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...