In a world where we've tamed our environment and largely protected ourselves from the vagaries of nature, we may think we're immune to the forces of natural selection. But a new study finds that the process that drives evolution was still shaping us as recently as the 19th century.
The finding comes from an analysis of the birth, death, and marital records of 5923 people born between 1760 and 1849 in four farming or fishing villages in Finland. Researchers led by evolutionary biologist Alexandre Courtiol of the Institute for Advanced Study Berlin picked this time period because agriculture was well established by then and there were strict rules against divorce and extramarital affairs. The team looked at four aspects of life that affect survival and reproduction, key signposts of natural selection: Who lived beyond age 15, who got married and who didn't, how many marriages each person had (second marriages were possible only if a spouse died), and how many children were born in each marriage. "All these steps can influence the number of offspring you have," says Courtiol.
Natural selection was alive and well in all of the villages the researchers surveyed. Almost half of the people died before age 15, for example, suggesting that they had traits disfavored by natural selection, such as susceptibility to disease. As a result, they contributed none of their genes to the next generation. Of those that made it through childhood, 20% did not get married and had no children, again suggesting that some traits prevented individuals from obtaining mates and passing on their genes to the next generation.
The numbers were about the same for landed and landless individuals, indicating that wealth did not buffer the environment enough to prevent natural selection from culling or favoring individuals. "Although there is agriculture and transmission of wealth, there is still as much room for evolution to proceed as in other animals," says Courtiol, whose team reports its findings online today in the Proceedings of the National Academy of Sciences .
The Finns were also subject to sexual selection, in that men who were able to attract new mates had more offspring. With one partner, the average was about five children; with four partners, that jumped to 7.5, Courtiol notes. Men benefited more than women in terms of begetting more children, most likely because they tended to remarry young women with good child-bearing potential. Thus sexual selection was more important in men than in women.
From the records they had, the researchers could not tell which traits were being selected for, but the variation in the number of offspring—from zero to 17—indicates there was a large opportunity for selection to occur. That variation is the grist for evolution.
The importance of sexual selection is well accepted in birds and fish, "but this is the first time that sexual selection has been so well documented in humans," says Stephen Stearns, an evolutionary biologist at Yale University. As for showing natural selection, "they are providing additional, confirmational evidence."
"Without a doubt, natural selection occurs in modern humans," agrees Jacob Moorad, an evolutionary biologist at Duke University in Durham, North Carolina, who was not involved in the study. He thinks this work will inspire other researchers with large databases of data on humans to look at how selection operates in populations.
Courtiol is not certain how strong natural selection is today, particularly in the developed world. But he says that at the very least, the data show that even as recently as 200 years ago, it still played a role in shaping humans as a species. As such, he notes, biological and cultural processes should both be considered in understanding how humans are changing through time.