The smaller and plumper a woman is, the more fertile she is

Medical advances mean that many people who would have died young in the past now live into their 80s, leading many to believe that natural selection no longer affects humans and that we have stopped evolving.

However, Stephen Stearns, a professor of biology at Yale University in the United States, said that although medical advances have made the lifespan of modern people no longer depend on human genes, there are still large differences in fertility. They want to know whether a trait such as women being able to give birth to more children can be passed on to future generations.

To find out, Stearns and his colleagues looked at data from the federal government's Framingham Heart Study, which has recorded the medical records of 14,000 residents of Framingham, Massachusetts, since 1948, during which time many families spanned three generations.

The research team studied 2,238 women who were past menopause and no longer able to have children. They tested the women's weight, height, blood pressure, cholesterol and other characteristics to see if these factors were related to the number of children they had. The researchers also controlled for social and cultural factors to calculate the role that natural selection played in shaping these traits.

The results show that smaller, fuller women are generally more fertile and have more children than tall, slender women; women with lower blood pressure and lower cholesterol also have more children; women who give birth to their first child at a younger age and women who enter menopause later also have more children. Surprisingly, these traits were also passed on to their daughters, who also had more children.

The researchers predict that if this trend continues for 10 generations, in 2409 women will be 2 cm shorter and 1 kg heavier than they are now, give birth to their first child 5 months earlier, and reach menopause 10 months later.

It is difficult to say what selects for these traits, or whether they are passed on through women's genes. But because the researchers controlled for many social and cultural factors, the results suggest that genes, not culture, play a role in evolution. Stearns points out that it is interesting that although cultural differences also affect human reproduction, biological characteristics still affect human evolution.