https://www.nytimes.com/2014/08/17/magazine/should-you-fear-the-pizzly-bear.html

"We might regard these developments as unintended consequences of intensifying human activity on the planet. Yet in the past decade or so, scientists have discovered that the genomes of many species — far more than previously thought — contain what seem to be snippets of DNA from other species, suggesting they were shaped not only through divergent evolution but also by occasional hybridization. "

"Polar bears and grizzlies appear to have hybridized before. Descendants from past intermixing live off the southeastern coast of Alaska, on the Admiralty, Baranof and Chichagof Islands. The ABC bears, as they’re called, look outwardly like brown bears, but their mitochondrial DNA comes from polar bears, as does a portion of their X-chromosome. No one agrees when exactly the ancestors of these bears — one an omnivore, the other a carnivore that specializes in eating seals — bred. But it seems to have happened only occasionally, which suggests to some that natural climatic fluctuations, in the form of advancing and retreating glaciers, pushed the bears together, encouraging intermixture."

"“Biodiversity has developed in a web of life rather than a tree of life,” Arnold told me. That interconnectedness lends strength. “It’s sort of cool that evolution is really messy.”

The scientists I spoke to expressed often a combination of anxiety and awe as they talked about watching animal life respond to multiple pressures with a plasticity they’re only now coming to appreciate. The abiding question is: Will hybridization further erode biodiversity, preserve it, augment it — or some combination of all three?"