The impossibility of species definitions

Sad news about the death of Sudan, the last northern male white rhino, prompted some discussion about whether the northern white rhino is a species or a subspecies. The TetZoo blog has a nice look at this specific issue. I’d like to take a broader look at the whole problem of why defining species is so hard.

Arguing over what defines a species is a long-running argument in biology. It’s practically its own cottage industry. There is much effort to define species precisely, for all sorts of good reasons. And that desire for clear, precise definitions often appears on websites like Quora. Questions come up like, “If Neanderthals bred with us, doesn’t that mean, by definition, they are the same species?”

But as much as we want clear definitions in science, there is a problem. You can’t always draw sharp dividing lines on anything that is gradual. (Philosphers know this as the continuum fallacy.)

To demand a precise definition of species is like demanding to know the precise moment that a man is considered to have a beard. For instance, I think we can agree that Will Smith, in this pic from in After Earth (2013), does not have a beard:

And that in Suicide Squad (2016), Smith pretty clearly does have a beard:

But does Smith have a beard in this pic? Er... there’s definitely some facial hair there.

What is the exact average hair length that qualifies a man to be “bearded”? There isn’t one. But that doesn’t mean that you can’t meaningfully distinguish After Earth Smith from Suicide Squad Smith.

It’s a problem that Charles Darwin recognized. In Darwin’s view, speciation was going to result from the slow, gradual accumulation of tiny, near imperceptible changes. Darwin recognized that speciation was a gradual process, and he  frequently made the point that “varieties” could be considered “incipient species.” At any given point in time, some groups would be early in that process of divergence, and some would be further along.

That’s why we shouldn’t expect there to be clear, consistent species definitions that apply across the board and are helpful in every case.

External links

The last male northern white rhino has died
How Many White Rhino Species Are There? The Conversation Continues