|Actual modern hybrid|
While some clades of brown bear, notably the Admiralty Islands (but also French ones) are more closely related to polar bear by mitochondrial DNA, by overall nuclear DNA they fall wholly within the brown bear clade. That’s because, mtDNA-wise polar bears are a subset of brown bears, while by nuclear DNA they are more clearly distinct.
The authors propose that this implies mtDNA introgression from brown bear into polar bears, up to the point of total displacement of the native polar bear lineages, but of course it may also be that some of their calculations are totally wrong. After all molecular-clock-o-logy is not rocket science, not at all and a total lineage replacement by just occasional inter-breeding seems a most unlikely event with the laws of probability in hand.
My impression is in fact the opposite: that the nuclear differentiation should have happened after that of the mtDNA but that molecular-clock speculations obscure this fact. But whatever. I may also be wrong, of course but I just can’t accept molecular-clock-o-logy as evidence of anything – doing that is pseudoscience.
|Phylogenetic labyrinth… or molecular-clock fanaticism?|
Update: PConroy mentions (see comments) a previous study (Current Biology 2011, Science Now article), which states that there is even closer mtDNA affinity between extinct Irish brown bears and modern polar bears than these have with the Alaskan ABC islander ones.