Yesterday was International Darwin Day to mark the celebration of Charles Darwin’s birthday on February 12, 1809. In honor of ol’ Darwin, I wanted to write a bit about beaver evolution.
Most people think there is one kind of beaver globally. Actually, the beaver of Eurasia (Castor fiber) and the beaver of North America (Castor canadensis) are different species. In fact, they are very different species. Many of the species Darwin used when developing his Origins of Species theory, both domestic animals like dogs and pigeons (he was a pigeon fancier) and wild animals like his famous finches, technically could breed together in many cases even though they either can’t because of isolation or don’t because of preferences. Castor fiber and Castor canadensis, however, are even more distant as species than Darwin’s examples: they have a different number of chromosomes (48 in the European and 40 in the North American). There is no known hybridization between the two species and it is assumed that they cannot produce viable offspring.
They did, of course, have a common ancestry. A study of beaver mitochondrial genomes showed that Castor canadensis branched off of Castor fiber about 7.5 million years ago when the animal migrated into the North American continent from Asia. It makes since that speciation would have occurred when the populations became geographically isolated à la Darwin.
As the figure shows there are also several recognised subspecies of the European beaver based on geographical isolation (the number varies in different publications from 5 to 8). The kind in Norway, which is the one involved in the reintroductions I study, is C. fiber fiber on the main branch.
I find it amazing–and disappointing–when I see misinformation about beaver species appearing even within the walls of natural history museums. When I visited the Zoology Museum at University of Aberdeen in Scotland last year, I noticed that the cute little Castor fiber specimen (at least it was tagged as such on its foot) had an informational card with a glaring error. It read:
There is one species, which is to be found in rivers and lakes in Europe, Asia and North America. The beaver is a water dwelling rodent…
BEAVER Castor fiber
Yikes! To see this museum sign claim that there is only one species of beaver globally is a huge error. And considering this museum is in Scotland where beaver reintroduction is an ongoing effort (and has been heavily), the museum really should make a point of saying that the species are not the same. I’ve noticed that the opponent discourse often references studies of C. canadensis to claim that salmon stocks will be damaged by the beavers, so a clear delineation between the two is imperative.
What’s not captured on the above genetic tree is the other, now extinct, branches of the Castoridae family. This included the awesome giant beaver of Pleistocene North America, castoroides. These beavers were probably twice as big as current C. canadensis, weighing in somewhere around 100 kg. I got to see the skeleton of castoroides at the Field Museum in Chicago. Beavers can bite if they feel threatened, so I can imagine that early humans 10,000 years ago were cautious around this guy (although I think the giant beaver is not nearly as scary as Zombeavers!).
The ancient speciation of beavers matters even in contemporary reintroduction projects. There were debates, for example, about which Castor fiber subspecies was best to choose for the Scottish reintroduction project. And in the latest decision about the free-living beavers in Devon, they will be allowed to stay if free from targeted parasites/disease and if they are European beavers (rather than C. canadensis). Finland has certainly had problems with its population of introduced North American beavers, so this is a wise precaution. George Monbiot mentions the loss of castoroides in his plea for rewilding the world, so its only a matter of time before someone gets to working on backbreeding it like the aurochs. Of course considering the objections many still make to regular beaver reintroduction, I can’t imagine many welcoming the giant beaver with open arms.
Darwin’s On the Origin of Species By Means of Natural Selection was a concerted effort to work out the mechanics of speciation. His insights have repercussions today in our understandings of what a species is and which species belong where. A beaver is not just a beaver. Time and space result in shifts in the population that split beavers into different things. Even ancient history matters for contemporary conservation.