In modern life, evolution comes down to a question of fertility, not fitness.

Evolution was once translated directly as "survival of the fittest," meaning those creatures best adapted to live longer were more likely to pass on their genes. Modern humans don't face the same threats our ancestors once did, however; we're all able to live long enough to reproduce.

Yet we're still evolving.

According to a study by Yale University evolutionary biologist Stephen Stearns, contemporary evolution is based on the genetic traits surrounding fertility.

"Variations in reproductive success still exist among humans, and therefore some traits related to fertility continue to be shaped by natural selection," Stearns told Time magazine. That is, women who have more children are more likely to pass on certain traits to their progeny.

Stearns' team examined the vital statistics of 2,238 postmenopausal women participating in the Framingham Heart Study, which has tracked the medical histories of some 14,000 residents of Framingham, Mass., since 1948. Investigators searched for correlations between women's physical characteristics — including height, weight, blood pressure and cholesterol levels — and the number of offspring they produced.

According to their findings, it was stout, slightly plump (but not obese) women who tended to have more children — "Women with very low body fat don't ovulate," Stearns explains — as did women with lower blood pressure and cholesterol levels. Using a sophisticated statistical analysis that controlled for any social or cultural factors that could impact childbearing, researchers determined that these characteristics were passed on genetically from mothers to daughters and granddaughters.

If these trends were to continue with no cultural changes in the town for the next 10 generations, by 2409 the average Framingham woman would be 2 cm (0.8 in) shorter, 1 kg (2.2 lb.) heavier, have a healthier heart, have her first child five months earlier and enter menopause 10 months later than a woman today, the study found. "That rate of evolution is slow but pretty similar to what we see in other plants and animals. Humans don't seem to be any exception," Stearns says.

For more on this study, see the full story in Time magazine.