Natural selection is causing gene mutations linked to conditions like Alzheimer's disease, asthma and heart disease to be weeded out, a study suggests.
Researchers at Columbia University in the US found that sets of genetic mutations that predispose people to heart disease, high cholesterol and asthma, also appear less often in people who lived longer and whose genes are therefore more likely to be passed down and spread through the population.
"It is a subtle signal, but we find genetic evidence that natural selection is happening in modern human populations," said Joseph Pickrell, an evolutionary geneticist at Columbia.
New favourable traits evolve when genetic mutations arise that offer a survival edge. As the survivors of each generation pass on those beneficial mutations, the mutations and their adaptive traits become more common in the general population, researchers said.
Researchers analysed the genomes of 60,000 people of European ancestry in the US and 150,000 people in Britain.
In women over 70 years, researchers saw a drop in the frequency of the ApoE4 gene linked to Alzheimer's.
Researchers saw a similar drop, starting in middle age, in the frequency of a mutation in the CHRNA3 gene associated with heavy smoking in men.
The found just two common mutations across the entire human genome that heavily influence survival.
This suggests that selection has purged similar variants from the population, even those that act later in life like the ApoE4 and CHRNA3 genes, researchers said.
"It may be that men who don't carry these harmful mutations can have more children, or that men and women who live longer can help with their grandchildren, improving their chance of survival," said Molly Przeworski, an evolutionary biologist at Columbia.
They then examined sets of mutations associated with 42 common traits, from height to Body Mass Index (BMI) and determined what value of the trait they would predict based on their genetics, and whether it influenced survival.
Researchers found that a predisposition for high cholesterol and LDL "bad" cholesterol, high BMI, and heart disease was linked to shorter life spans. To a lesser extent, a predisposition for asthma was also linked to earlier death.
They also found that those genetically predisposed to delayed puberty and child-bearing lived longer - a one-year puberty delay lowered the death rate by three to four per cent in both men and women; a one-year childbearing delay lowered the death rate by six per cent in women.
The study was published in the journal PLOS Biology.