Interspecies Hybrids Play a Vital Role in Evolution | Quanta Magazine
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Interspecies Hybrids Play a Vital Role in Evolution
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Interspecies Hybrids Play a Vital Role in Evolution
By
Jordana Cepelewicz
August 24, 2017
Hybrids, once treated as biological misfits, have been the secret saviors of many animal species in trouble. Reconciling that truth with conservation policies poses a challenge for science.
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The five big cats of the Panthera genus — leopards, tigers, snow leopards, lions and jaguars (as represented from left to right in the figure) — interbred repeatedly during the millions of years since their species diverged. Such hybridizations turn out to be surprisingly common and important in animal species evolution.
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August 24, 2017
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In 2006, a hunter shot what he thought was a polar bear in the Northwest Territories of Canada. Closer examination, however, revealed brown patches on its white fur, uncharacteristically long claws and a slightly hunched back. The creature was in fact a hybrid, its mother a polar bear, its father a grizzly. Although this cross was known to be possible — the two species had mated in captivity before — this was the first documented case found in the wild. Since then, it has become clear that this was not an isolated incident. Conservationists and others worry that if climate change continues to drive grizzly bears into polar bear territory, such interbreeding will become more common and will devastate the polar bear population. Some have even proposed killing the hybrids in an effort to conserve the species.
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But grizzlies and polar bears, as it turns out, have been mating since the species diverged hundreds of thousands of years ago. Polar bear genomes have retained mitochondrial DNA from ancient grizzly bears, and grizzlies have inherited genes from hybridizing with polar bears. “People worry that if they interbreed, polar bears will lose their beautiful white coats,” said Michael Arnold, an evolutionary biologist at the University of Georgia. “But the truth is these organisms have not been looking entirely like themselves for a long time now.”
“If this mixing is a common natural event,” he warned, “then killing hybrids to prevent them from mixing with the ‘pure’ parent genomes is not a management technique we should do lightly.” In fact, it may be that the genetic variation introduced by this kind of hybridization could save the polar bears, whose survival in the face of rising temperatures and melting ice may hinge on their ability to adapt to a rockier, less frozen habitat. Taking in some genes from grizzly bears is highly likely to be adaptive for polar bears, Arnold said, even though the results “won’t look exactly like a polar bear.”
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Grizzly-polar bear hybrids, like the one at left in this museum display, are born when grizzly bears mate with polar bears. Such interbreeding is relatively rare but happens often enough to have influenced the genetic heritage of both parental species.
Natural History Museum, London/Science Photo Library
Controversies like this one underscore the possibility that the bad reputation of naturally occurring hybrids is not entirely justified. Historically, hybrids have often been associated with the sterile or unfit offspring of maladaptive crossings (such as the mule, born of a female horse and a male donkey). Naturalists have traditionally viewed hybridization in the wild as a kind of irrelevant, mostly rare, dead-end fluke. If hybrids aren’t viable or fertile or common, how could they have much influence on evolution? But as genomic studies provide new insights into how species evolve, biologists are now seeing that, surprisingly often, hybrids play a vital role in fortifying species and helping them take on useful genes from close relatives.
In short, maladaptive pairings don’t tell the full story of interbreeding. The genetic transfer that takes place between organisms while their lineages are diverging has a hand in the emergence of adaptive traits and in...