Short, stout women have always been the picture of fertility. It was once assumed that women who fell into this size demographic were more fertile and thus natural selection purported fertility in the demographic. However, with reductions in fertility and mortality rates, natural selection may have a new demographic in mind. According to research data compiled over 55 years, natural selection is moving from short, stout women to tall thin women – a change that could have global implications, according to researchers.

In a study published in Current Biology, researchers have found that women in Gambia are changing and so is natural selection. Short, stout women used to be the most fertile, but with advances in healthcare and reduced fertility and mortality rates, a shift in fertility is being observed. Today, fertile women are taller and thinner than ever before, a fact researchers believe supports the theory that evolution doesn’t stop when mortality rates decline, it just changes.

While researchers are unsure how the shift will spread into western cultures, they believe the changes could affect global fertility and natural selection. Particularly, researchers believe how humans respond to this change will affect the future of evolution. According to study authors, “Our results are important because the majority of human populations have either recently undergone, or are currently undergoing, a demographic transition from high to low fertility and mortality rates. Thus the temporal dynamics of the evolutionary processes revealed here may reflect the shifts in evolutionary pressures being experienced by human societies generally.”

Source: Alexandre Courtiol, Ian J. Rickard, Virpi Lummaa, Andrew M. Prentice, Anthony J.C. Fulford, Stephen C. Stearns. The Demographic Transition Influences Variance in Fitness and Selection on Height and BMI in Rural Gambia. Current Biology, 2013; DOI: 10.1016/j.cub.2013.04.006