by John S. Wilkins
There are many narratives told about evolution. One of the most widely told is the Essentialism Story, replayed in textbook, popular storytelling and philosophy alike. It goes like this: Before Darwin, biologists were constrained by essentialist thinking, in which they were committed to species being natural kinds where there were essential characters shared by every member of the species. Darwin changed all this by adopting a kind of nominalism, in which every member of a species, and every species, was a unique object, and no species had members that shared characters that all members exhibited, and which no other species did. Darwin developed a view in which species were populations.
Later, Michael Ghiselin and David Hull developed an individualistic view of species, in which species were, like Darwinian individuals, particulars not classes. This is the new metaphysics of evolution. Anything else is “outmoded metaphysics” (as a review of a colleagues’ paper called it). If you aren’t with the new evolutionary metaphysics, you aren’t modern.
Only, it isn’t historically the case. As I discovered when I was doing my doctoral thesis, there is little evidence that anyone was what I now call a “taxic essentialist”. Sure, people talked about essences, of life, of organs, and so forth. But they never accepted that species had to have what we now call necessary and sufficient conditions, or that members of a species or other taxon would bear such essential properties. I am not the only person to think this. The alarm was sounded by Paul Farber in the 1970s, but recently historian of science Polly Winsor has made the same argument (I sent a copy of my thesis to her and she replied that I was “courageous”, a red flag term for a newly minted PhD if ever there was. Polly cited my thesis in her paper). I discuss the story and its falsity in my book Species.
So, when did the story arise? Polly argued that it was based on the ideas of Arthur J. Cain, taken up and disseminated by Ernst Mayr, Hull and many others. Hull was influenced directly and personally by Karl Popper, whose graduate seminar he had taken in the early 60s, resulting in the famous paper “The Effect of Essentialism on Taxonomy – Two Thousand Years of Stasis.”