or more of total variance is captured by the 300k SNPs used in recent GWAS studies.

It's very plausible that IQ is like height -- a large number of genes, each of small effect, control most of the variance. I've done some rough calculations, and it seems that the proposed BGI study (which will genotype both an ordinary and an extreme group) has a good chance of detecting a large number of those genes -- stay tuned.

See related post: Recent evolution in humans.

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