Species, Taxonomy, and Systematics 405all talk of classifying species taxa according to their species-specific essences [Hull,
1965; Sober, 1980; Mayr, 1982].^1 The view that species are kinds with essences is
part of a general view — kind essentialism. A natural kind is a class of entities that
share a kind-specific essence. Such essences capture the fundamental structure of
the world; or using Plato’s phrase, they “carve nature at its joints.”
Kind essentialism has two crucial tenets. First, all and only the members of
kind share a common essential property. If an entity has a certain essence, it
is member of a particular kind. If, for example, a piece of metal has a certain
atomic structure, it is a member of the kind gold. However, some properties occur
in all and only the members of kind yet they are not that kind’s essence. The
Aristotelian example is humans and the property featherless biped. All and only
humans are featherless bipeds, nevertheless that property is not the essence of
humans. Here is where the second tenet of essentialism comes into play. The
essence of a kind is a property that plays a fundamental role in explaining the
occurrence of other properties typically found among the members of a kind. For
example, the atomic structure of gold causes pieces of gold to dissolve in acid and
conduct electricity. That atomic structure, not any property that occurs in all and
only pieces of gold, is the essence of the kind gold.
Both philosophers and biologists have applied kind essentialism to species.
There are few essentialist biologists today, but essentialism remains a common
view among philosophers. A number of philosophers and biologists have challenged
species essentialism — the view that species are natural kinds with essences [Mayr,
1959; Hull, 1965; Ghiselin, 1974; Sober, 1980; Dupr ́e, 1981]. Here are two lines of
argument against species essentialism.
Recall the first tenet of essentialism: a property must occur in all and only the
members of a kind. Biologists have been hard-pressed to find a biological trait that
occurs in all and only the members of a particular species. Evolutionary theory
explains why. Suppose a genetically based trait were found in all the members
of a species. Mutation, random drift, and recombination can cause that trait not
to occur in a future member of that species. All it takes is the disappearance
of a trait in a single member of a species to show that it is not essential. The
universality of a trait among the members of a species is quite fragile.
Evolutionary forces also undermine the uniqueness of a trait among the mem-
bers of a species. Different species frequently live in similar habitats that cause
the parallel evolution of similar traits in different species. Birds and bats, for
example, each have wings, but the evolutionary path for each type of wing is dis-
tinct. Organisms in different species also share common ancestors, so they draw
(^1) Windsor [2003] has challenged the claim that prominent pre-Darwinian biologists were essen-
tialists. This raises an interesting issue concerning taxonomic thought. Linnaeus, for example,
wrote that organisms should be classified according to their taxon-specific essences. However, he
realized we do not have access to those properties, so he classified according to clusters of simi-
larities [Ereshefsky, 2001]. Was Linnaeus an essentialist or not? Linnaeus advocated essentialism
but could not implement it in practice. (Biologists often advocate taxonomic theory that they
cannot yet use.) So perhaps the proper answer is, in some ways Linneaus was an essentialist, in
other ways he was not.