Human Evolution

After analyzing the genomes of 210,000 people from the United States and Britain, scientists found that natural selection is weeding out unfavorable genetic variants from both populations. Specifically, they found a link between longer lifespans and less frequent genetic variants linked to Alzheimer’s disease and heavy smoking, but also some mutations linked to early puberty and asthma.

“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 in a statement.

Evolution in the making

The team set out on the lofty goal of directly measuring evolutionary fitness by examining individual genetic variants at the population level using the Genetic Epidemiology Research on Adult Health and Aging (GERA) cohort. A gene variant is an alteration in the most common DNA nucleotide sequence.

In other words, the objective was to find evidence of ongoing natural selection in humans. We know all species on Earth are constantly evolving with each generation by adapting to an ever changing environment, but how can we tell that humans are evolving per se? That’s a rather challenging line of work since gene mutations and variance are incredibly subtle. One approach would be to track age-specific mortality and fertility of hundreds of thousands of individuals “to directly observe the effects of genotypes on survival,” and that’s exactly what the international team of researchers set out to do.

“If a genetic variant does not influence viability, its frequency should be the same in individuals of all ages. We therefore tested for changes in allele frequency across individuals of different ages, while accounting for systematic differences in the ancestry of individuals of different ages (for example, due to migration patterns over decades) and genotyping batch effects,” the authors wrote in the journal PLOS Biology.

Out of all the mutations analyzed by the researchers at Columbia University and the University of Cambridge, two stood out at the entire population level.

One of them was the ApoE4 gene which is linked to Alzheimer’s. There are three types of the APOE gene, called alleles: APOE2, E3, and E4. Every person should typically have two copies of the gene and the alleles combination determines the APOE “genotype”.  The APOE4 allele characterizes approximately 10-15% of people and previous studies have found its presence is associated with a higher risk for Alzheimer’s and lowers the age of onset. Having one copy of E4 (E3/E4) can increase the risk by two to three times while two copies (E4/E4) can increase the risk by twelve times.

Weeding out the bad genes

In this case, the researchers found that women over 70 saw a drop in the frequency of the ApoE4 gene linked to Alzheimer’s.

Similarly, starting with middle age, US and UK populations experienced a drop in the frequency of a mutation in the CHRNA3 gene associated with heavy smoking in men.

“It was exciting to see that with the availability of large data sets, it is now becoming possible to actually observe natural selection in action in humans, and find things—like ApoE4—that make sense from what was already known,”  the study’s lead author, Hakhamenesh Mostafavi, told ZME Science.

When the team set out on the study, they expected their method to render far more variants that influence survival. The fact that they identified strong signals in only two genes suggests natural selection has purged similar variants from the populations.

“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 in a statement.

The scientists also found that people who had a predisposition for high cholesterol, high body mass index (BMI), and heart disease were linked to shorter life spans. Similarly, albeit to a lesser extent, a predisposition for asthma was linked to premature death.

Interestingly, people who were genetically predisposed to delayed puberty and child-bearing lived longer. Specifically, for every year of delayed puberty, the risk of an early grave dropped by 3 to 4 percent in both men and women while a one-year childbearing delayed lowered the risk of premature death by 6 percent in women. It’s not clear, at this point, if there’s any causal relationship between survival rates and delayed puberty.

“It could be that variants that influence puberty timing also influence another trait, and that trait is the one affecting survival. Without knowing the causal network, we cannot infer that “delayed puberty lowers the risk of death,” Hakhamenesh told me.

“Although natural selection may favor later puberty, environmental changes that may induce early puberty may override the genetics,” he added.

The team cautions, however, that while today such gene variants are being weeded out by natural selection the same can’t be said about the future. Adaptation is primarily driven by the environment, which is constantly changing — now more than ever due to technology, health care breakthroughs, and diet — so traits that are desirable now may not be any more in the future populations a couple of generations from now.

“Evolution just means changes in allele frequencies in the population. That can happen by chance or in response to pressures imposed by a particular environment. Like malaria being endemic: we know that has led to numerous adaptations in humans over the past thousands of years,” Hakhamenesh told ZME Sciene.

“In our study, we see that a variant that affects how much you smoke endangers survival. That is an example of evolution in a specific environment, because the survival cost of carrying that variant is not present in an environment in which people did not smoke.”

“But to say where we are headed as a species we’d need to know what the various environments would be like, not just what may or may not be beneficial in this one,” he concluded.

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