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We're Still Evolving--And We May Be Shrinking

A subset of women in Framingham, Massachusetts, is evolving at the same rate as the average animal and plant, and will become shorter and heavier over successive generations. That means that natural selection continues to exert its influence over humans, researchers argue in a new study, one of the more ambitious to assess evolution's impact on modern humans.

Soon after Darwin published his theory of evolution, Lawson Tait, a surgeon, wrote that the law of natural selection does not apply to people because medicine keeps adverse traits in the gene pool. Some doctors still think this today, says Yale University evolutionary biologist Stephen Stearns, but they're wrong. Natural selection continues to exert its pressure through our reproductive success: the more children we have, the more our traits spread through the population.

For evidence, Stearns and colleagues turned to the Framingham Heart Study, a classic source of family history data. Started in 1948, the study has followed the cardiovascular health of about 5000 residents of Framingham, Massachusetts, and their descendents over 3 generations. Stearns and colleagues followed only women from the study because they didn't initially have information on paternity.

First, the scientists looked for correlations between the number of children the women had and eight specific traits, such as age of menopause, which might contribute to overall reproductive success. When the researchers controlled for the influence of about 50 environmental factors, such as smoking and education, they found that selection acted on these traits and, for example, led women with lower cholesterol to have more children, although the reasons are unknown.

When the team compared the traits between generations within each of the more than 900 family trees, they could estimate the traits' rate of evolution and forecast 10 generations into the future. In 10 generations, Framingham female descendents' weight and onset of menopause will increase by about 1%, height and age at first childbirth will decrease by about 1%, and cholesterol levels will fall by about 4%, the team reports online this week in the Proceedings of the National Academy of Sciences. These slow rates of evolution are on par with what evolutionary biologists see in other species' traits. "The take home message from this is that the women of Framingham are pretty average--they're just like the other plants and animals other evolutionary biologists have studied," Stearns says.

Still, Stearns cautions that these predictions make some assumptions that may not turn out to be true, like that the environment these women live in won't change significantly. Also, the predictions won't hold for women worldwide, he says, since most of the Framingham women have Northern European ancestry. He and his colleagues hope to start analyzing more diverse datasets, such as a multigenerational one from Gambia, to investigate how selection varies across the globe.

The researchers' attempt to project evolution's direction is creative and ambitious, says behavioral geneticist Joseph Rodgers at the University of Oklahoma in Norman. "There hasn't been a study done quite like this one before."