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Because of advances in medical technology, fewer people are dying before they are able to have children. As a result, many people have believed that evolution had come to a halt in modern years. However, a study performed by researchers from the University of Sheffield in the United Kingdom, MRC Keneba in Gambia and Yale University in the United States has found that natural selection has continued into recent years. By studying two populations in rural Gambia, the researchers say that, as a result of natural selection, taller and thinner women are having more children. They say that trend can also be found in other areas around the world.

The study was performed by using 55 years of data obtained by the United Kingdom's Medical Research Council. The researchers analyzed thousands of women in two rural Gambian communities in East Africa. Over the time span studied, the communities experienced severe demographic changes, moving from a time period where families had a lot of children but experienced a high mortality rate, to a contemporary time period where families had fewer children but a low mortality rate.

Those demographic shifts had an impact on women's sizes. At the beginning of the study, shorter and stouter women had more children. In the present-day, taller and thinner women are having more children. Researchers believe that the change can be attributed to a number of factors, like environmental changes. They also believe that natural selection changed from impacting mortality rates to fertility rates. "Although we cannot tell directly, it may be due to health care improvements changing which women were more or less likely to reproduce," study author Alexandre Courtiol said in a statement, pointing to a clinic that was opened in the area in 1974.

However, while it may appear that the study may simply point to changes in those communities in Gambia, the researchers note that changes have occurred worldwide in recent years. As the Daily Mail reports, a recent study pointed to a similar change in the United Kingdom; taller men had more babies than shorter men, while thinner women tended to have more children.

"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," the researchers write in the study. "Thus the temporal dynamics of the evolutionary processes revealed here may reflect the shifts in evolutionary pressures being experienced by human societies generally."

The study was published in the journal Current Biology.