410 Marc Ereshefsky
that arose through multiple hybridization events. Boyd [1999b , 80] characterizes
this species as containing “distinct lineages” of organisms that have significant
similarities. This species is not a single historical entity, but a collection of sep-
arate lineages with separate origins. Boyd is quite clear that similarity and not
genealogical connectedness is the final arbitrator of species sameness. This as-
sumption makes sense given that Boyd believes that species are kinds and kinds
are ultimately similarity-based classes that play a role in induction. But this view
of the identity conditions of species taxa conflicts with the standard view in bi-
ological systematics that species are continuous genealogical lineages. Boyd, like
other participants in the debate over the ontological status of species, assumes
that ‘species’ is a theoretical term in evolutionary biology, so the status of species
is determined by their role in biological theory. Nevertheless, HPC theory’s prefer-
ence for similarity-based kinds conflicts with the biological assumption that species
are genealogical entities. Consequently, HPC theory fails to capture the proper
identity conditions for species taxa.
2.4 Population Structure Theory
An alternative approach to species is offered by Ereshefsky and Matthen’s [2005]
“Population Structure Theory” (PST). PST treats similarity as just one type of
trait distribution in species. PST does not privilege similarity over polymorphism,
so PST offers a more inclusive account of trait distributions in species than HPC
theory. In addition, PST highlights a common type of explanation in biology —
one that cites the population and inter-population structures of species. Such
population structure explanationsexplain trait distributions in species, whether
those distributions involve similarity or dissimilarity.
Population structure explanations are pervasive in biology. Consider E. O.
Wilson’s [1968] explanation of different social castes in some insect species. The
fitness of an insect colony is enhanced by its ability to respond to certain sorts
of contingency. There are several types of such contingency, and specialization
is required to deal with each of them, so there is selection for different castes.
In addition, there is selection for an optimal mix of castes. The optimal mix
of castes is calculated by figuring out how to keep the combined cost of various
contingencies at a tolerable level. Wilson’s theory explains the distribution of
difference and why some castes occur in small numbers and others occur in large
numbers. Wilson attributes the distribution of castes to group selection: different
colonies with different phenotypic distributions compete against one another, and
the ones with more optimal distributions are selected. Wilson’s account explains
population variation not uniformity. Furthermore, it explains such variation by
citing the structures of populations — here the distributions of castes in a colony.
Wilson’s explanation depends on group selection, but explanations citing popu-
lation structure need not appeal to group selection. Consider a population struc-
ture explanation that does not involve group selection and is aimed at explaining
similarity within a species. A prime example is sexual dimorphism. Male elk have