Timely, Pizzly Bears:
Scientists confirmed last week that a bear shot by an Inuvialuit hunter in the Northwest Territories is a second-generation grizzly-polar bear hybrid—a “pizzly” or “grolar” bear.
Not that big of a deal. It is likely that polar bears are simply a recent derived variant of brown bear. The main issue not noted in the Slate piece is that time until last common ancestor isn’t the only parameter to consider. Placental Invasiveness Mediates the Evolution of Hybrid Inviability in Mammals:
A central question in evolutionary biology is why animal lineages differ strikingly in rates and patterns of the evolution of reproductive isolation. Here, we show that the maximum genetic distance at which interspecific mammalian pregnancies yield viable neonates is significantly greater in clades with invasive (hemochorial) placentation than in clades with noninvasive (epitheliochorial or endotheliochorial) placentation. Moreover, sister species with invasive placentation exhibit higher allopatry in their geographic ranges, suggesting that formerly separated populations in mammals with this placental type fuse more readily on recontact. These differences are apparently driven by the stronger downregulation of maternal immune responses under invasive placentation, where fetal antigens directly contact the maternal bloodstream. Our results suggest that placental invasiveness mediates a major component of reproductive isolation in mammals.
For what it’s worth, primates tend toward invasive placentation.