Geography and history shape genetic differences in humans

Jun 05, 2009

New research indicates that 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 study, conducted by a team from the Howard Hughes Medical Institute, the University of Chicago, the University of California and Stanford University, is published June 5 in the open-access journal PLoS Genetics.

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. However, 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 history. "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 co-author Jonathan Pritchard. "In addition to selection, demographic history -- how populations have moved around -- has exerted a strong effect on the distribution of variants."

To determine whether the frequency of a particular variant resulted from natural selection, Pritchard and his colleagues compared the distribution of variants in parts of the genome that affect the structure and regulation of proteins to the distribution of variants in parts of the genome that do not affect proteins. Since these neutral parts of the genome are less likely to be affected by natural selection, they reasoned that studying variants in these regions should reflect the demographic history of populations.

The researchers found that many previously identified genetic signals of selection may have been created by historical and demographic factors rather than by selection. When the team compared closely related populations they found few large genetic differences. If the individual populations' environments were exerting strong selective pressure, such differences should have been apparent.

Selection may still be occurring in many regions of the genome, says Pritchard. But if so, it is exerting a moderate effect on many genes that together influence a biological characteristic. "We don't know enough yet about the genetics of most human traits to be able to pick out all of the relevant variation," says Pritchard. "As functional studies go forward, people will start figuring out the phenotypes that are associated with selective signals," says lead author Graham Coop. "That will be very important, because then we can figure out what selection pressures underlie these episodes of ."

But even with further research, much will remain unknown about the processes that have resulted in human traits. In particular, Pritchard and Coop urge great caution in trying to link selection with complex characteristics like intelligence. "We're in the infancy of trying to understand what signals of selection are telling us," says Coop, "so it's a very long jump to attribute cultural features and group characteristics to selection."

More information: Coop G, Pickrell JK, Novembre J, Kudaravalli S, Li J, et al. (2009) The Role of Geography in Adaptation. PLoS Genet 5(6): e1000500. doi:10.1371/journal.pgen.1000500, http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1000500

Source: Public Library of Science (news : web)

Explore further: Changes in scores of genes contribute to autism risk

add to favorites email to friend print save as pdf

Related Stories

Study: humans are still evolving

Mar 07, 2006

Even as controversy continues over the theory of evolution, scientists say they've found the strongest evidence yet that humans are still evolving.

Natural selection at single gene demonstrated

Apr 25, 2006

Biologists seeking elusive proof of natural selection at the single-gene level have a powerful new tool at their disposal. Chris Toomajian, postdoctoral researcher in molecular and computational biology in the USC College ...

Recommended for you

Changes in scores of genes contribute to autism risk

21 hours ago

Small differences in as many as a thousand genes contribute to risk for autism, according to a study led by Mount Sinai researchers and the Autism Sequencing Consortium (ASC), and published today in the journal Nature.

Dozens of genes associated with autism in new research

22 hours ago

Two major genetic studies of autism, led in part by UC San Francisco scientists and involving more than 50 laboratories worldwide, have newly implicated dozens of genes in the disorder. The research shows ...

Genetic link to kidney stones identified

Oct 29, 2014

A new breakthrough could help kidney stone sufferers get an exact diagnosis and specific treatment after genetic links to the condition were identified.

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.