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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.

Yale University news: http://opa.yale.edu/

chat10 Comments

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10 Comments

  1. jean

    Seems a little suspect to me. Age of mother when first child is born is so frequently a life-style choice.

  2. Peggy

    The decision tof if and when to try and bear children may be a lifestyle choice, but the fertility of the mother is a biological function, and therefore subject to natural selection.

  3. thomas

    Average onset of puberty in girls has been advanced by more than 6 years over the last 150 years. This is largely due to the fact that we don’t send our children to work in the mines and fields any more (so they have different fat to muscle ratios today and puberty sets in earlier). If there is any detectable genetic shift in the same direction, its much tinier than the societal/cultural changes.

    Note that humans also got taller (not smaller) over this time frame (just look at the basketball players today versus 50 years ago); I doubt that males would show trends opposite to females.

  4. Susanne

    It may be even that short, stocky women are able to bear children into their 40s…with longer fertility period. I had a short, stocky great, great, great grandmother who bore 17 children over from age 17 to mid-40s. Evolutionary possibility, anyway.

  5. Lawrence Turner

    Perhaps there is something in the water in Framingham, Massachusetts.

    Statistical anomaly perhaps?

    Or perhaps there is to much inbreeding in Framingham, Massachusetts.

  6. Laura

    Speaking as a woman who is short, chubby and has low blood pressure… I’m glad to hear this, and say “BOO” to all the naysayers.

  7. kevin

    @thomas, I like your analysis, but these guys know what they’re doing. Just because basketball players have gotten taller doesn’t mean the human population’s average height is increasing. Think of it like a roulette table. The more times you spin the wheel the more big numbers you’re going to hit…You’re also going to hit a bunch more small numbers, but even if someone rigs the wheel to hit smaller numbers more often, you’re still going to hit more big numbers with more spins of the wheel.

    In other words, the bell curve-shaped histogram of human height could be shifting towards the shorter end of the spectrum, but at the same time the sheer number of human beings in the world are increasing. This, by chance, allows for there to be taller basketball players just by increasing your odds. Let’s say your chances of having a Yao Ming-sized person with his athletic ability might be 10 million to one. In a world with 10 million people, the chances of that person being born and being in the right socioeconomic conditions to thrive is pretty small. In a world with 6 billion people, though, your chances are 600 times more likely.

  8. thomas

    Yes, you were right if we weren’t on a long-term historic road to taller people, and since a long time. Adam and Eva couldn’t have even reached today’s apple trees. Any cultural historical museum shows you couldn’t go upright through people’s doors 500 years ago, and couldn’t fit into their beds. So, this finding is something that is so unlikely and against all odds and must mean something different than what they present. The Framington study is known for oddities (= may be a pocket of society that is not the same as everybody else?).

    The probability argument is cool, though. If there wasn’t the nasty fact that today, such tall people are valued (and fed; see Jao-Ming’s history); a couple hundred years ago they were exhibited at state fairs and didn’t live long. If I take your argument and put it together with some epigenome stuff (the almost-Lamarque new version), there would also be unproportionally more taller people in the future (because they are unlikely to marry small people — see Jao-Ming’s parents). And what? This will up the average.

    The single one thing that counters all these genetics speculations is the secular trend to mature earlier and earlier. This extends the growth spurts and leads to taller people. We don’t send our kids to work any more — and people get taller. The phenome is not the genome… Now, if there’s a genetic counteracting trend, it has been smaller than the cultural change in the past, and it will continue to do so.

    Thomas

    Thomas

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