Geography and History Shape Genetic Differences in Humans
News Jun 08, 2009
In recent years, geneticists have identified a handful of genes that have helped human populations adapt to new environments within just a few thousand years-a strikingly short timescale in evolutionary terms. But new research indicates that in most cases, natural selection may shape the human genome much more slowly than previously thought.
Other factors - the movements of humans within and among continents, the expansions and contractions of populations, and the vagaries of genetic chance - have heavily influenced the distribution of genetic variations in populations around the world.
The research was carried out by Jonathan Pritchard, a Howard Hughes Medical Institute investigator at the University of Chicago, and a team of colleagues including Graham Coop at the University of California, Davis; Joseph Pickrell at the University of Chicago; and Marcus Feldman, Richard Myers, and Luca Cavalli-Sforza at Stanford University.
The team found that for most genes, it can take at least 50,000-100,000 years for natural selection to spread favorable traits through a human population. According to their analysis, gene variants tend to be distributed throughout the world in patterns that reflect ancient population movements and other aspects of population history – suggesting that the geographic distributions of these variants are determined only in part by local selection pressures such as climate or diet.
“We don’t think that selection has been strong enough to completely fine-tune the adaptation of individual human populations to their local environments,” said Jonathan K. Pritchard.
Natural selection occurs when a particular genetic difference - which researchers call a variant - gives an individual a greater opportunity to have children and pass on his or her genes to future generations. These genetic differences, which arise by mutation and then are inherited from parent to child, might confer a survival advantage in a given environment, such as being able to survive malaria or digest milk from animals. As individuals thrive and pass on their genes to their offspring, the variant can become more common in a population. This natural selection of advantageous genes-the raw material of evolution-leaves signals in our DNA that can be detected when researchers compare human genomes.
“But we don’t think that selection has been strong enough to completely fine-tune the adaptation of individual human populations to their local environments,” says Pritchard. “In addition to selection, demographic history - how populations have moved around - has exerted a strong effect on the distribution of variants.”
Pritchard and his colleagues published the results of their analysis on June 5, 2009, in Public Library of Science (PLoS) Genetics.
Pritchard says the genetic variants responsible for light skin are good examples of the effects of natural selection. Modern humans evolved in Africa more than 150,000 years ago and then spread throughout Africa and the rest of the world. As these humans moved into northern latitudes, natural selection favored traits that helped them survive in their new environment.
Dark skin became a disadvantage, possibly because it blocked too much of the sunlight that humans need to synthesize vitamin D for healthy bones. People with genetic variants that produced lighter skin therefore tended to be healthier and had more children, and today those variants are common in people of European and northern Asian ancestry.
Selection may also play a role in determining susceptibility to several common diseases. This is another reason why geneticists would like to identify genes that have undergone selection. Now that humans in many areas of the world have access to virtually unlimited amounts of food, genetic variants associated with this trait may be contributing to an epidemic of obesity and diabetes.
However, Pritchard points out that selection is not the only factor that influences the fate of genetic variants in populations. Unlike selection, the other mechanisms that are at work do not necessarily help populations adapt to their environments. The pool of genes within a population also tends to fluctuate due to chance events and random differences in the number of children people have and the particular genes they pass on to their children.
Pritchard and his colleagues set out to answer a fundamental question facing human geneticists: Is it possible to determine which genetic variants have increased because of selection and which have increased because of population changes or genetic chance? New data that became available last year from the Human Genome Diversity Project at Stanford University provided a much denser sampling of worldwide genetic differences than was previously available. Pritchard and his colleagues used this resource to carry out a new and more rigorous test for selection.