October 9, 2011 12:29:07 am
From parish records in a French-Canadian island,researchers have uncovered what may be the most recent known instance of human evolution in response to natural selection.
The island,Ile aux Coudres,lies in the St. Lawrence River 50 miles northeast of Quebec. Its church registries hold an unusually complete record of births,marriages and deaths. From this data,a team of researchers led by Emmanuel Milot and Denis Reale of the University of Quebec at Montreal have extracted the life histories of women born on the island between 1799 and 1940.
Over this 140-year period,the age at which a woman had her first childa trait that is highly heritablefell to 22 years,from 26. Because of this change,women on average had four more children during their reproductive lifetime,the researchers report. The finding supports the idea that humans are still evolving, the researchers write in a recent issue of The Proceedings of the National Academy of Sciences.
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Milot said statistical tests allowed the researchers to distinguish between the effects of natural selection and those of cultural practices affecting the age of marriage. The common view is that evolution is a slow process, he said. But evolutionary biologists have known for several decades that evolution can occur fast.
It was long assumed that people protected themselves from the forces of natural selection when they learned to put a roof over their heads and grow their own food. Data from the human genome in the last decade has shown this assumption is untrue: The fingerprints of natural selection are visible across at least 10 per cent of the genome. And this is selection that occurred in just the last 25,000 to 5,000 years,because the signal from older episodes of selection is muffled by constant mutation in the DNA sequence.
Geneticists examining that sequence cannot spot episodes of natural selection more recent than 5,000 years or so,unless the signal is particularly strong,because it takes many generations for a new and improved version of a gene to sweep through a population. But evolutionary biologists believe they can detect natural selection at work in the very recent past by looking at so-called phenotypic,or bodily,data.
These data are found in large medical studies like the Framingham heart study,in which many traits of a population are monitored over many years. Using sophisticated statistical techniques,biologists say they can distinguish traits that are changing under pressure of natural selection from both those caused by environmental effects and those due to genetic driftthe random genetic change that takes place between generations.
Summarising the results of 14 such studies in an article last year in Nature Reviews Genetics,a group led by Stephen C. Stearns of Yale wrote that the emerging picture is that selection is acting in postindustrial societies to reduce age at first reproduction in both sexes,to increase age at menopause in females and to improve traits such as total blood cholesterol that are associated with the risk of disease and mortality.
The study by the University of Quebec biologists is a good analysis of an extraordinary data set, Stearns said,and is the most recent known example of a genetic response to selection in a human population.
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