Evolution During Human Colonizations: Selective Advantage of Being There First

Research published in Science Nov. 3 reveals that the first individuals settling on new land are more successful at passing on their genes than those who did not migrate. According to Dr. Damian Labuda at the University of Montréal and Sainte-Justine Hospital, the study suggests that population expansion creates opportunities for natural selection to act.

The findings come from the utilization of a unique research infrastructure, the BALSAC population database which allows the reconstruction of the structure of the Quebec population over four centuries. In this research the descending lineages of all couples married in the Charlevoix-Saguenay Lac St-Jean region between 1686 and 1960 were analyzed. This genealogy comprises more than 1 million individuals.

Dr. Laurent Excoffier, University of Berne and Swiss Institute of Bioinformatics, Dr. Damian Labuda, and Dr. Hélène Vézina, Projet BALSAC, Université du Québec à Chicoutimi, who led the study, together with research associates Claudia Moreau, Michèle Jomphe and Ph.D. student Claude Bhérer, investigated the demographic history of this region to investigate the effects of rapid territorial and demographic expansion on the dynamics of colonization and human evolution.

"We find that families who are at the forefront of a range expansion into new territories had greater reproductive success. In other words, that they had more children, and more children who also had children," Labuda explained. "As a result, these families made a higher genetic contribution to the contemporary population than those who remained behind in what we call the range core, as opposed to the wave front.

The research confirms in humans a phenomenon that has already been observed in other species with much shorter generation spans. "We knew that the migration of species into new areas promoted the spread of rare mutations through a phenomenon known as 'gene surfing', but now we find that selection at the wave front could make this surfing much more efficient," Excoffier said. This evolutionary mechanism in combination with founder effects and social or cultural transmission of reproductive behavior could explain why some genetic diseases are found at an elevated frequency in the Charlevoix and Saguenay Lac Saint-Jean regions where the study was carried out, as rare mutations can also surf during a range expansion.

"It is exciting to see how a study on a regional population of Quebec can bring insight on a human process that has been going on for thousands of years. The BALSAC population is a powerful tool for social and genetic research and this study is a very nice demonstration of its possibilities," Vézina said.

The researchers also note that, although their study concerns a whole human population spread over several centuries, it only represents a short period of human evolution at a limited geographical scale. It thus appears difficult to directly generalize these results obtained in a farmer population to what happened during other range expansions, especially considering the differences between the ecological demography of hunter-gatherer and farmer communities. But given the highly successful history of the human colonization of our planet, it appears very likely that a considerable fraction of our ancestors have lived on the edge of expansion waves. Consequently, several human traits favoring dispersal and reproduction could have evolved during phases of range expansions rather than resulting from selection in constant environments.

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