Natural selection, one of the primary drivers of evolution, is still occurring in humans, according to extensive research conducted by geneticists at Columbia University.
Analysing the genomes of 210,000 people across the US and the UK, the researchers found that people with longer lifespans had a lower incidence of the genetic variants associated with both Alzheimer’s disease and heavy smoking. This suggests that these traits are being slowly ‘weeded out’, as those with longer lifespans have a better chance of ensuring their genes are passed on to future generations.
“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 study co-author Molly Przeworski, an evolutionary biologist at Columbia.
While this extra few years of life won’t make much of a difference over a generation or two, over thousands or even millions of years, such an ‘edge’ can have a profound impact on human genetics.
The notion that humans aren’t evolving is a common misconception among non-scientists; we are, although many had thought natural selection – one of several evolutionary mechanisms that also include genetic drift and sexual selection – was no longer in play in humans due to our radically adapted environment and tools such as modern medicine.
However, this research suggests that is not the case, and that natural selection is still shaping how our species evolves.
“It’s a subtle signal, but we find genetic evidence that natural selection is happening in modern human populations,” said study co-author Joseph Pickrell, an evolutionary geneticist at Columbia and New York Genome Center.
The research, which is published today in the journal PLOS Biology, involved the analysis of 60,000 genomes of Americans of European ancestry genotyped by California-based Kaiser Permanente, and 150,000 genomes of British people genotyped through the UK Biobank.
The researchers found that in women over 70 saw a notable drop in the frequency of ApoE4 – a gene linked to alzheimers – while in men a similar drop was observed in the frequency of a mutation in the CHRNA3 gene – associated with heavy smoking – at middle age.
The significance of just two common mutations was unexpected; with such extensive analysis the researchers had expected to find other variants, however their absence suggests they do not exist, indicating selection is in play.
However, whether this will lead to long-term evolutionary changes depends on whether the factors for selection remain the same.
“The environment is constantly changing,” said study 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.”