Humans, like all other organisms on Earth, are subject to the pressures of evolution. New research suggests that even in relatively modern societies, humans are still changing and evolving in response to the environment.
"Whether humans could or could not evolve in modern times could have interesting implications," study researcher Emmanuel Milot, of the University of Quebec in Montreal, told LiveScience. It could help us understand changing trends for the different traits of a population.
By studying an island population in Quebec, the researchers found a genetic push toward younger age at first reproduction and larger families. This is the first direct evidence of natural selection in action in a relatively modern human population.
Past studies have hinted our species continues to evolve, with research showing changes to hundreds of genes in the human genome over the past 10,000 years; in addition, skull measurements suggest our brains have been shrinking over the last 5,000 years or so.
An island population
The study used data from 30 families who settled on île aux Coudres, located in the St. Lawrence River outside of Quebec City, between 1720 and 1773. A church on the island held historical records of all births, deaths and marriages on the island, from which researchers were able to build intensive family trees.
The researchers analyzed the data from women who married between 1799 and 1940, comparing their relations, any social, cultural or economic differences, and the age they had their first child.
The researchers found that over a 140-year period, age at first reproduction dropped from 26 to 22, with somewhere between 30 percent and 50 percent of this variation being explained by genetic variation in the population, not by other factors, such as changes in cultures or social attitudes.
"We think, traditionally, that the changes in human population are mainly cultural, which is why a non-genetic hypothesis is given priority over a genetic or evolutionary hypothesis, whether or not there is data to support that," Milot said. "We have data that we analyzed from the genetic and nongenetic point of view, and we find that the genetic factors are stronger."
Naturally selected population
Because of the populations' lack of birth control, families in this population ended up being very large, and since fertility wasn't altered by outside influences, each couple was likely to reach maximum fertility.
The researchers didn't look at which genes might have changed over time, but they suggest reasons for the age change could include differences in fertility and how early a woman hits puberty, or even heritable personality traits that would nudge a woman to procreate earlier. These genetic factors would be changing in response to the natural selection for a higher number of kids overall.
"In that particular population, selective pressure seemed pretty constant for the study period," Milot said. "Maybe it has to do because it has a newly founded population and it was not disadvantageous to have big families."
A newly founded population would have the resources to support large families, and more kids mean the higher likelihood that one's genes would survive well into the future.
Seeing natural selection in modern populations is incredibly difficult. Because this population was pretty highly related and relatively cut off from outside populations, the correlation between genetic factors and age at first reproduction was easier to see.
"What we learn from that population is that evolution is possible in relatively modern times in modern humans," Milot said. "Where it is going to occur and in what ways is a different question."
Steve Stearns, a researcher from Yale University who wasn't involved in the study, told LiveScience in an email that the work "is an important advance, because it demonstrates a genetic response to selection in a recent, almost a contemporary, human population."
Welsh, Jennifer. 2011. "Humans Are Still Evolving, Study Says". Live Science. Posted: Available online: http://www.livescience.com/16358-human-evolution-natural-selection.html