Nature
Published online: 16 April 2007; |
doi:10.1038/446841a
Chimps lead evolutionary race More chimpanzee genes have been positively
selected for than human ones.
Michael
Hopkin
|
|
Chimpanzees have at least 233
genes thought to be shaped by selection for
beneficial mutations. Alamy
| | Humans are generally believed to be
more highly 'evolved' than our chimpanzee cousins. But in at
least one sense that isn't true, say geneticists who have
hunted for the hallmarks of natural selection in our
respective genomes - and found more of them in
chimps.
The discovery suggests that, since
our evolutionary paths diverged 6 million years ago, greater
numbers of chimpanzee genes have been shaped by 'positive
selection', in which natural selection favours beneficial
mutations.
Researchers at the University of
Michigan, Ann Arbor, combed through 14,000 matching genes from
the human and chimpanzee genomes. As they report in
Proceedings of the National Academy of Sciences this
week, 233 chimp genes showed signs of having been shaped by
positive selection (M. A. Bakewell, P. Shi and J. Zhang
Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0701705104;
2007). The corresponding figure for our own genes was just
154.
The result overturns the view that, to promote humans
to our current position as the dominant animal on the planet,
we must have encountered considerable positive selection, says
lead author Jianzhi Zhang. "We think we're very different from
animals, with our large brain size and speech," he says.
The gene discrepancy might be due to the fact that,
for much of our histories, chimpanzees had the larger
population size. Humans, with a smaller and more fragmented
population, may have been shaped by random, erratic changes.
 It is possible that the genetic
changes underlying brain size are very few.
 | | |
 |
| It is difficult to put together a coherent picture,
says Zhang, because it is hard to know which genes would have
been crucial in shaping traits such as our large brain size.
"It is possible that the genetic changes underlying brain size
are very few," he says.
A sample of 14,000 genes does not
tell the whole story. The team could not compare the entire
genome as the chimp sequence has not been completed to the
same level of detail as the human one. But for genes with good
sequences, they were taken to show signs of positive selection
if they had a high proportion of 'non-synonymous mutations'
DNA changes that alter the protein sequence produced by the
gene which could be a 'lever' for natural
selection.
Zhang admits it is difficult to
spot genes that have been the subject of more recent positive
selection. Such genes could have been responding to selection
pressures such as changes in climate and food source
encountered by humans as they began to move out of Africa and
across the planet over the past 100,000 years.
There
also seems to be little pattern to the functions of the
selected genes, says Zhang. Among those favoured in chimps are
genes for protein metabolism and stress responses, whereas the
human genes are involved in processes such as fatty-acid
metabolism.
Victoria Horner, who works with
chimpanzees at Yerkes National Primate Research Center in
Atlanta, Georgia, says: "We assume that chimpanzees have
changed less than us, when that's actually not the
case."
Visit our newsblog to read and post comments about
this story.
Article brought to you
by: Nature
|
