416 Marc Ereshefsky
so the Lineage Species Concept captures an important similarity of species taxa.
However, all genera, families, and other Linnaean taxa are also lineages. The
Lineage Concept is too inclusive because it captures all Linnaean taxa. In an
attempt to provide an over-arching conceptualization of species taxa, de Queiroz
and Mayden have cast their net too widely.
Consider a final monist response to pluralism inspired by advances in molecular
genetic sequencing. Alex Rosenberg (in conversation) suggests that perhaps the
correct species concept could be based on genetic similarity. As more molecular
studies are performed, we may discover the distinctive genome of each species. We
could then use that information to construct a single classification of the organic
world. The problem with this suggestion is that classifying by molecular data
would not unify biological taxonomy, instead it would add further classifications.
As Ferguson [2002] observes, classifications based on overall genetic similarity and
classifications based on the ability to interbreed do not coincide. The result is
two different classifications: one classifying organisms by interbreeding, another
classifying organisms by overall genetic similarity. The same sort of disunity is
found when classifications based on overall genetic similarity and ecological adap-
tiveness are compared. Wu and Ting [2004] cite cases where classifications based
on genes for ecological adpativeness fail to coincide with classifications based on
overall genetic similarity. Bringing molecular data to the table does not reduce the
number of classifications but increases their number and brings further disunity
to biological taxonomy.
4 THE LINNAEAN HIERARCHYThe Linnaean hierarchy should be familiar to all readers of this volume. The
hierarchy contains a series of ranks — species, genus, family, and so on — that
serve as the primary categories for biological classification. The Linnaean ranks are
also cited in biological theory. Prey-predator models in ecology refer to species,
and hypotheses concerning the tempo and mode of macroevolution often refer
to families, classes and more inclusive ranks. There is more to the Linnaean
hierarchy than just a series of ranks. The Linnaean hierarchy also includes rules of
nomenclature that tell us how to name taxa. For example, the Linnaean hierarchy
tells us to give all and only species taxa binomial names, such as the name of our
species,Homo sapiens.
Given the pivotal role of the Linnaean hierarchy in biology, one might be sur-
prised to learn that a number of biologists and philosophers question the usefulness
of that hierarchy. Critics of the Linnaean hierarchy offer a long list of problems
[Hull, 1966; Hennig, 1969; de Queiroz and Gauthier, 1992; Ereshefsky, 2001]. Some
critics believe that the problems facing the Linnaean hierarchy are so severe that
the Linnaean hierarchy should be replaced with an alternative system of classifi-
cation [Hennig, 1969; de Queiroz and Gauthier, 1992; Ereshefsky, 2001; Cantino
et al., 2003]. The Linnaean hierarchy has its defenders as well. Defenders are
well aware of the problems facing the Linnaean hierarchy, but they argue that