Natural selection in humans is happening more than you think

In an age when humans in industrialized countries have good access to birth control and are seldom eaten by predators, is natural selection still directing our evolution? Probably more than you might think, a new study suggests.

Natural selection is still influencing the evolution of a wide variety of human traits, from when people start having children to their body mass index, reports a study published Monday in the journal Proceedings of the National Academy of Sciences.

"It's surprising to some scientists. I think it's probably even more surprising to the general public," acknowledges lead author Jaleal Sanjak, who just completed his PhD in evolutionary biology at the University of California Irvine. "It's pretty neat."

Many traits vary among different members of a population of humans and animals, from body size to hair colour, and those differences are often linked to differences in genes. Natural selection occurs when some of those traits help some individuals survive and reproduce more than others. That causes their genes to become more common in the population over time, and it's the way species evolve to adapt to changes in their environment.

To see what traits were being selected for in humans, the researchers used DNA and medical data from more than 200,000 women over the age of 45 and 150,000 men over the age of 50 — that is, people who had mostly finished having babies — from a huge database of volunteers in the U.K. called the U.K. Biobank.

Because there aren't many differences in survival among humans in our modern lives, Sanjak and his colleagues looked at what traits were linked to a person having more children over their lifetime.

Then the researchers checked to see which of those traits had a genetic component and could therefore undergo natural selection. They did that by looking at people with similar traits and measuring how similar those people were genetically. That is, they didn't know exactly which genes were linked to the traits, which are influenced by a variety of genetic and environmental factors.

Lots of traits

The study found 13 traits in women and 10 traits in men that were linked to having more children and had a genetic component. Those included having:

• Their first child at a younger age.
• A higher body mass index.
• Fewer years of education.
• Lower fluid intelligence, which is the capacity to solve problems that require logic and reasoning.

The researchers noted that some of those traits were linked — for example, people who had their first child at a younger age tended to have fewer years of education. But interestingly, among women who had their first child when they were older, those with more education had more children.

While it might be surprising that people heavier relative to their height are having more children, their body mass index was measured after they already had children, and Sanjak notes that it's not clear whether having kids itself causes a higher body mass index or whether having a higher body mass index helps increase the number of children you have.

Just because some traits are linked to more children and have a genetic component doesn't mean they'll cause changes in the population over time either. Men had more children if they were taller, while women had more children if they were shorter, but because height genes affect men and women the same way, those two types of selection should cancel each other out, Sanjak says.

Stabilizing selection

While a lot of traits do look like they'll move in one direction or the other, the researchers also noticed a lot of traits under "stabilizing" selection, where extreme traits reduce your reproductive success. For example, being a slightly taller than average man may increase your chance of having lots of children, but being seven feet tall will drastically decrease it.

Sanjak says that's exciting not just because it hasn't been observed much in humans, but because it allows researchers to better calibrate mathematical simulations of human evolution.

Overall, he says, the study's take-home message is that humans are still evolving under natural selection, but the effects are not that strong.

He added, "It's probably true that sociological factors or secular trends in these traits are going to kind of swamp the effects of natural selection."

University of Manitoba evolutionary biologist Trevor Pemberton called Sanjak's paper a "really interesting study." Pemberton said among modern humans, traits linked to having more children are probably based on human preference rather than any actual survival or reproductive advantage. Pemberton, who studies natural selection in central African hunter gatherers, added that he'd be interested to compare the results of the new paper to what's happening in populations that continue to follow traditional ways of life.

Emmanuel Milot, an evolutionary biologist at the University of Quebec at Trois-Rivières, says many of the findings in the new study are consistent with other, smaller regional studies.

"But it's always nice to have better, more precise data to confirm that."

Milot himself has done studies of natural selection in small populations in Quebec based on church records of marriages and births.

He said the fact that Sanjak's study deals with a very recent population shows that natural selection can occur — though perhaps more slowly and weakly — even when the birth rate is very low.

"Selection never really stops," he said. "So there's a lot of room for more evolution in humans, that's for sure."