Well, I have news for you people, about grizzles and polar bears, from my “botanist,” Glen. He has an account here as Cirbryn He was with the Wis.dm migration. He sz:
“Alright, you asked, so it’s your own fault if this is long: (This was the only thing that he was wrong about. It wasn’t my fault. It was Rarebear’s fault. He referred me to him.)
What it means is that the inability to produce viable hybrid offspring isn’t the only determining factor, even though a lot of people think it is. If two populations can’t produce viable hybrids, then they will be separate species, but if they can, then they won’t necessarily be the same species.
The determining factor is the presence of genetically-based “isolating mechanisms” that prevent significant mixing of the gene pools. The most effective such mechanism is complete inability to produce viable hybrids, but there are other mechanisms that can still prevent significant mixing. Examples include preference for a particular type of habitat (as with polar and brown bears), preferences for a particular type of song or color pattern or body shape, preferences for a particular time or place of mating, etc. There’s also partial reproductive incompatibility in which hybrids on average have reduced viability, but some remain capable of producing young (See http://darwiniana.org/mayrspecies.htm).
As just one example, we don’t know for certain that humans and chimps are completely incapable of producing viable hybrids. But the preferences of each for mating within its own species are so strong that it’s a moot point. The likelihood of significant numbers of chimp genes getting into the human genome (or the other way around) is so small that no one would suggest we might be the same species, regardless of whether we might hypothetically be able to produce a viable hybrid.
So what that means is that during times of severe environmental change, some of the things separating certain species can break down. If polar bears start losing their habitat, some of them may try to do the best they can in brown bear habitat, which might mean their only mating opportunities are with brown bears. Similarly for northern spotted owls losing their old-growth forests and having to mate with barred owls, or gray wolves losing their forest habitat (or losing all their pack members to hunters) and so occasionally interbreeding with coyotes. Possible results of such interbreeding range from business as usual (but with occasional hybrids) to hybrid populations supplanting one or both parent species. The supplantation could be due to competition or due to genetic swamping, which is what happens when so many outside genes keep entering the species gene pool that the species loses the characteristics that had made it distinct.
As it happens, my favorite example of genetic swamping involves polar bears and brown bears. A 1996 study of mitochondrial DNA found that a particular lineage of brown bears from the Admiralty, Baranof, and Chichagof (“ABC”) Islands near Alaska, are more similar to polar bears than they are to other brown bears. The interpretation was that polar bears had evolved from ABC brown bears. But the indicated time of separation was surprisingly recent. Even so, the example of speciation provided in the second episode of Neil De Grasse Tyson’s “Cosmos” reboot involved brown bears evolving into polar bears (https://www.youtube.com/watch?v=9XXFUKJBOlM).
However, subsequent studies didn’t find the same relationships when looking at nuclear DNA as a whole, but did find the relationship (though not as strongly expressed) when looking only at the nuclear DNA on the X chromosome. (Remember most DNA is in the nucleus, but a little bit is in a cellular organelle called a mitochondrion, which is responsible for providing energy. Mitochondria are only inherited from the mother.) A 2013 paper finally made sense of it all: the ABC brown bears had originally been a population of polar bears. When the ice from the last glaciation receded, male brown bears (which range farther than females) entered the area and interbred with the female polar bears that had stayed on the islands despite the retreating ice. The hybrids in the next generation were better able to deal with the warmer climate, and they interbred, year after year, with new waves of male brown bears coming in from the mainland. The result was that what originally had been a polar bear population turned into a brown bear population. But because it was generally male brown bears coming in, the population kept much of their original polar bear DNA whenever that DNA was in packets (such as mitochondria) passed on primarily by the females.
It’s still possible that polar bears evolved from brown bears. But since it happened much further back in time, it’s also possible that some previously-existing species gave rise to both of them.”
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