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“I love getting huge boxes of blood,” says genetic ornithologist Rachel Vallender as she pulls open a drawer full of small plastic vials in her laboratory at the in Ottawa, where she’s a visiting scientist. Each tube, carefully labeled and organized, holds a blood sample from a single warbler. Whether the bird is actually a hybrid is the question Vallender seeks to answer.

Hybrids of golden-winged and blue-winged warblers are increasingly popping up across the Northeast and into Canada. The physical differences between the mixed progeny and their pure counterparts can be subtle. A bird might, for instance, have the distinctive yellow patches on its wings, the golden head, and the jet-black collar of a golden-winged warbler but with the yellowish belly of a blue-winged warbler. So individual scientists and conservation groups, including and , are gathering samples from across eastern North America and sending them to Vallender, who analyzes mitochondrial DNA in the blood to determine the birds’ genetic history. She examines the shipments she receives in free moments—on nights, weekends, and vacation days from her full-time job with Environment Canada, a government agency. The research is revealing how prevalent this intermingling of genes is and helping bring to light some of the potential dangers it poses.

Records of blue-wingeds spreading into golden-winged territory, hybridizing with them, and gradually replacing them extend back to the early 20th century. Such mixing isn’t unusual in the avian world: Nearly 10 percent of all bird species are known to occasionally interbreed. But the genetic work of Vallender, who has been studying warbler hybridization for more than a decade, backs up the observations of birders and scientists who, during the same time period, have reported growing numbers of hybrids while conducting population surveys. She’s found that in many places across the United States and Canada, hybrids now make up as much as 30 percent of golden-winged warbler populations. “This isn’t just some sporadic event anymore,” she says.

This shift, says Vallender, correlates with the onslaught of climate change. Biologists have long known that habitat loss is a major factor driving blue-winged warblers to expand their range. The bird’s preferred scrubland habitat is disappearing as abandoned farmland reverts to forest. Warming temperatures might be adding additional pressures, causing blue-wingeds to move north in search of cooler climes and into habitat already occupied by golden-wingeds.

For reasons unknown, the golden-winged warblers seem to suffer most from the interaction. While blue-winged populations are experiencing declines, golden-winged populations are plummeting, and scientists are wary of the species’ chances for long-term survival. “If [this decline] continues at the rate it has been going, we could see drastic reductions in their populations or, worst-case scenario, extinctions,” says Vallender. “We need to do this research now.”

What’s happening to the two warblers isn’t unique. Polar bears and grizzly bears are mating, as are different species of everything from butterflies to sharks.

In some instances, it’s clear that climate change is playing a role. More than 1,700 animal species across the globe have shifted their ranges northward and upward in elevation, searching for colder temperatures and following as the plants and other animals they rely on shift as well. Ice sheets and other physical barriers that once kept species apart are disappearing. All of these changes are expected to accelerate as we spew ever more greenhouse gases into the atmosphere, driving up the earth’s temperature.

Climate-driven intermixing is raising challenging conservation issues. Should hybrid offspring be protected if one parent species is threatened or endangered? Ecologically, does it matter if the world loses purebred species to hybridization? Is it best to get involved, or to let nature take its human-altered course, creating new species and eliminating others? These are the questions experts are just beginning to ponder, even as the planet continues to warm.

In 2006 an American big-game hunter from Idaho shot and killed the first documented wild polar-grizzly bear hybrid, a mostly white male covered in patches of brown fur, with long grizzly-like claws, a humped back, and eyes ringed by black skin. Four years later a second-generation “pizzly” or “grolar” was shot. After hearing reports of the bears, Brendan Kelly, then an Alaska-based biologist with the National Oceanic and Atmospheric Administration, started to wonder which other species might be interbreeding as a result of a changing Arctic landscape.

Snow and sea ice hit record lows in 2012, and the Arctic has warmed more than 3.6 degrees Fahrenheit since the mid-1960s, more than twice the global average.

To gauge what kinds of effects these shifts were having on Arctic animals, Kelly teamed up with biologist David Tallmon at the University of Alaska and conservation geneticist Andrew Whiteley at the University of Massachusetts-Amherst. The trio coauthored a for the journal Nature in 2010 that chronicled the hybridization that wildlife managers and First Nations communities had been seeing in the Arctic, including the mixing of beluga whales and narwhals, bowhead and right whales, Dall’s and harbor porpoises, hooded and harp seals, spotted and harbor seals, and North Atlantic minke and North Pacific minke whales, in addition to polar and grizzly bears. They also outlined the devastating effects the new genetic exchanges could have on biodiversity, such as parent species being driven to extinction or creating hybrids unable to survive in the environments they are born into.

The scientific community at large quickly recognized that the genetic mixing wasn’t limited to animals in the rapidly changing Arctic. Today they’re finding it all over the place, in owls, petrels, squirrels, big cats, and wild canines.

Between 2007 and 2009 researchers from several Australian universities caught 57 hybrid blacktip sharks while doing routine marine surveys off the northeast coast of Australia. Genetic tests confirmed that they were crossbreeds of Australian and common blacktips. The result of several generations of interbreeding, they were found south of the tropical areas where Australian blacktips typically live.

Elsewhere, scientists are discovering that hybridizing species are exchanging behavioral and physiological traits, not just physical ones. Mark Scriber is an entomologist and professor emeritus at Michigan State University who studies swallowtail butterflies. In 1999 he began noticing hybrids in northern ranges that could and were eating plants previously tolerated only by southern swallowtail species. He also discovered hybrids in the north whose emergence had been delayed by four or five weeks, so that they arrived too late to mate with the previous generation of butterflies and too early to mate with the next. They could mate only with each other, essentially creating a new species.

These sorts of interactions are, in their purest form, a kind of evolution, points out Kelly. For millennia, wildlife was forced together and pushed apart as climate, ecosystems, and landscapes changed. During these periods of upheaval, genes flowed between animals, creating new species and driving others to extinction. But genetic mixing that frequently takes centuries now takes only decades or even years, because modern climate change is altering the earth so quickly and drastically.

Regardless of the cause, Jim Mallet, an evolutionary biologist at Harvard University who has studied hybridization in European and South American butterflies, argues that we should let nature take its course. And while he isn’t completely alone in his thinking, most other scientists interviewed for this story were divided over whether to take action or let the interactions play out unimpeded. “My feeling is that hybridization is natural,” Mallet says. “It is the result of a mating decision by an individual, and different individuals have different desires and interests. You don’t want to label a mating decision as unnatural when it’s found in the wild.”

Still, recent human-driven hybridization could have catastrophic results for species. “The climate warming that we have induced is closer to a meteor strike [for species] than to the gradual evolution of green plants,” says Kelly, who is now the assistant director of polar science for the . “We’re forcing change to happen so quickly that it is more likely to promote extinctions than provide adaptive responses.”

Unnaturally speeding up the hybridization process can significantly affect biodiversity and the animals themselves. Pairings in which one parent species is threatened usually hastens its decline, though scientists aren’t certain why one set of genes wins out over the other, as Vallender has seen with blue-winged warblers surviving while golden-winged warblers die off.

Across the continent, Eric Forsman, a biologist with the U.S. Forest Service, has watched closely while spotted owls, a threatened species in the United States, lose their tenuous foothold in the Pacific Northwest, in part because of interbreeding with newly arrived barred owls. Barred owls have expanded their range from their native Midwestern homeland to the Pacific Coast, likely due to wildfires and climate change. “One hypothesis is that because of warming temperatures, the forests of northern Canada expanded,” scattering woodlands across the Great Plains and creating a migration corridor, Forsman says. “That may have allowed barred owls to expand west- ward across what was once a physical barrier and into spotted owl territory.” What spurs the two species to interbreed isn’t well understood, says Forsman. It may be that since there are fewer of their own species to mate with, spotted owls pair up with the first owl they see—and that’s likely to be one of the many barred owls that have moved into the area. Most barred owls, meanwhile, continue to mate with their own species. The end result is fewer spotted owls.

The more genetically similar two species are—in terms of chromosome numbers or reproductive proteins—the easier it is for them to reproduce. Dogs and cats, for example, or lions and lambs are just too different genetically to produce offspring. But even when interspecies pairing is successful, the hybrid off- spring face a series of unique challenges. Just as most mules—a cross between a male donkey and a female horse—are sterile, many hybrids cannot reproduce and are therefore genetic dead ends, says Mallet. Others inherit traits from their parents that render them ill-equipped to thrive or even survive. Polar-grizzly bear hybrids bred in captivity, for instance, can’t swim as well as genetically pure polar bears, which could pose grave risks in an ecosystem where ice sheets—the frozen platforms from which they hunt seals—are smaller and farther apart.

One of the biggest debates is about whether hybrids should be eligible for legal protection, particularly if one or both parent species are threatened or endangered. Currently, the Endangered Species Act doesn’t address hybrids. Same goes for the . (Hybrids may often be unknowingly protected because they can be difficult to distinguish from their safeguarded parents.) The U.S. Fish and Wildlife Service drafted a hybrid policy in 1996 but ultimately decided not to approve it, says J.B. Ruhl, an environmental lawyer and expert in climate change and the Endangered Species Act at Vanderbilt Law School. The agency instead adopted a policy of dealing with these animals on a case-by-case basis. Neither a Fish and Wildlife Service press officer nor several conservation lawyers could name any hybrids currently protected by the agency.

The problem, Ruhl says, is that the Fish and Wildlife Service’s current policy addresses individuals, while the real issue is populations. Scientists and conservation experts are split as to whether these legal policies should be changed to deal with the growing number of hybrids. Some see no value in keeping hybrids around at all. Stuart Pimm, a species extinction expert at Duke University, says that wiping out hybrids is the best way to protect threatened species—though doing so would be tricky, he admits. “An unfortunate aspect of all this is that hybridization is a major cause of species endangerment and disappearance,” he says. “This is not one of those circumstances where the choices are easy ones, but these hybrids are a threat to many valued species.” Hybrids have value, too, argues Richard Kock, a conservationist and member of the IUCN’s Species Survival Commission. “We should see [hybrids] as holding genes, some of which represent original species and therefore are of value,” he says. “With modern genetic understanding, breeding back to an original geno-type is not impossible. So they have a place in conservation.”

Ultimately, how we deal with hybrids will be decided among lawmakers and wildlife managers, in courtrooms and at inter- national meetings. “It becomes a value question,” says Kelly. “Do you like having a white bear that specializes in hunting seals in the ice? That’s what’s in peril.”

This story originally ran in the November-December 2013 issue as "Mix-Up."