Though ongoing human evolution is difficult to see, researchers believe they've found signs of rapid genetic changes among the recent residents of a small Canadian town.

Between 1800 and 1940, mothers in Ile aux Coudres, Quebec gave birth at steadily younger ages, with the average age of first maternity dropping from 26 to 22. Increased fertility, and thus larger families, could have been especially useful in the rural settlement's early history.

According to University of Quebec geneticist Emmanuel Milot and colleagues, other possible explanations, such as changing cultural or environmental influences, don't fit. The changes appear to reflect biological evolution.

"It is often claimed that modern humans have stopped evolving because cultural and technological advancements have annihilated natural selection," wrote Milot's team in their Oct. 3 Proceedings of the National Academy of Sciences paper. "Our study supports the idea that humans are still evolving. It also demonstrates that microevolution is detectable over just a few generations."

How humans evolve in modern societies is an enduringly fascinating question. It's easy to assume that, in an age of developed-world plenty, biological evolutionary pressures have ceased, and biological evolution is supposedly too slow to detect in time scales of a few generations.

In fact, genetic signals of recent human evolution have been found, and there's even reason to think it's speeding up. But such genetic signatures, unconnected for now to identifiable traits, are far less glamorous and tangible than fertility.

Milot's team based their study on detailed birth, marriage and death records kept by the Catholic church in Ile aux Coudres, a small and historically isolated French-Canadian island town in the Gulf of St. Lawrence. It wasn't just the fact that average first birth age – a proxy for fertility – dropped from 26 to 22 in 140 years that suggested genetic changes. After all, culture or environment might have been wholly responsible, as nutrition and healthcare are for recent, rapid changes in human height. Rather, it was how ages dropped that caught their eye.

The patterns fit with models of gene-influenced natural selection. Moreover, thanks to the detailed record-keeping, it was possible to look at other possible explanations. Were better nutrition responsible, for example, improved rates of infant and juvenile mortality should have followed; they didn't. Neither did the late-19th century transition from farming to more diversified professions.

"I am inclined to have faith in the analyses since they are established within the quantitative genetic community," said University of Utah anthropologist Henry Harpending, who has studied changing rates of human evolution. "Evolution, gene frequency change, can work in a hurry and is working all the time in our species."

According to Harpending, the findings are part of a trend away from assuming that changes in populations are always environmental. "Here and elsewhere we are discovering that changes are due to genetic changes, not changes in the environment," he said.

Milot recommended that natural selection not be dismissed out-of-hand by demographers and anthropologists. However, while heritability appeared to account for between 30 and 50 percent of first birth age, social factors still accounted for the other 50 to 70 percent.

"Culture shapes the selection pressures acting on the age at first birth and the reproductive history of women in this population," he said. "The cultural context was favoring the selection of some genes."

Image: Tamaki Sono/Flickr

Citation: "Evidence for evolution in response to natural selection in a contemporary human population." By Emmanuel Milot, Francine M. Mayer, Daniel H. Nussey, Mireille Boisvert, Fanie Pelletier, and Denis Réale. Proceedings of the National Academy of Sciences, Vol. 108 No. 40, Oct. 4, 2011.

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