Since 1996 it’s been known that, according to mitochondrial-DNA-based phylogenies, polar bears (Ursus maritimus) are actually nested within brown bears (Ursus arctos) rather than being a separate lineage. In other words, the mtDNA of some populations of brown bears—in particular, those from the Admiralty, Baranof, and Chichagof (ABC) islands of southwest Alaska—is more closely related to the mtDNA of polar bears than to the mtDNA of other brown bears.
This makes brown bears “paraphyletic” with respect to polar bears. That is, the brown bear species U. arctos does not include all of the descendants of its most recent common ancestor, since some of these descendants are placed within polar bears.
This conclusion was just confirmed by complete mtDNA sequencing of the bears. A new study in PNAS by Lindqvist et al. used “fossil DNA” from a subfossil polar bear jaw to look at the evolution of polar bears vis-à-vis their relatives. The jaw, from Norway, was estimated at about 130,000-110,000 years old, and was sufficiently well preserved that a complete polar bear mtDNA genome could be extracted and sequenced.
This sequence was compared to mtDNA sequences from two living individual polar bears and four living brown bears. The phylogeny based on this sequence is given below. The main result is that this jawbone came from a polar bear living about the time (estimated at 152,000 years ago) when the ancestors of modern polar bears diverged from those of brown bears. The authors also did stable carbon-isotope analysis of a tooth from the subfossil’s jaw, and found that the isotope values for carbon-13 were close to that of modern polar bears, suggesting to the authors that the individual engaged in “marine feeding,” i.e., ate seals.
The figure below also confirms the results of earlier phylogenies, using smaller segments of mtDNA, showing that brown bears—at least these three brown bears—from the ABC islands are more closely related to polar bears than to brown bears from other places. Again, brown bears seem to be paraphyletic.
Fig. 1 (Fig. 3 of Lindqvist et al.) A. Maximum clade probability tree of bear mtDNA using BEAST anaysis. B. Phylogenetic network of complete mtDNA genomes. Note that brown bears from the “Adm” (Admiralty) and “Baranof” populations are more closely related to polar bears (including the subfossil specimen shown in red) than to other brown bears.
The authors conclude:
The stable isotope data, phylogenetic analysis, and the geological and molecular age estimates of the Poolepynten specimen indicate that ancient polar bears adapted extremely rapidly both morphologically and physiologically to their current and unique ecology within only 10–30 ky following their split from a brown bear precursor and, subsequently, within the course of ~100 ky, spread to the full perimeter of the polar basin. As such, the polar bear is an excellent example of evolutionary opportunism within a widespread mammalian lineage (33). Moreover, the extreme proximity of the Poolepynten specimen to the polar bear ancestor provides a unique case of a morphologically and molecularly validated fossil link between living mammal species.
Now this is pretty interesting, but I don’t find it terribly exciting. I think its acceptance in PNAS is based more on the novelty of using subfossil DNA than on any new and pentrating insight into bear evolution. But I want to discuss the “paraphyly” of brown bears highlighted here and in previous work.
If this DNA-based tree really reflected the species tree, then the ancestry of the groups shows true “species paraphyly“: that is, some living populations of brown bears are more closely related to living polar bears than to other living populations of brown bears. And if that were the case, then hardcore cladists, who employ a species concept based only on “monophyly,” would not recognize the two species “brown bears” and “polar bears.” They would have to lump them together into a single species of bear. We would no longer have polar bears.
Of course, cladists aren’t rushing to do this, even though the paraphyly has been known for 14 years. Why not? Well, a lot of cladists aren’t interested in “alpha taxonomy,” the practice of naming species. But anyone who’d lump polar with brown bears would also be derided. That’s because, regardless of the genetic ancestry of these groups, the two species now seem to be independent evolutionary units, presumably isolated from each other by reproductive isolating barriers such as habitat and mate preference.
But if this is true “species paraphyly,” how could it have come about? How could one or a few populations of species X be more genetically related to members of species Y than to members of its own named species? Well, it’s possible that the ancestor of all polar bears came from only one geographic population of brown bears (that population represented by the ABC localities), and so the ancestry of polar bears reflects this origin. If gene flow were sufficiently restricted among all populations of brown bears, then the species phylogeny (which, after all, is only a formalization of evolutionary history) could reflect this localized origin.
This probably happens quite commonly, as it cannot be all that rare for a widespread species to bud off a new descendant from only one or a few of its populations. (Migration of a few individuals to an island or a distant new habitat, for example, must involve such a process.) Usually, however, gene flow among members of that big, interbreeding species would soon efface this history.
But all this presupposes that the mtDNA phyogeny gives us the true species phylogeny—the evolutionary history of the populations themselves rather than just that of mtDNA segments. Does the “gene tree” of mtDNA—which, since all the DNA in a mitochondrion is physically linked, behaves as if it were a single gene—reflect the “species tree” of bears?
It may not. We’ve known for a while that hybridization between species can occasionally move DNA between them, even after they’re formed, if reproductive barriers aren’t complete. And, for reasons we don’t understand, mitochondrial DNA (or chloroplast DNA) seems to move between species more easily than does nuclear DNA. If the ABC populations of brown bears exchanged, some time in the past, mitochondria with polar bears, though rare hybridization (and this is known to occur between the species), then sequencing mtDNA might tell us, erroneously, that for all genes, ABC populations are more closely related to polar bears than to other brown bears. And, importantly, such hybridization, which might have occurred after the polar and brown bear lineages diverged, would give us an erroneous idea of when the lineages diverged.
Such hybridization isn’t rare. There are lots of cases—Allen Orr and I list many of them in the appendix of our book Speciation (Sinauer, 2004)—in which mitochondrial-DNA based trees give a false diagnosis of paraphyly, while nuclear DNA, consisting of lots of independent genes and not just one, shows a nonparaphyletic tree. This is true for oak trees, birds, fruit flies, and many other species. Sometimes, as in the Drosophila species I work on, movement of mtDNA between different species makes them seem genetically identical, while independent nuclear genes show well-demarcated species. Hybridization between species can make it very risky to use just one gene to reconstruct their history.
Yet somehow people continue to accept mtDNA trees as equivalent to species trees. To be sure, Lindqvist et al. formally recognize that hybridization between polar and brown bears could produce an illusory species paraphyly, although they, like earlier authors, don’t give the possibility much weight (the PNAS paper gives the caveat,”Although mtDNA capture cannot be excluded to have happened between ABC bears and polar bears, these estimates nevertheless affirm with strong support a very recent divergence of polar bears from brown bears.”) But it’s time for biologists to stop calling species paraphyletic when what they mean is that genes (e.g., mtDNA) are paraphyletic.
To determine if brown bears are really paraphyletic with respect to polar bears, and thus whether cladists would designate (brown + polar) bears as a single (very variable!) species, we’d have to look at a lot more genes—and genes from the nucleus. If the consensus phylogeny from all these genes still shows the paraphyly, then systematists can worry about nomenclature. (But even if there were true species paraphyly, I’d still vote on retaining the two named species of bears, since I adhere to the “biological species concept” that is based not on phylogenies but the presence of reproductive barriers.)
For now, brown and polar bears are phylogenetically safe. But I wish that systematists would worry more about the problem of equating gene trees with species trees, and would stop relying solely on mitochondrial DNA when they can also use nuclear DNA. The more genes the better!
Lindqvist, C. et al. 2010. Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. Proc. Nat. Acad. Sci. USA 107:5053-5057,
Shields, G. F., D. Adams, G. Garner, M. Labelle, J. Pietsch, M. Ramsay, C. Schwartz, K. Titus, and S. Williamson. 2000. Phylogeography of mitochondrial DNA variation in brown bears and polar bears. Molecular Phylogenetics and Evolution 15:319-326.
Talbot, S. L., and G. F. Shields. 1996. Phylogeography of brown bears (Ursus arctos) of Alaska and paraphyly within the Ursidae. Molecular Phylogenetics and Evolution 5:477-494.