Nature selects for shorter, stouter women

YALE (US)—Researchers have detected the effects of natural selection among two generations of contemporary women and predict their descendants will be slightly shorter and chubbier, have lower cholesterol and blood pressure, and have their first children earlier in life.

The predictions, which were made in the Oct. 19 online edition of the Proceedings of the National Academy of Sciences, were based on an analysis of women who have participated in the famous Framingham Heart Study, that began in 1948. The results illustrate the medical value of evolutionary biology principles, 150 years after Darwin published the Origin of the Species, the authors say.

“The idea that natural selection has stopped operating in humans because we have gotten better at keeping people alive is just plain wrong,” says Stephen Stearns, senior author of the paper and Edward P. Bass Professor of Ecology and Evolutionary Biology at Yale University.

The reason is that traits that enable women to have children will continue to be subject to selection. As a first step, the Yale researchers measured the individual reproductive success of two generations of more than 2,000 women who participated in the Framingham study and had reached menopause. They then surveyed the traits that conferred reproductive success.

After adjusting for environmental factors such as income, education, and lifestyle choices such as smoking, the researchers estimated the heritability of traits by applying correlations among all relatives. They also adjusted for the indirect effects of selection by measuring the impacts the traits have on each other—such as whether high blood pressure is correlated with lower or higher age of sexual maturity.

The statistical analysis allowed researchers to predict which of those traits were likely to be conferred by natural selection upon the third generation of women participating in the Framingham study. The results showed that the effects of natural selection are slow and gradual, but trend towards shorter, chubbier women with lower blood pressure and cholesterol and who give birth earlier in life.

For instance, the women in the third generation of the study are predicted to begin their periods a month earlier and enter menopause a month later than their mothers and grandmothers.

However Stearns points out that the rate of change driven by natural selection found in this group of women does not differ much from rates observed in nature. “The paper drives home the point that humans aren’t different, that we are evolving at about the same average rate as other life on the planet,” Stearns says.

Researchers from the University of Pennsylvania and the Boston University School of Medicine contributed to the research, which was funded by Yale University.

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