Modern medical advances and access to contraception have lowered both fertility and infant mortality rates, but according to a study published in Evolution this week, natural selection will continue to shape human populations. Despite artificial influences, the genetic differences between us will continue to fuel evolution.

An international team led by Elisabeth Bolund of Uppsala University in Sweden analyzed centuries' worth of genealogical records collected from churches in Finland dating back to the early 1700s. (One of the Finnish families is pictured below.) The researchers assembled the family trees of over 10,000 people to look at how much of a trait’s variation is due to genetic influences versus environmental influences. They also looked at how the determining factors for successful traits have changed over time. In the 1860s, only 67 percent of children survived into adulthood; that number went up to 94 percent during the 1940s. Meanwhile, people went from having 5 children to 1.6 children on average during their lifetime.

In the 18th and 19th centuries, they found that 4 to 18 percent of the differences between individuals’ lifespan, family size, and ages at first and last birth were influenced by genes. The rest of the variation was driven by differences in various aspects of culture and the environment.

“This is exciting because if genes affected differences between individuals in these traits, it means they could also change in response to natural selection,” Bolund explains in a news release. “But we know that the environment has changed rapidly and dramatically, so we investigated the genetic basis of such complex traits and their ability to continue changing through evolution.”

In fact, the team found that genetic influences on timing of reproduction and family size were actually higher in recent times—meaning our modern societies can still respond to selection. “It is possible that we in modern societies have more individual freedom to express our genetic predispositions because social and normative influences are more relaxed,” Bolund adds, “and this leads to the genetic differences among us explaining more of the reproductive patterns.”

Images: shutterstock.com (top), University of Sheffield (middle)