Species

Philosophy and Species 281

The species category is heterogeneous because there are two main approaches to the

demarcation of species taxa and within each of these approaches there are several

legitimate variations. One approach is to group organisms by structural similarities.

The taxa thus generated are useful in certain kinds of biological investigations and

explanations. However, there are different levels at which structural similarities can be

sought. The other approach is to group organisms by their phylogenetic relationships.

Taxa resulting from this approach are appropriately used in answering different kinds

of biological questions. But there are alternative ways to divide phylogeny into evolu-

tionary units. A pluralistic view of species taxa can be defended because the structural

relations among organisms and the phylogenetic relations among organisms provide

common ground on which the advocates of different taxonomic units can meet.^14

I have defended a similar view myself:

In one class of species concepts—the reproductive isolation concepts of species—being

a species depends upon a natural kind: sex. We tend to generalise from zoological, and

particularly mammalian, sexual modalities, but the biological reality is that sexuality

is pluriform. Nothing that we can set up as necessary and sufficient criteria captures

all and only sexual reproduction. On the cladistic account, the only natural group is

a monophyletic group, known as a clade (a single taxon plus all and only its descen-

dants). Sex is not a trait of only a single monophyletic group, and hence is polyphy-

letic (and indeed paraphyletic). ... However, the many different modalities of sexual

reproduction are due to the many monophyletic origins of these modalities. It follows

that biospecies are evolved modes, and that there are therefore many different modes

of biospecies. Since biospecies have independent origins, on the cladistic convention

different modes of sexual species must be seen as distinct groups within the tree of

life, and the more general kind of species is the more inclusive class of asexual spe-

cies, or agamospecies. Hence, there is no natural group of biospecies. But the actual

processes that isolate the many kinds of sexual species are empirically determinable,

and so just in terms of this one concept we are limited pluralists, constrained by the

evidence. By induction, other concepts that aim to capture biological realities, will be

likewise constrained.^15

In short, ways of being species evolve, just like ways of being vertebrates, or
mammals, or angiosperms.^16 This of course raises the issue of what it is that makes
some group a species, to which I shall return (see Chapter 14, Species Realism).

(^14) Kitcher 1984, 309.
(^15) Wilkins 2003, 625.
(^16) See also Ereshefsky 1998.
TABLE 12.1
Monism, Pluralism, Realism, and Antirealism
Species... Monist Pluralist
Realist Monist realist Pluralist realist
Antirealist Monist antirealist Pluralist antirealist