Species, Taxonomy, and Systematics 421Consider the case of taxonomic revision. Taxonomic revision is the activity of
revising a classification. Such revision is the norm not the exception in biological
taxonomy. A taxon may be reassigned to another higher taxon, for example, a
species may be reassigned to another genus. Or revision may involve giving a
taxon a new rank, for example, a taxon thought to be a tribe is reassigned as a
family. Taxonomic revision occurs for a couple of reasons. New empirical informa-
tion may be gathered, such as new DNA evidence or the discovery of new fossils,
that causes revision. Or revision may occur when taxonomic theory changes and
old classifications are updated to cohere with the new theory. Taxonomic revision
causes instability in classification, and this is to be expected. Classifications are
hypotheses and are open to revision in light of new evidence or theoretical con-
siderations. Unfortunately, the Linnaean rules of nomenclature are themselves a
cause of instability because they require that the names of taxa be changed when
revision occurs. Recall that in the Linnaean hierarchy, the names of taxa reflect a
taxon’s rank and taxonomic position. A change in a taxon’s classification requires
a change in a taxon’s name. This may not sound like much of an inconvenience,
but it is. A case of taxonomic revision can involve the renaming of hundreds of
taxa (Ereshefsky 2001). The Linnaean rules themselves are a source of instability.
Another area where the Linnaean rules of nomenclature make classifying taxa
harder than need be is that of taxonomic disagreement. In some cases biologists
disagree over the classification of a taxon. The disagreement may be over the
rank of a taxon, or the disagreement may be over the taxonomic placement of
a taxon (does it belong to this or that more inclusive taxon). The Linnaean
rules require that the rank and placement of a taxon be reflected in its name. If
biologists disagree over a taxon’s rank, for instance, the Linnaean rules require
those biologists to assign different names to what they agree is the same taxon.
Consider a disagreement between E.O. Wiley and G. G. Simpson concerning the
placement of our genus,Homo. They agree thatHomobelongs to a more inclusive
taxon, call it ‘X,’ but they disagree on X’s Linnaean rank. They agree on the
members and properties of X, but because they belong to different taxonomic
schools they assign X different ranks. Wiley thinks X is a tribe. Simpson thinks
it is a family. Following the Linnaean rules, Wiley calls XHominini, and Simpson
calls XHominidae. The Linnaean rules force Simpson and Wiley to give different
names to what they agree is the same taxon. The Linnaean rules are, thus, a
source of semantic confusion.
4.3 Alternatives to the Linnaean Hierarchy
In light of the problems facing the Linnaean hierarchy, several alternative sys-
tems of classification have been suggested in the last 40 years. Hull [1966] and
Hennig [1969] offered alternative systems, and more recently a system of phyloge-
netic nomenclature called the “PhyloCode” has been suggested [de Queiroz and
Gauthier, 1992; Cantinoet al., 2003. All of these alternatives to the Linnaean hier-
archy recommend eliminating Linnaean ranks from biological classification. They