Like defining the term “life,” defining the biological category “species” has been the subject of interminable debate. Literally dozens of definitions of “species” exist, which is probably best explained by the ineliminable role of philosophy in the debate. When differing philosophies are competing, any definition represents an act of war (so to speak) — laying a claim to the whole conceptual territory by trying to drive out the competitors using semantic fiat.

So what is the definitional equivalent of the ultimate act of war? Blow the concept “species” to smithereens. Just say that species don’t exist — at all. One cannot define what does not exist. To complete the act of destruction, coin a term such as “speciesism” to attach as a pejorative to one’s opponents. They are the baddies who insist that the unreal (i.e., the standard taxonomic category of species) actually exists out there in the world.

From the Latin

The sober name for this “blow it to bits” approach is radical nominalism. “Nominal” comes from the Latin root for name (nomen). On this view, species are artificial categories, on which, for our own purposes, we hang names. Darwin promoted radical nominalism in the Origin of Species (1859, p. 52):

I look at the term species as one arbitrarily given, for the sake of convenience, to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, for convenience’s sake.

Radical nominalism didn’t fare very well in the 20th century, however, mainly because evolutionary biologists found they needed a category with some correspondence to real groups, out there in the nature, simply to have a coherent science of living things. But the blow-it-to-bits approach never went away. 

A Program of Radical Nominalism

Recently, biologist Brent Mishler at UC Berkeley and his colleagues have laid out a full program of radical nominalism, presented in two open access volumes:

• Speciesism in Biology and Culture: How Human Exceptionalism is Pushing Planetary Boundaries
• What, if anything, are species?

So what is real, according to Mishler? Only phylogeny — the tree of evolutionary descent. We slice the tree into arbitrary units of description, but ALL the units, including the species, are unreal.

Sigh. A long sigh. Biology without species? Like chemistry without the periodic table.

Anyway, thanks to Mishler and his publishers for making the texts open access.