Most human populations are the product of a series of range expansions havingoccurred since modern humans left Africa some 50,000 years ago to colonize the rest ofthe world, but how have these processes influenced today's population diversity? Aninternational research team led by Damian Labuda at the University of Montreal, HélèneVézina from the University of Quebec at Chicoutimi (UQAC) and by Laurent Excoffierfrom the University of Bern and the SIB Swiss Institute of Bioinformatics have studiedthe effects of rapid territorial and demographic expansions on recent human evolution.
Using genealogies including more than one million individuals in a recently colonizedregion of Quebec, they show that pioneer individuals on the edge of the colonizationwave had a selective advantage, such that their genes are now predominantly found inthe population. Similar processes are likely to have occurred in other regions of theworld, so that this study suggests that range expansions played a key role in humanevolution. The results of their study are published today in the prestigious journalScience.
The exact mechanisms of population expansions are difficult to study as they extend overmany generations and hundreds or thousands of years. The expansion of humans into theCharlevoix Saguenay-Lac-Saint-Jean area of Quebec offered researchers a uniqueopportunity to study a range expansion in real time, thanks to the availability of deep andcomplete genealogies reconstructed from parish registers. The descending genealogies of allcouples who married in these regions between 1686 and 1960 were reconstructed thanks tothe BALSAC database managed by Hélène Vézina. The analysis of this huge genealogyincluding more than one million individuals shows that the genes present in today's populationwere mostly transmitted by ancestors who were living on or close to the wave front of theexpansion.
"We knew that the migration of species into new areas promoted the spread of raremutations through a phenomenon known as 'gene surfing', but now we find that selection atthe wave front can make this surfing much more efficient. There is thus a long-termevolutionary success of people living on the edge", Excoffier said.Women on the wave front had a selective advantage"We find that families who are at the forefront of a range expansion into new territories had agreater reproductive success.", Labuda explained. Women on the front of the expansionindeed married about one year earlier than women in the range core and had 15% morechildren and even 20% more married children.The higher fertility on the wave front is compatible with an increase in resource abundance andlowered competition among individuals to access these resources. "People could indeed marryyounger as more farm land was available on the wave front than in the core, where good landswere mostly already occupied", says Excoffier.
Human curiosity, also an inherited trait from past range expansions?
Some human traits others than those the team has measured may have also evolved duringrange expansions. More specifically, if there are some traits favoring dispersal andcolonization, it is highly likely that they have also evolved during past range expansions. Inother words, human curiosity and the desire to look over the next mountain or hilltop might beone of these inherited traits. "It is exciting to see how a study on a regional population ofQuebec can bring insights on human processes that have been going on for thousands ofyears. The BALSAC genealogical database is a powerful tool for social and genetic researchand this study is a very nice demonstration of its possibilities", Vézina said.