Natural Selection In The Modern Human Species

Yale University’s Stephen Stearns argues we’re not done evolving:

Stearns hired a postdoc, Sean Byars, and together with Govindaraju and Douglas Ewbank, a University of Pennsylvania demographer, set to work analyzing a handful of medically relevant traits for their effects on women’s lifetime reproductive rates. They measured the statistical associations between the traits and family size in the first two generations of Framingham women to estimate the strength of natural selection and the potential genetic response to selection. Early results show that women with lower cholesterol, lower blood pressure, lower blood glucose, and women who conceive earlier in life and reached menopause at a later age, all had more offspring. As a result, a model based on the data (that also controlled for social factors that influence fertility) predicts that levels of all these genetically based traits will change over the next generation. “People, myself included, may have written off evolution in humans,” says Ewbank. “But it’s still there. It’s still happening.” But the same response won’t continue indefinitely. Obviously, says Stearns, “if selection continued to reduce cholesterol, we couldn’t build a brain.”

The researchers also plan to study several other traits of medical interest, including high-density lipoproteins, triglycerides, and bilirubin levels. The researchers have not yet looked at the genetics behind the trend, but neither did Darwin when he was studying natural selection, which can be demonstrated solely by showing statistically that a trait can be heritable and lead to larger family sizes.

“The findings are startling,” says Govindaraju. “This will set the stage for really a new way of thinking about modern populations,” and allow researchers to make short-term predictions about humanity’s future evolution. Peter Ellison, a Harvard evolutionary biologist who was not involved in the study, agrees. “We can now think about human phenotypes much more dynamically than we normally do,” he says.

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