(CN) – Natural selection actively weeds out unfavorable gene variants linked to heavy smoking and Alzheimer’s disease, according to a large-scale study that reveals humans continue to evolve.
A genomic analysis of 210,000 people in the United States and Britain shows that such harmful traits are less common among older people, which suggests that genetic mutations that lead to heart disease, obesity and other health issues are not being passed down as often.
“It’s a subtle signal, but we find genetic evidence that natural selection is happening in modern human populations,” said co-author Joseph Pickrell, an evolutionary geneticist at Columbia University and the New York Genome Center.
Favorable traits emerge and develop when genetic mutations arise that offer a higher likelihood of survival. As such genes are passed down to future generations, these beneficial mutations become more common in the general population. While it can take millions of years for more complex changes to evolve, natural selection itself occurs each generation.
Following the genomic revolution, biologists can now see evolution in action by studying genetic blueprints. By tracking the ebb and flow of certain mutations across generations, researchers can identify which traits are spreading or disappearing.
The team analyzed the genomes of 150,000 people in Britain genotyped at the U.K. Biobank, and 60,000 people of European ancestry genotyped by Kaiser Permanente in California. Given the relative lack of older people in the Biobank, the researchers used the participants’ parents age at death as a proxy as they investigated the impact of specific mutations on survival.
The researchers identified two population-level mutation shifts. Women over 70 had a drop in the frequency of ApoE4, a gene linked to Alzheimer’s. This finding is consistent with earlier research suggesting that women with one or two copies of the gene tend to die much earlier than those without it. The team also noted a drop, starting in middle age, in the occurrence of a mutation in the CHRNA3 gene, which is associated with heavy smoking in men.
The team was surprised to find just two common mutations that largely influence survival across the entire human genome. The level of analysis performed in the study should have detected other variants, if they existed, the researchers said. This suggests that natural selection has already removed similar mutations from the population, including those that manifest later in life like the ApoE4 and CHRNA3 genes.
“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 co-author Molly Przeworski, an evolutionary biologist at Columbia.
However, the team added that many traits are determined by dozens to hundreds of mutations. Even larger samples, including the one the team conducted, can have trouble identifying such genes’ effect on survival. To address this limitation, the researchers examined sets of mutations associated with 42 common traits, including height and body mass index, or BMI, and calculated what value of the trait the team would predict based on the participants’ genetics. They then studied whether the traits influenced survival.
They found that a genetic predisposition for high BMI, high cholesterol and LDL, the “bad” cholesterol, and heart disease was associated with shorter lifespans. To a lesser extent, predisposition for asthma was also linked to premature death.
The researchers also determined that people genetically predisposed to delayed childbearing and puberty lived longer. A one-year puberty delay reduced the rate of death in both men and women by 3 to 4 percent. While the team believes these results are evidence that genetic variants that influence fertility are developing in certain American and British populations, they also caution that environment plays a role as well.
“The environment is constantly changing,” said lead author Hakhamenesh Mostafavi, a graduate student at Columbia. “A trait associated with a longer lifespan in one population today may no longer be helpful several generations from now, or even in other modern-day populations.”
The study may the first to examine how the human genome is evolving in a period as short as one or two generations.