Adventures in Very Recent Evolution - NYTimes.com
Ten thousand years ago, people in southern China began to cultivate rice and quickly made an all-too-tempting discovery — the cereal could be fermented into alcoholic liquors. Carousing and drunkenness must have started to pose a serious threat to survival because a variant gene that protects against alcohol became almost universal among southern Chinese and spread throughout the rest of China in the wake of rice cultivation.
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The variant gene rapidly degrades alcohol to a chemical that is not intoxicating but makes people flush, leaving many people of Asian descent a legacy of turning red in the face when they drink alcohol.
The spread of the new gene, described in January by Bing Su of the Chinese Academy of Sciences, is just one instance of recent human evolution and in particular of a specific population’s changing genetically in response to local conditions.
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.
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.
So much natural selection has occurred in the recent past that geneticists have started to look for new ways in which evolution could occur very rapidly. Much of the new evidence for recent evolution has come from methods that allow the force of natural selection to be assessed across the whole human genome. This has been made possible by DNA data derived mostly from the Hap Map, a government project to help uncover the genetic roots of complex disease. The Hap Map contains samples from 11 populations around the world and consists of readings of the DNA at specific sites along the genome where variations are common.
One of the signatures of natural selection is that it disturbs the undergrowth of mutations that are always accumulating along the genome. As a favored version of a gene becomes more common in a population, genomes will look increasingly alike in and around the gene. Because variation is brushed away, the favored gene’s rise in popularity is called a sweep. Geneticists have developed several statistical methods for detecting sweeps, and hence of natural selection in action.
About 21 genome-wide scans for natural selection had been completed by last year, providing evidence that 4,243 genes — 23 percent of the human total — were under natural selection. This is a surprisingly high proportion, since the scans often miss various genes that are known for other reasons to be under selection. Also, the scans can see only recent episodes of selection — probably just those that occurred within the last 5,000 to 25,000 years or so. The reason is that after a favored version of a gene has swept through the population, mutations start building up in its DNA, eroding the uniformity that is evidence of a sweep.
Unfortunately, as Joshua M. Akey of the University of Washington in Seattle, pointed out last year in the journal Genome Research, most of the regions identified as under selection were found in only one scan and ignored by the 20 others. The lack of agreement is “sobering,” as Dr. Akey put it, not least because most of the scans are based on the same Hap Map data.
From this drunken riot of claims, however, Dr. Akey believes that it is reasonable to assume that any region identified in two or more scans is probably under natural selection. By this criterion, 2,465 genes, or 13 percent, have been actively shaped by recent evolution. The genes are involved in many different biological processes, like diet, skin color and the sense of smell.
A new approach to identifying selected genes has been developed by Anna Di Rienzo at the University of Chicago. Instead of looking at the genome and seeing what turns up, Dr. Di Rienzo and colleagues have started with genes that would be likely to change as people adopted different environments, modes of subsistence and diets, and then checked to see if different populations have responded accordingly.
She found particularly strong signals of selection in populations that live in polar regions, in people who live by foraging, and in people whose diets are rich in roots and tubers. In Eskimo populations, there are signals of selection in genes that help people adapt to cold. Among primitive farming tribes, big eaters of tubers, which contain little folic acid, selection has shaped the genes involved in synthesizing folic acid in the body, Dr. Di Rienzo and colleagues reported in May in the Proceedings of the National Academy of Sciences.
The fewest signals of selection were seen among people who live in the humid tropics, the ecoregion where the ancestral human population evolved. “One could argue that we are adapted to that and that most signals are seen when people adapt to new environments,” Dr. Di Rienzo said in an interview.
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