Species, Taxonomy, and Systematics 413B
CA
Figure 1. A branching event on a phylogenetic tree. If species must be mono-
phyletic, then A+B cannot form a species. Some of the descendant’s of A+B’s
ancestor are not contained in A+B but are in C.
for the PSC, a species must contain all and only the descendents of a common
ancestor. The ancestral species consisting of A and B violates that requirement on
species taxa: some of its descendents belong to the new species C. Thus, according
to the PSC, either there is one species present (the combination of A, B, and C),
or there are three species (the ancestral species A, which went extinct, and two
new species, B and C). Either way, the PSC and the BSC cross-classify the very
same group of organisms. Take an organism, X, in B. According to the BSC, X
belongs to a species consisting of A and B. According to the PSC, X either belongs
to a species containing only B or a species containing A, B, and C. Each species
concept places X into two different taxa.
The above examples are just the tip of the iceberg of examples where species
concepts provide different classifications of the same group of organisms. General-
izing from these examples, different species concepts give rise to different classifi-
cations of the organic world. Pluralists believe that examples like these show that
we should take a pluralistic approach to biological classification: different species
concepts provide different but equally legitimate classifications. Monists disagree.
Before turning to monist responses, let us focus on the various brands of pluralism
in the literature. This will help further articulate the pluralist’s argument.
3.2 Varieties of Pluralism
The argument for pluralism suggested above is motivated by ontological and not
epistemological considerations. Some authors (for example, Rosenberg [1994]) sug-
gest that we adopt pluralism because of our epistemological limitations. The world
is exceedingly complex and we have limited cognitive abilities, so we should ac-
cept a plurality of simplified classifications of the world. The taxonomic pluralism