My reasoning is this Beag.
The Polar Bear and it's NA cousins can interbreed, therefore they must have had a common ancestor.
If natural selection is to work at all then both forms will continue to refine their adaptions to their environments, I see no reason to assume that evolution will halt within their separate species.
The logical outcome would then be that eventually they would no longer be able to interbreed. Also their dependence on their specific environments would make them less adaptable to changes in their environent.
Darwin believed that natural selection worked to fix speciation unless a change in environment occurred, which seems logical, it it ain't broke. don't fix it!
If natural selection is to work at all on hybrids then continous shuffling of genes must work to make that hybrid better suited to its environment, and hybrids must have a better chance of re-shuffling their genes than species that has bred true for many generations.
The ultimate 'goal' of natural selection must be to make a species less and less able to survive changes, Koala Bears, Giant Pandas, host specific parasites, all seem to be the end game.
First people deny a thing, then they belittle it, then they say it was known all along! Von Humboldt