March 7, 2006

Human selection is alive and kicking

Geneticists track evolutionary forces in three populations.

by Michael Hopkin
Nature News

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Humanity's response to the challenges of the past few millennia, from adapting to different environments to taming crops and animals, are writ large in human society. Now geneticists have shown that they are also writ small - in our DNA.

Researchers at the University of Chicago, Illinois, have identified the regions of our genetic sequence that show the strongest marks of natural selection. Their work highlights the genes that have been most important in adapting to new lifestyles, and could help to identify the genetic factors involved in complex medical conditions such as high blood pressure and alcoholism.

Genes that show the most evidence of recent selection include those involved in milk digestion. Although most mammals drink milk only in infancy, humans seem to have adapted genetically to digest it throughout life.

Genes for skin pigmentation also bear the hallmarks of rapid evolution - highlighting the fact that many populations have become more fair-skinned as they have colonized more extreme latitudes with less sunshine.

It's a snip

The team used data from the International HapMap Project, which collates and maps out 'single-nucleotide polymorphisms' (SNPs). These are sites in the genome at which DNA varies between people by just a single letter of genetic code. The team sampled data from more than 200 unrelated individuals in three different racial groups: East Asians, Europeans and the Yoruba of Nigeria.

Genetic mutations that confer an advantage on a given population spread much faster than they would through natural, random mixing of genes. These portions of DNA should carry certain SNPs along with them as they are selected for over time. So if researchers find a string of SNPs that are mostly the same letters within a given population, they can say that the accompanying genes have come under strong selection pressure.

The method reveals changes that have occurred since various populations split to colonize different areas of the globe, says Jonathan Pritchard, who led the research published in the journal PLoS Biology.

Food and sex

Many of the genes that showed evidence of selection involve food metabolism, notes Pritchard. This shows that adapting to different diets has been a key trend in recent human evolution.

Around 20% of the genes identified showed evidence of selection in more than one of the populations. Chief among these were genes involved in reproductive processes such as sperm manufacture, showing that these were equally important in different settings.

All three racial groups showed equal amounts of recent evolution. This is interesting, says Pritchard, given that African populations have remained in the birthplace of our species, while others have moved away. Travelling populations might be expected to encounter challenges favouring more selection pressure than those that stay still.

But, notes Pritchard, the environment can change as much with time as it does with distance. "It is perhaps naive to think of Africans as staying 'in the same place'," he says.

Pritchard is not convinced by the predictions of some experts that advances in medicine will negate evolutionary pressures from now on. "Even today there is plenty of scope for natural selection, such as in genes that impact fertility or fetal survival."

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