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In the classic book series, Little House on the Prairie, Pa's wanderlust repeatedly drives the Ingalls family westward past the edges of civilization. That craving for open space is probably what drove Homo sapiens to leave Africa in the first place and spread across the globe. According to new research, the desire to expand into new territory may have provided an evolutionary advantage to those who had it over those who lacked it.
The study, published November 4 in Science, analyzed the genealogies of settlers in Canada's Charlevoix Saguenay Lac-Saint-Jean region, northeast of Quebec City. Since the colony's initiation in 1608, it underwent several waves of geographic expansion. The researchers, led by population geneticist Laurent Excoffier of the University of Montreal, looked at the colony's marriage and birth records between 1686 and 1960. The analysis found that families living on the edges of the expansions had 20 percent more children than families living at the settlement's core. They also married one year earlier, on average, and contributed up to four times more genes to the region's current population.
"This is a lovely paper," said Henry Harpending, an anthropologist at University of Utah, who did not participate in the study. Although the researchers could only include births registered in church records, which most likely excluded illegitimate births, Harpending said the researchers "did a thorough job, and analyzed lots of data."
The notion that pioneers tend to have more babies is consistent with the behavior of other species. Expose a bare patch of land, and the first plants to colonize it will most likely be species that grow quickly, reproduce early, and create many offspring. But these early colonizers eventually cede space to other plants that are slower growing but more efficient at using resources such as water, nutrients and space. Shrubs and trees, for instance, grow slowly and produce fewer offspring, but invest enough energy and resources in those offspring to make them highly competitive in the long run.
Humans are generally more like shrubs and trees: slow growing (children take more than a decade to reach adulthood) and efficient consumers of resources. (Quick-breeding rabbits and mice, by contrast, are the weeds of the mammal world.) But a change in environment can turn a slow grower into a weed. That is what happened, Harpending says, when North American settlers found themselves on the fringes of civilization.
Although ecologists have studied the dichotomy between fast versus slow growers since the 1960s, they only recently started considering how range expansion might affect those strategies; in a 2010 paper in the journal Ecology, Benjamin Phillips, a research fellow at James Cook University in Australia (who was not involved in the research), was the first to explore expansion’s effects on life-history traits. He theorized that populations have an incentive to grow exponentially when there are plentiful resources and space. Once those niches fill up, individuals switch to a more slow-growing, competitive strategy. But some individuals will always be on the outskirts, with greater space and more resources. For them, it makes sense to return to an exponential growth pattern. As populations expand outward and then space gradually fills, Phillips's theory goes, groups on the edge should experience rapid evolution between life-history strategies.
Support for the theory comes from several other ecological studies. Pine trees in expanding populations have shorter generation times and smaller, more dispersive seeds. Invasive purple loosestrife plants grow more rapidly, and presumably reproduce earlier, than loosestrife within the plants' native range. "It's quite amazing to see that the model fits for humans as well," Phillips says, "although it's not entirely surprising, since you would expect just about any species to be governed by the same natural laws."
What was surprising to Harpending was that the increased fertility on the expansion front seemed to have a genetic component. Pioneering ancestors with high fertility had children who also eventually had high fertility, although those effects were moderated by whether the offspring lived on the frontier or within the colony's core.
Although Phillips's model does not account for such cultural complexities, it looks as if the simple laws of nature explain the colonial behavior of humans just as well as that of weeds and pine trees.