Polar bears aren’t a species? Since when?

15 03 2007

In honor of Carlous Linnaeus‘ 300th birthday, the journal Nature has issued a special edition focusing on the current state of taxonomy and the debates that often rage therein. What struck me most, however, was an article entitled “The species and the specious,” kicking off with a description of the debate over whether Polar Bears (Ursus maritimus) are actually species or not.

Looking at the polar bear, it seems exceedingly obvious that it is its own distinct species, just like Pandas are obviously true bears. Even so, while the debate over the true affinites of pandas were settled through genetic studies, the position of polar bears have come into question as a result of mitochondrial DNA testing, suggesting that some populations of Brown Bears (Ursus arctos)are more closely related to polar bears than other brown bears. While polar bears and brown bears may have shared a more recent common ancestor than previously believed and can produce fertile hybrids, I still don’t see how this undermines the status of the polar bear as a distinct species.

While polar bears have Hibernation Induction Trigger in their blood (inherited from their common ancestor with brown bears), there is an entire suite of morphological and behavioral adaptations that set them apart. The most striking example of this is their skin and hair, their hair actually being translucent and their skin is black, these adaptations allowing the animals to keep warm. There is a danger of becoming overheated, however, and thus polar bears do not do well in temperatures over 50 degrees Fahrenheit. Polar bears also lack the fatty hump that is indicative of brown bears, as well as having differently shaped molars. It is also important to keep in mind that brown bears do not do well in polar bear habitat and vice versa, each having their own sets of behavior. How all of this can be ignored to suggest the two are of one species, I don’t know.

Overall the debate seems to be one not of species definitions per se but of the “traditional” approach vs. cladistics/phylogentic study. Rather than sharing information and the genetic studies being taken into account with paleontology, morphology, behavior, etc., each side seems to have their own standpoint, some preferring genetics over morphology and some preferring morphology over genetics. Indeed, I have heard the argument before that our current classification system is so packed full of sub-orders and super-families that there are simply no more labels to give, and a cladistic approach is the only way to accurately reflect evolution. For my own part, I’m trying to find a way to bridge the gap in my own mind, to understand the bearing of the genetic studies on what is apparent from morphology, which I am more comfortable with. I don’t wish to dismiss genetics, but there is lots of work yet to be done and there is much we don’t get understand (and much we haven’t sequenced either), so I don’t think that the phylogenetic studies done on polar bears can rightfully undo what is visually apparent; polar bears and brown bears are different species. It might not be professional, but I think this polar bear exemplifies the expression I would use in an argument with a “lumper” of the two species;




4 responses

16 03 2007
Sarda Sahney

It is often the case with the advent of new technologies (such as our ability to investigate genomes) that we forget the common sense approach of our forbearers. While there has been (rare) evidence of polar bears hybridizing (actually I am working on a grizzly/polar bear posting right now), this is true of many other organisms that blur the line of species, for example, the captive mating of lions and tigers. As I mentioned in another posting about clouded leopard, the ability to look at genomes has not clarified our definition of species, it has compounded it.

17 03 2007

Hi Sarda, thanks for the comment! I agree that the current state of phylogenetic studies often works almost independently of other fields of biology. While I think that as we learn more about genomes we will be able to use the knowledge more accurately and effectively, there seems to be an air of “genetic studies trump all” amongst some scientists. We need more scientists who can effectively look at morphology, genetics, ecology, etc. if we are to truly understand nature, being that when scientists only act within their own discipline and don’t try to illuminate the big picture through communication and discussion, science easily turns into “stamp collecting.”

18 07 2007
Splitting lumps, lumping splits, and other taxonomic shenanigans « Laelaps

[…] about a Nature article in the special “Linnaeus issue” involving the problem with determining what actually constitutes a species. In fact, the very same day I also mentioned that a new species of Clouded Leopard (Neofelis […]

24 01 2010
Richard Zander

You have it right, Brian. Cladistics uses sister-group relationships, solely. These are indirect. Direct relationships are ancestor-descendant relationships, which can often be inferred from cladograms and other data, e.g. polar bears come from (ancestral) brown bears, obviously. Check out the Evolutionary Systematics web site at:
for more on this. I’m hoping that alert students like yourself will stop the rapid Lysenko-ish degradation of systematics.

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