The researchers used DNA taken not from the nucleus, but from mitochondria, the power plants of the cell. Each cell contains many mitochondria, but only one nucleus.
"If you are working with very degraded, ancient DNA like that from museum skins, you are way ahead using this DNA with lots of copies," said Pergams. Mitochondrial DNA evolves much more rapidly than nuclear DNA, he said, though this evolution was previously thought only to occur over thousands of years.
"We did not expect to find the rapid, consistent and directional change that we did find," he said.
While evidence of such fast change has been cited in studies of fruit flies, this is the first reported study to document such quick evolution in a mammal.
What may account for this change?
"We think it likely that the new gene sequence was either unconditionally advantageous, or that it was advantageous relative to environmental changes caused by humans," Pergams said.
"Settlers may have brought in mice with the favorable gene that were able to out-compete mice with the native variant. A less likely possibility is that mice with these new gene sequences were already present, and that dramatic changes that humans caused in the environment allowed the new gene sequence to be selectively advantageous."
Since all the mice studied were caught in forest preserves and parks, Pergams and Nyberg consider the second alternative unlikely. Future studies should reveal if the favorable gene is in older mouse specimens held by museums in other parts of the country.
In any event, Pergams thinks this research may have broad implications.
"It suggests that humans are a likely cause of such rapid evolution," Pergams said, "and that much of current phylogenetic and phylogeographi
Contact: Paul Francuch
University of Illinois at Chicago