A few years ago a paper came out which suggested that the brown bears of the ABC Islands of southeastern Alaska were more closely related to polar bears than they were to other brown bears. More precisely, polar bears and ABC brown bears formed a distinct clade set apart from other brown bears, so that the class “brown bear” was not monophyletic. This meant that all the descendants of the hypothetical ancestral lineage of brown bears are not brown bears. Like reptiles, brown bears may then be paraphyletic. If this is correct polar bears can be thought of as a derived and specialized lineage of brown bears, despite all their morphological differences.
This is not just systematic arcana. The phylogenetic relationships of species has important implications for their conservation status, something all the more salient due to changes in the arctic habitat of the polar bear.
But there is a catch with the science: it focuses on mitochondrial lineages. In other words, the matriline, the female line of descent. There are technical reasons for this, primarily having to do with the tractability of generating phylogenetic trees from nonrecombining data sets of mtDNA as well as the ease of extractions of this genetic material (it’s abundant). And, in the case of ancient DNA abundance is still critical.
Last week a new paper in Current Biology reexamined the phylogenetic relationships of polar bears and brown bears using ancient DNA samples. Unfortunately it resulted in some weird titles: ‘Polar bear’s ancestor is Irish brown bear, study finds’. We’re revisiting the problem of ‘mitochondrial Eve’ all over, conflating mtDNA lineages with the total history of the species (granted, the fine print of the journalism usually alludes to this detail, but the headlines do not).
Let’s look at the paper itself. Ancient Hybridization and an Irish Origin for the Modern Polar Bear Matriline: