The study was the work of researchers from Yale University, the University of Pennsylvania and the Boston University School of Medicine who are part of a working group sponsored by the National Evolutionary Synthesis Center in Durham, North Carolina.
A paper on the study is due to be published online this week in the Proceedings of the National Academy of Sciences. The senior author, Dr Stephen C Stearns is Edward P Bass Professor of Ecology and Evolutionary Biology at Yale.
Stearns said in a separate statement that:
"The idea that natural selection has stopped operating in humans because we have gotten better at keeping people alive is just plain wrong."
For the research, Stearns and colleagues accessed records from the 60-year Framingham Heart Study that started in 1948. They looked at data on more than 2,000 North American women who had reached menopause and analyzed about half a dozen or so traits that relate to human health in relation to the number of children they had (a measure of "reproductive success") over their lifetime.
After adjusting for possible confounders such as income, education and smoking, using statistical models, the researchers estimated the heritability of traits by applying correlations among all relatives and made short-term predictions of how each trait was likely to be passed on to the third generation of women in the study.
They also adjusted for indirect effects of selection by measuring the effect the traits have on each other: such as the extent to which high blood pressure may be linked to lower or higher age of sexual maturity.
The results suggested that:
- The women's descendents will be slightly shorter and heavier.
- Their descendents will also have lower blood pressure and cholesterol.
- The women will have their first child earlier, and will reach menopause later in life.
- For example, they predicted that the third generation of women in the study will begin their periods a month earlier and experience their menopause a month later than their mothers and grandmothers.
"The take-home message is that humans are currently evolving."
"Natural selection is still operating," he added.
While the changes may be slow and gradual, the rates they predicted are much the same as we see elsewhere in nature, said the researchers.
"These results place humans in the medium-to-slow end of the range of rates observed for other living things," explained Stearns.
"But what that means is that humans aren't special with respect to how fast they're evolving. They're kind of average," he noted.
"Natural selection in a contemporary human population."
Byars, S., D. Ewbank, et al.
Proceedings of the National Academy of Sciences 106(42), October 2009.
Sources: National Evolutionary Synthesis Center (NESCent), Yale University.