November 4, 2011
‘First Wave’ Settlers Had Key Role In Human Evolution
A new study examining the genealogies of early human pioneers suggests that settlers who were first to colonize a new region of the world produced more offspring than the settlers who followed them, giving them a selective advantage.
The international team of researchers, who specialize in studying the effects of rapid territorial and demographic expansions on recent human evolution, analyzed the growth of human colonies in an area of Quebec, Canada, between 1686 and 1960.They found that pioneer individuals on the edge of the colonization wave had a selective advantage, such that their genes are now predominantly found among the population.
In other words, genes present in today's population were mostly transmitted by ancestors who were living on or close to the wave front of the expansion.
Similar processes are likely to have occurred in other regions of the world, suggesting that range expansions played a key role in human evolution, said the researchers.
While the exact mechanisms of population expansions are difficult to study as they extend over many generations and hundreds or thousands of years, the expansion of humans into the Charlevoix Saguenay-Lac-Saint-Jean area of Quebec offered researchers a unique opportunity to study a range expansion in real time, thanks to the availability of deep and complete genealogies reconstructed from parish registers.
The researchers used the BALSAC genealogy database — which includes the genealogies of more than one million individuals -- to reconstruct the descending genealogies of all couples that married in the region between 1686 and 1960.
"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 can make this surfing much more efficient. There is thus a long-term evolutionary success of people living on the edge,” said Laurent Excoffier from the University of Bern and the SIB Swiss Institute of Bioinformatics, one of the researchers on the project.
The analysis also found that the settlers who colonized this region of Quebec had significantly more children than those who lived within the center, or core, of an existing colony.
"We find that families who are at the forefront of a range expansion into new territories had a greater reproductive success,” said lead researcher Damian Labuda at the University of Montreal.
Indeed, women on the front of the expansion married about one year earlier than women in the range core, and had 15% more children and even 20% more married children.
The higher fertility on the wave front is compatible with an increase in resource abundance and lowered competition among individuals to access these resources.
"People could indeed marry younger as more farm land was available on the wave front than in the core, where good lands were mostly already occupied,” said Excoffier.
The researchers also said it was likely that some specific traits, such as human curiosity that favors dispersal and colonization, might be inherited from past expansions
"The wave front is a moving edge," Excoffier explained.
"This wave front is always at the periphery of the range. So, individuals begin by colonizing a given region, which becomes the wave front by definition. Then, people send migrants toward new regions, which become the wave front in turn“¦ and when a given territory has been fully settled, the wave front disappears since there is no wave of advance anymore."
Moreau and the researchers found that the majority of people currently living in the Saguenay-Lac-Saint-Jean region of Quebec are related to ancestors who lived on – or close to – the wave front of colonization.
"The registries gave us information on the parents of all married individuals, including the time and place of their marriage," said Excoffier.
"The number of children those individuals had could be deduced from the data“¦ We could actually count how many children each woman had, both on the wave front and in the core of the population."
Since this increased reproductive success only seems to occur on the wave front, the researchers argue that fertility is a trait that can rapidly evolve during range expansion, passing from one generation to the next.
"We think this heritability on the wave front and not in the core population is due to the fact that there is some competition for resources in the core, which prevents members of large families to access land and to get married early," said Excoffier.
"Since there is more land available on the front, there is less competition and, thus, this correlation persists there."
The study was be published online November 3 in the journal Science.
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