OncologyGeography And History Shape Genetic Differences In Humans
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 population 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 natural selection."
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."
CITATION:
"The Role of Geography in Human Adaptation."
Coop G, Pickrell JK, Novembre J, Kudaravalli S, Li J, et al. (2009)
PLoS Genet 5(6): e1000500. doi:10.1371/journal.pgen.1000500
PLoS Genetics