Genetically Brown and polar bears are subspecies not separate species. They have also had 3+X as many generations since the last ice age vs people.
> A comparison of the nuclear genome of polar bears with that of brown bears revealed a different pattern, the two forming genetically distinct clades that diverged approximately 603,000 years ago,[25] although the latest research is based on analysis of the complete genomes (rather than just the mitochondria or partial nuclear genomes) of polar and brown bears, and establishes the divergence of polar and brown bears at 400,000 years ago.
> ... However, because neither species can survive long in the other's ecological niche, and because they have different morphology, metabolism, social and feeding behaviours, and other phenotypic characteristics, the two bears are generally classified as separate species.[29]
Ed: I don't disagree with what you're saying, I just don't think it's relevant to discussions of DNA. We can call Pluto a potato, does not change anything. As to classification the trend is clearly to call them the same species we just need to let the old scientists die off ;)
So, in theory you can have genetically compatible separate species as soon as they are no longer capable of interbreeding. However, as Humans and Brown bares are both biologically capable of interbreeding and live in such a manner that the do interbreed they don't fit that definition. Temporary barriers are not necessarily enough as they may exchange DNA in the future.
PS: Ring species are considered a single species because a mutation can start at one end and given time make it's way around the ring. Thus they are a single pool that can and will exchange DNA.
No, that was one long discredited idea of species. It has not been used in decades. Why are you so insistent on pushing this false claim that brown bears and polar bears are the same species? Look it up, they are not. One is Ursus arctos and the other is Ursus maritimus. There is no ambiguity or dispute.
That analogy is not useful at all. Continents are not biological organisms.
>Genetically brown and polar bears can and in reality often do interbreed producing viable offspring and exchanging genetic information between populations
And yet they are separate species. Just like the 4 separate species of giraffe the article is about, which can and do all interbreed. The concept of species is in no way reliant on an inability to produce fertile offspring.
At most you can say brown and polar bears are "generally classified as separate species" But, there really is debate on this topic.