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8

Reproductive

Isolation Concepts

They opened Buffon again and went into ecstasies at the peculiar tastes of

certain animals.

...

They wanted to try some abnormal mating.

...

They made fresh attempts with hens and a duck, a mastiff and a sow, in the

hope that monsters would result, but quite failing to understand anything about

the question of species.

This is the word that designates a group of individuals whose descendants

reproduce, but animals classified as different species may reproduce, and oth-

ers, included in the same species, have lost the ability to do so.^1

The notion that species are kinds of organisms delineated not by the decisions of the
classifiers but by the reproductive behaviors and results of the organisms themselves
is an old one. As we have seen, it was suggested as part of John Ray’s definition
in 1688, and also by Buffon in 1748, but it goes back in the form of the generative
conception of species to the time of the Greeks. To a greater or lesser degree, it has
been a component of nearly all species concepts since Linnaeus (his sexual system
implicitly required reproductive isolation), and even now, it is a key component of
phylogenetic species concepts and most other operational definitions.^2
The generative conception of species has been applied to living beings effectively
back to Epicurus and the neo-Platonists. That is to say, there has always been a
requirement not only of constancy of form, but of the reproduction of form, in defini-
tions of living species. This is surprising, since the implication or tacit assumption
of many discussions, such as Mayr’s^3 o r H u l l’s ,^4 has been that species had been for
much of the history of the concept arid definitional constructs based on “essences.”
There can be no doubt that essence has played a role in species concepts. However, as
we have seen, at least since Locke there has always been a tacit distinction between
the real essence of a species (that is, what causes a species to be a species, its “real

(^1) Flaubert 1976, 87. Thanks to Neil Thomason for bringing this wonderful passage, first published in
1881, to my attention.
(^2) Cracraft 2000.
(^3) Mayr, 1982.
(^4) Hull 1965a, 1965b, 1988b.