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.

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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 drift — the random genetic change that takes place between generations.

Summarizing 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,” Dr. Stearns said, and is “the most recent known example of a genetic response to selection in a human population.”

“Our culture is changing and our biology is trying to keep up,” he said. “But culture changes faster — genes can’t change fast enough to keep up with iPads.”

Dr. Milot said the genetic changes in his study showed up so clearly because other factors that might cloud them had been held to a minimum by the particular social conditions on Île aux Coudres. The island was granted by royal decree to the priests who managed the Quebec seminary and was settled by 30 families who arrived between 1720 and 1773. The families took up farming, then other professions, like fishing. Throughout the period, considerable equality was maintained, and the population lacked the gradations of wealth that can influence who has how many children.

Also, because most people married locally, the island’s population became considerably inbred, despite a ban on marrying first or second cousins.