A massive genetic study that aimed to find out how the human genome is evolving suggests that natural selection is getting rid of harmful genetic mutations that shorten people’s lives. The study analysed DNA from over two hundred thousand people and is one of the first attempts to probe directly how humans are evolving over one or two generations.
The major new study has analysed DNA from 215,000 people and is one of the first attempts to probe directly how humans are evolving over one or two generations.
The work, which was recently published in PLoS Biology, aimed to identify which bits of the human genome might be evolving, researchers scoured large US and UK genetic databases for mutations whose prevalence changed across different age groups.
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In the study, each person, the parents’ age of death was recorded as a measure of longevity, or their own age in some cases. People carrying a harmful genetic variant die at a higher rate, so the variant becomes rarer in the older portion of the population.“If a genetic variant influences survival, its frequency should change with the age of the surviving individuals,” says Hakhamanesh Mostafavi, an evolutionary biologist at Columbia University who led the study.
Mostafavi and his research team tested more than eight million common mutations, and found two that seemed to become less prevalent with age. 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 fell off in the population starting in middle age.
Individuals without these mutations have a survival edge and are more likely to live longer, the researchers suggest. Why such mutations might lower a person’s genetic fitness — their ability to reproduce and spread their genes — remains an open question for the researchers.
They suggest that for men, it could be that those who live longer can have more children, but this is not conclusive. They are also considering two other explanations for why longevity is important.
Number one, 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 after they can have children.
Second, it’s possible that genetic variants that are explicitly bad in old age are also detrimental — but not as obviously — earlier in life.
Moving forward, the researchers intend to broaden their study into the millions of people. They conclude:
These analyses will provide a comprehensive answer to the question of which loci affect survival, helping to address long-standing open questions such as the relative importance of viability selection in shaping genetic variation and the extent to which genetic variation is maintained by fitness trade-offs between sexes or across ages.
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