Thousands of years ago, the researchers suggest, a few wolves mated with dogs—possibly kept as pets by Native Americans—creating hybrid animals that passed on their genes.
The study appears in this week's issue of the journal Science.
The new work's genetic data is "as good as it gets," said Marc Bekoff, an ecological and evolutionary biologist at the University of Colorado at Boulder who was not involved in the study.
But the research can't answer some of the larger questions, including why darker wolves tend to live in forests.
"It does seem natural to say, Oh, well there's more black wolves in the forest because that helps them blend into their environment," study author Barsh said.
But wolves don't depend on camouflage to hunt, biologists have found, suggesting that the benefit of being genetically coded for dark fur might not be related to color.
"That's an intriguing idea, because it turns out [the same] gene is involved in humans in helping bolster the immune system to fight off infection," Barsh said.
Who's a Wolf?
The new study may also help conservation biologists think beyond traditional views on genetic diversity, Barsh added.
"There is sometimes one school of thought that to preserve species diversity is to keep the natural population pure," he said.
"This is an example where preserving genetic diversity has in fact been facilitated by a hybridization event with a domesticated animal."
The newly identified dog gene may also "ask us to consider who's a wolf," Colorado's Bekoff added.
"It's going to ask us to revise what we think about what is wild and natural [in animals], and how we protect those species."
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