Given enough time, chronic conditions such as obesity, high cholesterol, Alzheimer’s disease and even heavy smoking might disappear from humanity without the need for medical intervention or major lifestyle changes.
The only problem is, the process will take a large number of years – enough, in fact, for scores of generations to live, die and, critically, reproduce.
The researchers analysed genomes of more than 210,000 people in the US and UK. In doing so, they discovered that genes, or groups of genes, associated with some chronic conditions were less common in people who lived to older ages.
On a basic metric, people who live into old age are more likely to have successfully passed on their genes more often than people who die relatively young.
People who lacked gene-groups thought to be associated with the chronic conditions were better represented in the older cohort than those with the suspect genes. It is reasonable to assume that, over many generations, their descendants will eventually outnumber everyone else.
“It’s a subtle signal, but we find genetic evidence that natural selection is happening in modern human populations,” says study coauthor Joseph Pickrell.
Pickrell and colleagues looked at the genomes of 60,000 people in the US and 150,000 people in the UK. All were of European ancestry.
Two mutation-related patterns were particularly prominent.
Looking at women over 70 the researchers found a drop in those carrying a gene called ApoE4, which has been linked to Alzheimer’s.
The finding was consistent with several earlier pieces of research that found carrying one or two copies of ApoE4 was strongly correlated with early death.
The other major pattern that was found concerned a gene called CHRNA3, which is linked to heavy smoking in men. The genomes revealed that the presence of the gene became less frequent in older people, starting in middle age.
“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,” says coauthor Molly Przeworski.
Of course, many chronic conditions are influenced by hundreds of genes and mapping their effect is challenging. To moderate this phenomenon Przeworski and her colleagues looked at 42 common conditions in tandem with the genomes of each person in the study.
They found that a predisposition to high cholesterol or a BMI at obese level were associated with early deaths. Interestingly, they also found that, for women, delayed puberty and delayed age of first childbirth were both associated with small but significant increases in longevity.
The findings suggest several targets for research, but, the geneticists warn, genetic resilience (or vulnerability) is also heavily influenced by environmental factors.
“The environment is constantly changing,” says co-author Hakhamenesh Mostafavi.
“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.”