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Genetic Mutations Offer Insights on Human Diversity
Each person's collection of these changes (called "single nucleotide polymorphisms, or SNPs) contributes to his or her individuality. People with a common ancestry, however, tend to have similar collections of SNPs (pronounced "snips").
"There is no single gene, no single DNA marker, that would distinguish one population from another," Myers said. Instead, he said, "it is a pattern, like a bar code with thousands of lines on it," that allows researchers to tease apart the fine points of relatedness among populations.
He and his colleagues looked at 938 individuals from 51 different populations whose DNA is in a repository in France. The group lead by Rosenberg and Andrew B. Singleton, of the University of Virginia, studied 485 people from the same collection. Each person studied had a clear-cut ethnic identity and in most cases came from a family that had lived in the group's homeland for generations.
With such diverse and abundant starting material, the researchers were able to sketch a picture of ethnicity far more detailed than previously known.
For example, Africa's surviving hunter-gatherers -- two groups of pygmies and the San people of southern Africa who were formerly known as Bushmen -- are closely related to one another and quite distinct on a genetic basis from all other black Africans. The Hazara of Afghanistan and Pakistan and the Uygur of northwestern China are also close genetic relatives, despite living far apart. On the other hand, China's dominant ethnicity, the Han, is actually two genetically distinct groups, the northern and the southern Han.
The research shows that populations' genetic footprints on the planet are deep, sharp and not easily covered over by time.
Both research teams using the French DNA collection found geographic distance from East Africa is a major determinant of genetic differences among groups.
"Each group carried only a subset of the genetic variation from its ancestral population. So there is a loss of genetic diversity with the distance from Africa," Rosenberg said.
One of the more interesting consequences of that pattern is the subject of the third study, also published in Nature.
Carlos D. Bustamante of Cornell University and his colleagues measured SNPs in 20 European Americans and 15 African Americans. They found that the average person carries at least 2,000 SNPs that change the meaning of a genetic "word." However, in the European Americans, a larger proportion of those changes were likely to be unhealthy or unfavorable.
The reasons for this curious finding are not fully known, although there are theories.
The chief explanation is that the ancestors of Europeans (and most white Americans) suffered repeated population "bottlenecks" in which their numbers crashed as result of epidemics, environmental catastrophes and genocide. Each time that happened, the population lost a lot of its genetic diversity simply because a lot of people died.
The survivors, like their ancestors, carried a certain random collection of deleterious SNPs -- genes that caused disease or increased the risk of disease. When the population rebounded, those genes were spread widely as the small number of survivors gave rise to all living descendants.
But if they were potentially bad, why weren't they flushed out by natural selection? That is the mystery.
It may be that in the rebound after the bottleneck, the slight hazardousness of these SNPs did not make much difference. New conditions, perhaps territory free of competitors or a new technology for getting food, allowed people who carried them to flourish just as well as people who did not have them.
It is even possible that some deleterious SNPs were "dragged along" into the future because they were physically close on chromosomes to newly arising SNPs that increased a person's biological fitness.
With time, natural selection will tend to flush the deleterious SNPs out, as it has done -- relatively speaking -- in African populations. But in the case of Europeans, not enough time has passed.