414 Marc Ereshefsky
advocated by Kitcher [1984], Ereshefsky [1992], and Dupr ́e [1993] is not episte-
mologically driven. It is motivated by the idea that evolutionary theory, a well
substantiated theory, tells us that the organic world is multifaceted. Pluralism is
an ontological implication of one of our best scientific theories.
Though Kitcher, Ereshefsky, and Dupr ́e agree that species pluralism should be
adopted for ontological reasons, they adopt different versions of pluralism. Ereshef-
sky’s form of pluralism, as outlined in the previous section, asserts that the tree
of life is divided into different types of lineages — interbreeding species, ecological
species, and phylogenetic species. Furthermore, these lineages cross-classify or-
ganisms on the tree of life (as illustrated by the examples in the previous section).
Why are there different types of species lineages? They are the result of different
evolutionary forces: interbreeding species are the result of interbreeding; ecological
species are caused by natural selection; and, phylogenetic species are the result of
genealogy. To highlight only one or two of these types of lineages is to give an
impoverished account of evolution. Of course, this picture of evolution could be
mistaken, but it is our current best picture of evolution.
Kitcher and Dupr ́e adopt a different version of ontological pluralism than the
pluralism advocated by Ereshefsky. Both Kitcher and Dupr ́e recognize the three
species concepts explored here — the biological, ecological, and phylogenetic species
concepts. Kitcher and Dupr ́e also accept species concepts based on shared sim-
ilarities among the members of a species even when such species do not form
continuous genealogical entities. For example, Kitcher posits ‘structural’ species
concepts that allow species to be spatiotemporally disconnected entities (see Sec-
tion 2.2). As should be familiar by now, ‘species’ is a theoretical term in biology,
thus the ontological status of species taxa is determined by the role that term plays
in biological theory. According to evolutionary biology, species taxa are first and
foremost genealogical entities. A quick search of the biological literature shows
that the species concepts suggested by biologists assume that species taxa form
continuous genealogical entities. Given these considerations, the form of species
pluralism advocated by Kitcher and Dupr ́e is too liberal. It posits species concepts
outside of contemporary biology.
Another type of species pluralism is suggested by Mishler and Brandon [1987].
While Kitcher and Dupr ́e’s form of pluralism is too liberal, Mishler and Bran-
don’s is too conservative. Mishler and Brandon advocate a version of the phy-
logenetic species concept. They require that all species form monophyletic units
— lineages containing all and only the descendents of a common ancestor. Fur-
thermore, Mishler and Brandon suggest that different evolutionary forces cause
monophyletic lineages to be species taxa. Some phylogenetic species are main-
tained by interbreeding, others by ecological forces, and others by developmental
constraints. The result is one classification of the tree of life, but different forces
affect different branches on that tree. This form of pluralism is too conservative
because it is wedded to a phylogenetic approach to species and rules out the possi-
bility of interbreeding species that are not monophyletic. As we saw in the case of
ancestral species, many non-monophyletic interbreeding species meet population