Tracking Genetic Data
Using previously published genetic data to track migration patterns, the researchers developed a statistical model to examine the differences in mitochondrial DNA (mtDNA) and Y chromosome data among the six groups.
Population geneticists study mtDNA and Y chromosome data to determine an individual's genetic history.
Mitochondrial DNA is passed on from mothers to both sons and daughters. Y-chromosomes are passed only from fathers to sons.
Every so often random mutations in a DNA sequence occur. These mutations, which happen naturally and are usually harmless, are called markers.
Once a marker has been identified, geneticists can count the differences within and among populations, defining a unique lineage of descent. These lineages can be used to track migration patterns.
Prior to this study it was not possible to estimate male and female migration separately. The genetic data allowed the authors to compare the movements of males and females within the same society.
What they found is that among the hill tribes of northern Thailand, on average, fewer than one male entered a patrilocal society per generation, but there were lots of new femalesmore than seven per generation.
The ratio of female immigrants versus male immigrants was much more balanced in matrilocal hill tribes. Here, the number of incoming men and women was close to the same, with around four males and three females entering per generation.
The study raises interesting questions about how social organization affects genetic diversity and suggests that genetic data can be used to interpret social organization in the past. The study is published in this week's issue of the Proceedings of the National Academy of Sciences.
Looking at Prehistoric Societies
Future studies may enable scientists to look at genetic data and determine the social organization of a particular group.
"One idea is that, perhaps earlier in human history, most groups were matrilocal or at least there wasn't a great deal of control over whether males or females migrated," Stoneking said.
According to the theory, only when resources became abundant did patrilocality arise. Stoneking said that by staying put and keeping other males out, men controlled those resources.
Under this hypothesis, patrilocality would emerge, depending on the region, between 6,000 and 12,000 years ago, as groups converted from hunting and gathering societies to agricultural societies.
"What we'd known before is that in human populations, we find, in general, bigger differences between groups for the Y chromosome than for mitochondrial DNA, which is consistent with patrilocality," Stoneking said. "In this study we were able to test what we already knew and to actually estimate male and female migration rates."
"The novel conclusion of this study is that patrilocality and matrilocality are not just simply the opposites of each other," he said. "There's a lot more going on in terms of social control of movement of individuals. ..."
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