Ligers and Tions and Wholphins, Oh My

I know I’m late to the birthday party, but I wanted to comment on the wholphin story. That whales and dolphins can interbreed did not really surprise me. We do have mules and hinnies, after all. Not to mention the tion and the liger, bred for its skills in magic.

No, what surprised me was that the new wholphin’s mother was… a wholphin. The newborn calf is actually three quarters dolphin, one quarter (false killer) whale. Now, the term species is often defined – at least for high school students and undergrads – as a group of organisms that can interbreed and produce fertile offspring. According to this definition, horses and donkeys aren’t the same species, even though they can mate to produce mules, because mules are infertile. But if the wholphin is fertile, the same definition means dolphins and (false killer) whales are the same species.

Part of the problem here is just a bad definition. As this article explains, the fertile-offspring requirement results from a misapplication of the notion of “reproductive isolation.” While inability to produce fertile offspring is sufficient to distinguish between two species, it is not necessary.

But there’s something more intriguing here: that even superior definitions cannot “cut reality at the joints.” Nature has an annoying tendency to produce gradients instead of sharp lines. Reproductive isolation is not a zero-or-one proposition. Sometimes bottlenose dolphins and false killer whales do mate, so they must not be fully isolated. (So far as I can tell, the cetaceans’ handlers did not force them to mate, though they may have played some Barry White.) If they were completely isolated, we might expect their genomes to have diverged more over the millennia.

These observations are, I assume, pedestrian stuff for professional biologists. Yet the free and easy use of the term ‘species’ (e.g., the article’s statement that “Although false killer whales and Atlantic bottlenose dolphins are different species, they are classified within the same family by scientists”) indicates that journalists and the general public still labor under the misapprehension that a ‘species’ is some kind of natural category in the world. In fact, ‘species’ is a purely mental category, whose purpose is to impose greater order on a rather messy natural world.