People still evolving
25 October 2011 by Tom Marshall
Natural selection is still at work on modern humans, and it acts quickly, according to a groundbreaking new study.
Many people think we've stopped evolving, because we now use technology to overcome problems and deal with new situations quickly, rather than waiting for many generations for evolution to help us adapt.
But research published in Proceedings of the National Academy of Sciences shows this isn't necessarily true - in the right circumstances, natural selection can still cause noticeable changes over a relatively short period.
"It's the first evidence for evolution taking place in a reasonably modern human population over such a short timescale that I'm aware of," says Dr Dan Nussey of the University of Edinburgh, one of the paper's authors.
Researchers at the Universities of Quebec and Sherbrooke in Canada and Edinburgh in the UK examined historical records of the French-Canadian population of the Isle de Coudres, northwest of Quebec, between 1799 and 1940. After collecting this data and entering it into a computer, they used complex statistical models to analyse it.
The results show that over this 140-year period, women's average age of first reproduction (AFR) - the age at which they bore their first child - dropped from 26 years to 22.
Using detailed records of relatedness among individuals in the population, the study also shows that AFR is heritable and under some degree of genetic control - a prerequisite for evolution to take place.
Remarkably, the study also suggested that an evolutionary response to natural selection, which favours earlier age at first reproduction in this population, played a major part in the decline in AFR that the researchers observed over time.
The Isle de Coudres' small population is an ideal setting to investigate evolutionary change. Church registers provide very detailed records of births, marriages and deaths, and the relatively static nature of the population, with little migration onto or off the island, make it easier for scientists to track the effects of genetic inheritance and natural selection over time.
The population is also relatively homogenous in cultural terms; there is little variation in class, culture, education or other factors that could confuse the study's results.
Experiments with animal populations have succeeded in showing evolutionary changes in response to natural selection already, but this is the first evidence for such adaptations in humans.
And many of the animal experiments have faced criticism for using insufficiently conservative statistical methods that make it too easy to find what looks like evidence for evolution. This new work on humans uses more rigorous statistical methods that mean its conclusions should be more reliable.
"In some ways this is a step forward from the studies on animals, because we use a far more conservative set of techniques that really stack the odds of finding something against us - to get a result with these methods there needs to be an extremely strong signal," Nussey explains.
The drop in AFR seen on the island is a little puzzling, as in most areas of the world the trend has been firmly in the other direction - women have in general been having children later and later in modern times as societies have become more urban and industrialised. It could be that cultural and economic factors are behind this more general trend, while much more mobile populations have diluted the impact of natural selection.
In an isolated population, all other things being equal, it's an evolutionary advantage to have as many children as possible, and one way of doing that is to start as early as you can. As the island's women started reproducing earlier, the records show they ended up with more children over their lifetimes - in biological terms, the change increased their 'lifetime reproductive success'.
It's possible that other factors besides evolution could have contributed to the change. Women could have become better-nourished and so healthier, for example, and this could have increased their breeding success.
But the evidence doesn't seem to support this; if it was true, you would expect infant mortality to fall too as the whole community enjoyed better nutrition, and this doesn't seem to have happened. Similarly, cultural or social factors could conceivably have played a role, but the study's authors have tried to take this possibility into account, although it is difficult to completely exclude such factors in studies of this kind.
They believe that studies of human population genetics and demography need to take the possibility of this kind of evolutionary change into account - at the moment they tend to assume that if evolution happens at all in modern people, it's so slow that it can safely be ignored.
The study was led by Dr Emmanuel Milot and Professor Denis Réale, both of the University of Quebec at Montreal.
'Evidence for evolution in response to natural selection in a contemporary human population' - Emmanuel Milot, Francine M Mayer, Daniel H Nussey, Mireille Boisvert, Fanie Pelletier, and Denis Réale. PNAS 11 October 2011 vol. 108 no. 41 17040-17045. DOI: 10.1073/pnas.1104210108