Species, Taxonomy, and Systematics 409the criticisms of species essentialism are avoided. Furthermore, HPC allows that
external relations play a significant role in inducing similarity among the members
of a kind. Traditional essentialism assumes that the essence of a kind is an internal
or intrinsic property of a kind’s members, such as the atomic structure of gold
or the DNA of tigers. Such intrinsic essences are ultimately responsible for a
kind’s similarities. HPC theory is more inclusive because it recognizes that both
the internal properties of organisms and the external relations of organisms are
important causes of species-wide similarities. For instance, HPC theory but not
essentialism cites interbreeding as a fundamental cause of similarity among the
organisms of many species.
While HPC theory is better at capturing the features of species than essential-
ism, a question remains: Does HPC theory provide an adequate account of species
as natural kinds? Some argue that it does not [Ereshefsky and Matthen, 2005].
Here are two problems with HPC theory. HPC theory’s objective is to explain
the existence of stable similarities within groups of entities. However, species are
also characterized by persistent differences. While HPC theory gives an account
of the similarities among the members of a species, it provides no account of the
persistent differences among a species’ members. As such, it provides an impov-
erished account of species. Polymorphism — stable variation within a species —
is an important feature of nearly every species. Species polymorphisms are easy
to find. Consider sexual dimorphism. Within any mammalian species there are
pronounced differences between males and females. Or consider polymorphism in
the life cycles of organisms. The lives of organisms consist of dramatically different
life stages, such as the difference between the caterpillar and butterfly stages of
a single organism. HPC theorists recognize the existence of polymorphism, but
they do not recognize polymorphism as a central feature of species in need of ex-
planation. HPC theorists only privilege and attempt to explain similarities. In
addition to Boyd’s ‘homeostatic’ mechanisms we need to recognize ‘heterostatic’
mechanisms that maintain species variation.
A second problem with an HPC account of species concerns the identity condi-
tions of species. The members of a species vary in their traits. Moreover, they vary
in their homeostatic mechanisms. Over time and across geographic regions, the
members of a single taxon are often exposed to different homeostatic mechanisms
[Ereshefsky and Matthen, 2005]. Given such variation, what causes organisms with
different traits and exposed to different homeostatic mechanisms to be members
of the same species? The common answer is genealogy: the members of a species
taxon form a continuous genealogical entity on the tree of life. A species’ home-
ostatic mechanisms are mechanisms of one species because they affect organisms
that form a unique lineage.
Boyd and promoters of HPC theory recognize that the importance of genealogy
and see historical relations as one type of homeostatic mechanism. However, Boyd
does not see genealogy as the defining aspect of species taxa, and this goes against
a fundamental assumption of biological systematics: that species are first and
foremost continuous genealogical entities. Consider Boyd’s example of a species