Species, Taxonomy, and Systematics 419terion for defining the higher Linnaean ranks. Instead, biologists use a patchwork
of criteria for defining the higher ranks. As a result, taxa of the same rank can
vary dramatically. Families can vary in their age, their phylogenetic structure,
their phenotypic diversity, and the breadth of their adaptive zone. Calling a taxon
a ‘family’ merely means that within a particular classification that taxon is more
inclusive than a genus and less inclusive than a class. There is no definition for
‘family’ that applies to all or even most classifications of families. ‘Family,’ like
the other Linnaean ranks, refers to a heterogeneous set of taxa. Some authors
have suggested that the Linnaean hierarchy of ranks is a fictitious grid we place
on nature [de Queiroz and Gauthier, 1992; Ereshefsky, 1994].
The heterogeneity of the Linnaean ranks has practical implications. Consider
the use of Linnaean ranks in biodiversity studies. Biologists tend to measure bio-
diversity in Linnaean terms; an area or higher taxon is surveyed for the number
of species or families present. However, the Linnaean ranks mask important bi-
ological differences. Suppose we want to measure the biodiversity of two groups
of organisms and we measure that diversity by numbers of families. One group
consists of snail families, and the other contains mammalian families. Snail fami-
lies and mammalian families are biologically different. Snail families have denser
phylogenetic structures than mammalian families; and, mammalian families ex-
hibit more ecological diversity than snail families. Snail and mammalian families
are not comparable. Counting biodiversity by the number of families present in a
group masks important biological differences. Biodiversity studies should instead
use parameters that capture such biological phenomena as phylogenetic structure
or ecological breadth. Then we would have proper measures of biodiversity.
4.2 Linnaean Names
As mentioned earlier, the Linnaean hierarchy is a general system of classification
containing both hierarchical ranks and rules for naming taxa. The Linnaean ranks
are not the only aspect of the Linnaean hierarchy that is problematic, so are
the Linnaean rules of nomenclature. Critics believe that the Linnaean rules of
nomenclature lead to a number of practical problems in taxonomy [de Queiroz
and Gauthier, 1992; Ereshefsky, 2001]. What follows is a quick introduction to
those problems.
Linnaeus’s best-known naming rule is his binomial rule. Each species’ name
has two parts: a generic name and a specific name. InHomo sapiens,Homois
the name of our genus andsapiensis the specific name of our species. Binomials
clearly indicate the classification of a species: Homois the name of our species’
genus.Sapiens, the specific name, distinguishes our species from other species in
Homo. Linnaeus’s motivation for assigning binomial names was his belief that a
biologist should memorize the classification of all species in a kingdom. Linnaeus
realized that there were too many species to do that; for example, he recognized
approximately 10,000 plant species. He believed that the number of plant genera
in the world was much smaller, approximately 300 genera. Furthermore, he did