404 Marc Ereshefsky
not correspond to categories in nature, as many critics of the Linnaean hierarchy
argue, then most biodiversity studies are based on faulty measures.
1.1 Terms and Distinctions
We have already used the termsclassification,taxonomy,andsystematics, but
what do they mean? What distinctions do they highlight? We are familiar with
classifications: organisms are sorted into species, species are sorted into genera,
genera into families, and so on up the Linnaean hierarchy. A classification is a
hypothesis concerning how organisms or taxa are related. Taxonomy is the disci-
pline that tells us how to sort organisms into taxa and taxa into more inclusive
taxa. Taxonomic theory offers principles for constructing classifications. System-
atics does not tell us how to construct classifications but studies how organisms
and biological taxa are related in world. Classification, taxonomy, and systematics
line up in the following way. Systematics, the study of the relations among organ-
isms and taxa, guides our choice of taxonomic theory; taxonomic theory guides
biologists in constructing classifications of the organic world. The path from sys-
tematics to classification, however, is far from smooth. Biologists do not agree
on systematic theories, so they posit different taxonomic theories (i.e., different
principles for constructing classifications). Moreover, even when there is agree-
ment on taxonomic theory, how to implement that theory can be controversial.
For example, biologists tend to prefer more parsimonious classifications, but they
disagree on how to measure parsimony.
Another distinction is betweenspecies taxaand thespecies category. Species
taxa are groups of organisms;Homo sapiensandDrosophila melanogaster(a fruit
fly) are examples of species taxa. The species category is a more inclusive entity:
it is the class of all species taxa. When biologists and philosophers discuss the
definition of ‘species’ they are discussing the definition of the species category. In
Section 2, we focus on the ontological status of speciestaxa: is a species taxon a
kind or an individual? In Section 3, we focus on the speciescategory: is there a
single, unified species category? In Section 4 we move up the Linnaean hierarchy
and discuss the reality of the other Linnaean categories –genera, families, and so
on. Does the Linnaean hierarchy reflect a hierarchy of natural categories, or are
the Linnaean ranks merely useful instruments for organizing information about
the organic world?
2 SPECIES2.1 Essentialism
Traditionally philosophers have treated biological species as natural kinds with
essences. This approach to species is found in the work of Kripke [1972] and
Putnam [1975], and has its roots in Aristotle and Locke. Biologists have been es-
sentialists as well. John Ray, Maupertuis, Bonnet, Linnaeus, Buffon, and Lamark