What, if anything, is a beaver?

The title of this post is a reference to this paper (only the first page is available for free, but only the first two paragraphs are relevent) and to a Stephen Jay Gould essay. Both address the question of whether the group of animals corresponding to a colloquial name is actually an evolutionary entity, a monophyletic group—meaning they are all the descendents of a single common ancestor, and all the descendents of that ancestor are included in the group. As you’ll soon see, the answer in the case of beavers is yes.

The question of where in the family tree of rodents the two extant beaver species should be placed has been an open question for some time. A recent paper in PLoS ONE sheds further light on the answer. By comparing the complete mitochondrial* genomes of both North American (Castor canadensis) and Eurasian (Castor fiber) beavers to those of other rodents, the authors determined that

• beavers are a monophyletic group within the “mouse-related” rodents
• of the other species used to build the evolutionary tree, beavers are most closely related to a type of flying squirrel called the scaly-tailed squirrel (genus Anomalurus)
• the rodents may or may not be monophyletic; the glires (rodents plus lagomorphs, i.e. rabbits and their kin) are

Having complete mitochondrial genomes allowed the authors to do some other interesting things. First, they were able to calculate the substitution rate—how many base pairs of DNA are likely to mutate, by chance, every million years. This rate could be considered a proxy for the rate of evolutionary change in the species as a whole. Conversely, if we know this rate for some species, we can use it to estimate the age of other related species. This study found that in beavers, the substitution rate is much lower than in other rodents, possibly because they’re much longer lived. (Think about it: all rodents might have the same substitution rate per generation, but if beavers live much longer, they’ll have fewer generations in a given time period and thus fewer total substitutions than a shorter lived species. It would be interesting to see if the substitution rate is also low in the capybara, the largest living rodent.)

With substitution rates for many different kinds of rodents, the authors were also able to date the origin of the rodent order (with the assumption that they’re monophyletic). The data, 67 million years ago, fits rather nicely among other published estimates of the age of rodents and conveniently falls right around the great extinction at the end of the Cretaceous—i.e. the end of the dinosaurs. It’s only a correlation, but it’s fun to think of the end of the dinosaurs allowing rodents—the most speciose group of mammals—to take over the world.

Finally, the authors note that the closest living relative of the beavers is still not known. Although they’re most closely related to Anomalurus in this study, there was no complete mitochondrial genome available for the pocket gophers and kangaroo rats, which have also been proposed as close relatives of the beaver. And, of course, the fossil beavers—there are quite a few, including some truly massive species—aren’t represented in this study. Still, it seems likely that they will fall on the same branch of the evolutionary tree of their similar-looking (but much smaller!) modern relatives.

*The mitochondria are parts of cells that generate energy from food. They have their own DNA, separate from the 46 chromosomes that we more commonly call the human genome. The mitochondrial genome has some interesting properties, including the fact that it’s only transmitted matrilineally.
Horn, S., Durka, W., Wolf, R., Ermala, A., Stubbe, A., Stubbe, M., & Hofreiter, M. (2011). Mitochondrial Genomes Reveal Slow Rates of Molecular Evolution and the Timing of Speciation in Beavers (Castor), One of the Largest Rodent Species PLoS ONE, 6 (1) DOI: 10.1371/journal.pone.0014622