Researchers looked at church records from 1799 to 1940, which provided detailed accounts of marriages, births and deaths. Because the vast majority of families have remained on the island with few newcomers arriving, it was possible to build extensive family trees from the demographic data. The team studied the age at which women are capable of giving birth, a trait that’s heritable between generations.

They found that the average age at which women on the island had their first child fell from 26 years to 22 years of age over the time period. But what makes the finding unique — an example of microevolution — is the relationship between younger childbirth and family trees. The trend closely follows genetic ties, and it increased over time, suggesting the trait was passed down and favored by natural selection…

Natural selection, as suggested by a researcher not involved in the study, may have even acted on a group of genes, not just one gene, responsible for sexual maturity, according to a New York Times article.