to improve fitness in unexpected ways.
suggests variants that shorten life
are being selected against.
The work (Identifying
Genetic Variants that Affect Viability in Large Cohorts),
published in PLoS Biology, analyzed DNA from 215,000 people
and is one of the first attempts to probe directly how humans are
evolving over one or two generations.
For each person, the parents' age of death was recorded as a measure of longevity, or their own age in some cases.
People who carry a
harmful genetic variant die at a higher rate, so the variant becomes
rarer in the older portion of the population.
A variant of the APOE gene, which is strongly linked to Alzheimer's disease, was rarely found in women over 70.
And a mutation in the
CHRNA3 gene associated with heavy
smoking in men petered out in the population starting in middle age.
People without these mutations have a survival edge and are more
likely to live longer, the researchers suggest.
So harmful mutations that
exert their effects after reproductive age could be expected to be
'neutral' in the eyes of evolution, and not selected against.
That such a large study found only two strongly suggests that evolution is "weeding" them out, says Mostafavi, and that others have probably already been purged from the population by natural selection.
So scientists are considering two other explanations for why longevity is important.
First, parents surviving into old age in good health can care for their children and grandchildren, increasing the later generations' chances of surviving and reproducing.
This is sometimes known
as the 'grandmother
hypothesis', and may explain why humans tend to live long
The researchers also found that certain groups of genetic mutations, which individually would not have a measurable effect but together accounted for health threats, appeared less often in people who were expected to have long lifespans than in those who weren't.
predispositions to asthma, high body mass index and high
cholesterol. Most surprising, however, was the finding that sets of
mutations that delay puberty and childbearing are more prevalent in
The link between longevity and late fertility has been spotted before, but those studies could not discount the effects of wealth and education, because people with high levels of both tend to have children later in life.
The latest genetic evidence makes Pritchard think there is an evolutionary trade-off between fertility and longevity, which had previously been studied only in other animals.
Studying ongoing evolution in humans is notoriously difficult.
Scientists who want to observe selection directly would need to measure the frequency of a mutation in one generation, and then again in all that generation's children and, better still, grandchildren, says Gil McVean, a statistical geneticist at the University of Oxford, UK.