[ExI] Why Pioneers Breed Like Rabbits

[Eugen Leitl]

(same thing applies for edge of populated space)

http://www.scientificamerican.com/article.cfm?id=why-pioneers-breed-like-rabbits&WT.mc_id=SA_WR_20111110

Why Pioneers Breed Like Rabbits

Families that colonized the Canadian frontier contributed more genetic
material to the modern population than folks who stayed home, says a new
study

By Sarah Fecht  | Thursday, November 3, 2011 | 11

Pioneers at the forefront of expansion may have an evolutionary edge over
those who come later Image: Wikimedia Commons

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.