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Have humans stopped evolving? No, say scientists studying key life events
Humans are still evolving, according to new research that contradicts the
prevailing view that modern society's healthcare and technology means
human populations are not changing genetically.
In particular, the scientists say that, in industrialised societies,
behavioural traits that cause women to have children earlier are becoming
Ian Owens from Imperial College in London and his colleagues used data
about the lives of 2710 female twins in Australia to calculate the
evolutionary "fitness" of each woman - a measure of the number of
descendants her lineage would leave.
Cultural factors such as religion and education had a big impact on
fitness, but they could not explain everything. "If you remove everything
that's cultural, there's still an enormous difference between women,"
says Owens. He estimates that while 50 to 60% of fitness is
environmentally determined, 40 to 50% of it is genetic.
This finding is crucial because genes that increase the number of
descendants an individual leaves are bound to become more widespread with
In societies without medicine or technology, factors such as surviving
through childhood and resistance to illness are important in determining
fitness. Genes that diminish the physical ability to have lots of
children are weeded out by natural selection.
With better medical care, that kind of selection has been much reduced.
Instead natural selection is now acting on other factors, such as how
many children we choose to have, the new research suggests.
Sarah Blaffer Hrdy, an evolutionary biologist from the University of
California in San Diego, says the work is "extremely provocative", while
her colleague Christopher Wills is excited too. "Data on the heritability
of fitness traits in industrialised human populations are very unusual
indeed," he says.
Religion and education
To find out which genes were involved, the researchers looked at the
different traits that affected fitness, to see if any of them were
inherited. They worked out how much was due to genes by comparing data
from identical twins, who share all of their DNA, with non-identical
twins, who share only half of it.
Cultural factors such as religion and education had virtually no genetic
component. But one major determinant which did was the age at which women
started having children. The earlier women had their first child, the
"fitter" they were. This makes sense - women who have their first child
early have more time to have more children, and a shorter generation time
also means more descendants.
The surprise was that across all groups in society, the age at which
women had their first child was genetically inherited. After accounting
for cultural factors, 23% of the variation in age was down to genes.
Owens says the genes involved probably determine psychological or
behavioural traits that make women more likely to start having children
younger. He warns that while society is encouraging women to have
children later, the biological urge to have kids early will get stronger.
"The genes are pushing in the other direction," he says. "There's a
fierce conflict between career, and wanting to reproduce."
More at: Evolution (vol 55 p 423)
Correspondence about this story should be directed to
1307 GMT, 24 April 2001
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