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Study examines how humans are still evolving
Courtesy University of Chicago Medical Center
and World Science staff
By scanning the human genome, researchers say they have found more than 700 genetic variants that evolution may have favored during the past 10,000 years, illustrating how human evolution is continuing.
“There have been a lot of recent changes—the advent of agriculture, shifts in diet, new habitats, climatic conditions” during that time, said the University of Chicago’s Jonathan Pritchard,
one of the scientists. “We’re using these data to look for those signals of very recent adaptation.”
The data consisted of DNA from 209 unrelated individuals: 89 East Asians, 60 Europeans and 60 Yorubans from Nigeria. The researchers said they found roughly the same number of
“signals” of recent evolution in each population.
Evolutionary theory predicts that researchers can find genes favored by recent evolution because
the DNA surrounding these genes has specific features.
According to the theory, the most favored genes should spread so rapidly through a population that there’s little time for a gene shuffling that
occurs each generation, called recombination, to change nearby genes. This produces a characteristic homogeneity among the chromosomes carrying that gene variant. Pritchard and colleagues scanned the data for genomic regions fitting this pattern.
“We found many of the strongest signals in the genomes, but there may be many more that we’ve missed,”
Pritchard said. The study was published online March 7 in the research journal
Public Library of Science - Biology.
Among the favored gene variants that the team identified were previously known sites of recent adaptation, the researchers said. These included the salt-sensitive hypertension gene,
associated with a condition in which salt intake raises blood pressure, and the lactase gene,
which the scientists called the strongest signal in the genome hunt.
The lactase mutation, which lets adults digest milk, appeared in about 90 percent of Europeans, the researchers reported. “Presumably,” Pritchard said, “a few thousand years from now, if selection pressure remains the same, everyone will have” this mutation. Selection is the process in which evolution weeds out genes less fit for a given environment in favor of more useful ones, which then spread through populations.
The genes found to have the strongest evidence of “positive selection”—that is, of being evolutionarily favored—included genes related to smell, reproduction-related processes and carbohydrate metabolism, including the lactase gene, the researchers said.
Also showing signals of selection were genes linked to metabolism of foreign
compounds, they reported, as well as to brain development and form. Five
such genes, involved in skin pigmentation, appear to have been favored in
Europeans specifically, they added.
Genes affecting reproduction and sexual competition tend to evolve quickly in many organisms, including primates. Signals of selection in these areas were found in all three populations, according to Pritchard.
“Many of the signals, however, seem to be more specific to modern human adaptation,” he said, “like skin pigmentation, which may respond to changes in habitat, or metabolism genes, like lactase, which may respond to changes in agriculture.”
Among East Asians, the researchers said they found a strong signal of selection in alcohol dehydrogenase genes, enzymes that break down alcohol. Many East Asians have a mutation in a related gene that disrupts this process, so they can’t metabolize alcohol, Pritchard said. But such mutations “must have some additional positive effect that has been favored by natural selection.”
Evolutionary changes linked to modern diets may affect how the body uses and stores food, the researchers said.
An idea known as
the “thrifty gene” theory suggests some genes encourage efficient food storage, leading to weight gain in times of plenty. Before modern agriculture, it was important for the body to keep extra resources, but today, those genes have been linked to obesity. The researchers
said they found signals of selection in several such genes, including the leptin receptor gene, responsible for regulating fat deposits.
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