Really? I thought the requirements for species classification were: (1) must be able to reproduce and (2) offspring must be fertile.
Is it less objective than that?
This is really interesting, thank you. I've never heard of "ring species" before.
How many species are there? This is why the term "species" can never be entirely objective. I remember the eureka moment I had when I finally understood this (admittedly somewhat simple) point.
That said, I don't think that means that "species" is entirely subjective or meaningless.
in general, there are multi organizational levels of reproductive incompatability.
in this case, the geographic distance, orogenic blockade, and ecological confounds of arctic conditions preclude easy mingling of U.arctos x U. maritimus.