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14

Species Realism

1

It is really laughable to see what different ideas are prominent in various
naturalists’ minds, when they speak of “species;” in some, resemblance is
everything and descent of little weight—in some, resemblance seems to go
for nothing, and Creation the reigning idea—in some, descent is the key,—in
some, sterility an unfailing test, with others it is not worth a farthing. It all
comes, I believe, from trying to define the undefinable.
Darwin to Hooker
24 December 1856^2
No one definition has satisfied all naturalists; yet every naturalist knows
vaguely what he means when he speaks of a species.
Darwin
Origin of Species^3
Many scientists and philosophers have said something like “species are the units
of evolution” or “biodiversity”; it is even in the titles of some well-known books on
the subject.^4 What does this even mean? In this section I argue that species are real,
phenomenal, objects rather than objects of any biological theory, let alone of evolu-
tionary theory.
In addressing the impact of evolution upon the concept of species we must first
ask what are the units of evolution? The ontology depends a lot on what theory is
being employed. When talking about population genetics, the basic units, of course,
are the allele and the locus.^5 When talking about development, then the unit is the
organism or lifecycle, as it also is when you are talking about ecological interactions,
although “species” is used here as the term for a class of ecologically exchangeable
organisms; that is, organisms that play roughly the same role in the local ecosystem.
Although organisms are pretty well all different (which is the point of population
genetics), for the purpose of trophic webs (the food webs of ecology), conspecifics
are treated as being interchangeable elemental units. Then there are the larger units
of evolution: populations,^6 and the particular revision of that concept, the “deme.”^7
A deme is basically the population that can interbreed—the term in the equations
of population genetics is Ne, the number of effective, or reproductive, individuals.
However, non-breeders also play a role in many species in contributing to the fitness
of their kin, by helping raise them, or finding food, so the ontology here depends

(^1) This section is revised from Wilkins 2018, with permission from the editor.
(^2) Darwin 1888, vol. 2, 86.
(^3) Darwin 1859, 38.
(^4) Ereshefsky 1992, Claridge et al. 19 9 7.
(^5) Griffiths and Stotz 2007.
(^6) Godfrey-Sm ith 20 09.
(^7) Winsor 2000.