To biologists, polar bears and grizzlies are distinct species - not only do they look different, but a grizzly could never survive in the polar bears' icy habitat, where swimming talent is required as well as the ability to hunt seals and whales.
But don't tell that to the bears.
New DNA evidence shows that polar bears carry genetic material that came from, of all places, Ireland, where grizzlies, a.k.a. brown bears, once roamed around 30,000 years ago. The finding, published in last week's Current Biology, could change our picture not only of polar bear evolution but of the role of cross-species breeding more generally in shaping the living world.
Mixing between bear species has been reported recently, with several DNA confirmations of "pizzlies" or "grolar bears." Their appearance has been connected to the way climate change is shifting bear habitats.
The scientists refer to this kind of mixing as "opportunistic" breeding. That sounds a bit desperate, but these bears have nothing to be ashamed of; a similar DNA analysis suggests that human ancestors mixed it up with Neanderthals around the same time, leaving many of us with some bits of Neanderthal DNA.
"Hybridization seems to have occurred in the history of polar bears, and it's completely normal and natural," said biologist David Tallmon of the University of Alaska Southeast, who was not part of the team. "But my concern is the rate and extent of this is much greater now," he said, thanks to much more rapid climate change.
The new finding doesn't mean that all polar bears originated in the Emerald Isle, said Penn State biologist Beth Shapiro, who was part of the team. But it does show that, like many of us, they have some Irish blood.
Limited genetic comparisons suggest the two groups may have branched off from a common ancestor as early as 500,000 years ago, she said.
This new DNA analysis suggests that between 20,000 and 50,000 years ago, brown bears from Ireland not only interbred with polar bears, but some of their DNA spread through the entire polar bear population.
"When two closely related species overlap in range and they can mate, they often do," Shapiro said.
Though schoolchildren often learn that species are defined by who can produce fertile offspring together, that's something of an oversimplification, said biologist Michael Hofreiter of the University of York, who was not part of the team. "Speciation is a continuous process," he said. Eventually two diverging species will not have fertile offspring, and further down the line won't have offspring at all.
The team doing this latest analysis relied on a specific part of the genetic code - the so-called mitochondrial DNA, which is outside the cell nucleus and is passed down only through eggs, thus tracing out maternal lines. The team took samples from more than 200 bears living at different periods, using fossil bones and teeth.
York geneticist Mark Thomas, who was part of the team, explained how it's possible for all polar bears to have some Irish grizzly DNA without the whole species having originated from Irish grizzlies. That can happen if a few frisky hybrids mix back into the polar bear population, and some stretches of DNA inherited from brown bears spread through the next generations of polar bears by chance. It's a process that could be called luck of the Irish, or, more technically, genetic drift.
In retrospect, we shouldn't be that shocked by this, said Thomas. "If you look down at the globe, you see polar bears surrounded by a ring of brown bears." The findings, he said, might help scientists better understand the role of hybrids in evolution. In the past, they've been seen as freaks that can undermine conservation efforts by blurring a species' distinct characteristics.
Scientists should be careful how they interpret these latest findings, said biologist Charlotte Lindqvist of the University at Buffalo (SUNY). People who are opposed to taking action against global climate change may take it as evidence that polar bears will be fine, since they have mixed with grizzlies in the past, so we need not worry about them.
She said the new paper examines a tiny fraction of the DNA from these bears - just 1 percent of the mitochondrial DNA. While she finds the result intriguing, she'd like to see more of the DNA compared before drawing any sweeping new conclusions about the relationship between species.
"Hybrids have always been a difficult issue," she said. "How do we define hybrids and should they be protected?"
Alaska's Tallmon agrees. In some cases, depleted, inbred species can gain new genetic diversity by mating outside the species boundary. But mixing it up can also lead to extinction, he said. "If hybridization is occurring rapidly, to a large extent you can get one species swamped by another."
That's a serious possibility for polar bears this century. The polar bears' sea ice habitat is disappearing, Tallmon said. That may not have been such a threat when polar bears and grizzlies were hooking up thousands of years ago.
Tallman said that instead of thinking about these different bears as occupying different species categories, he sees them as branches growing apart on an ever-changing evolutionary tree.
Now, we may be seeing those branches grow back together. That's not just a threat for polar bears but for many species, as human activity changes and destroys habitats that the animals have occupied for millennia.
"From a conservationist perspective, we want to preserve the tips of those branches, and the concern is we're going to see a lot of them blend back together."