406 Marc Ereshefsky
on common stores of genetic material and developmental constraints. These com-
mon genetic and developmental resources cause the members of different species
to share similar characteristics. Just think of all those organisms in the world with
four limbs. Just as the forces of evolution work against the universality of a trait
in a species they also work against the uniqueness of trait in a species.
The above considerations pose a strong challenge to the essentialist requirement
that a trait must occur in all and only the members of a species. This does not
mean that the occurrence of a biological trait in all and only the members of species
is empirically impossible. Nonetheless, consider what conditions must be met for
that requirement to hold. For essentialism to work, a trait must occur in all the
members of species, for the entire life of that species — from its initial speciation
event to its extinction. Moreover, for a trait to qualify as an essential property
of a species, it must occur only in that species. That is, the trait in question
cannot occur in any member of any other species for the entire span of life on this
planet, indeed, for the entire span of life in this universe. This is a tall order!
The occurrence of a trait in all and only the members of species is an empirical
possibility. But given current biological theory, that possibility is unlikely.
Sober [1980], following Mayr [1969], offers a different objection to species es-
sentialism. Sober illustrates how essentialist explanations have been replaced by
evolutionary ones in biology, thereby rendering essentialist explanations theoret-
ically obsolete. Suppose we want to explain variation in a population, say varia-
tion in height. The essentialist explanation cites the essence of the organisms in
a species, and then cites ontogenetic interference that prevents the manifestation
of that essence in some or all members. Organisms of that species would have the
same height if they were exposed to the same environment, but they are exposed
to different environments, hence there is variation from a common type. Variation
is explained by essence and interference. In contrast, the evolutionist explains
variation in a population without positing essences. Evolutionists cite the gene
frequencies within a population and the evolutionary forces that affect what fre-
quencies occur in the next generation. Variation in height is explained by citing
the relevant gene frequencies in one generation plus the occurrence of such evo-
lutionary forces as selection and drift. No essences are posited. In contemporary
biology, the positing of species essences has become theoretically superfluous.
2.2 Species as Individuals
If species taxa are not natural kinds with essences, what is their ontological status?
The prevailing view is that species are not natural kinds but individuals. Ghiselin
[1974] and Hull [1978], the most prominent advocates of this view, contrast natural
kinds and individuals in terms of spatiotemporal restrictedness. Membership in
a kind requires that the members of a kind share a set of qualitative properties.
A drop of liquid is water so long as it has the molecular structure H 2 O. It does
not matter where that liquid is located, whether it is on Earth now or in a distant
galaxy in a million years. So long as that drop has the molecular structure H 2 O