Evolution__3rd_Edition

490 PART 4 / Evolution and Diversity
Summary
1 There are two main principles, and three main
schools, of biological classification: phenetic and phylogenetic
principles, and phenetic, cladistic, and evolutionary
schools. The schools differ in how (if at all)
they represent evolution in classification.
2 Phenetic classification ignores evolutionary relations
and classifies species by their similarity in appearance;
cladism ignores phenetic relations and classifies
species by their recency of common ancestry; evolutionary
taxonomy includes both phenetic and phylogenetic
relationships.
3 Phylogenetic inference is uncertain and phenetic
classification has the advantage that it is not subject to
revision when new phylogenetic discoveries are made.
4 Phenetic classification is ambiguous because there is
more than one way of measuring phenetic similarity
and the different measures can disagree.
Further reading
5 Cladism is unambiguous because there is only one
phylogenetic tree of all living things.
6 The cladistic philosophy specifies the pattern of
groups within groups in a classification, but not the
ranking of those groups. The large number of nodes in
a fully resolved phylogeny can be difficult to represent
in a Linnaean classification.
7 Evolutionary taxonomy avoids some of the extraordinary
properties of cladism. But it suffers from the
ambiguity of phenetic taxonomy, and its argument for
excluding one kind of phenetic relation (convergence)
works equally well against the kind of phenetic relation
(differential divergence) that it includes.
8 Living things show a diverging, tree-like pattern of
relationships. Darwin explained this by his “principle
of divergence”: that competition is stronger between
more similar forms, forcing them to evolve apart.
Schuh (2000) is a recent book about biological systematics. I have previously discussed
many of the points discussed in this chapter, at an introductory level and at greater
length, in Ridley (1986). Sneath & Sokal (1973) is the standard work on numerical
taxonomy; Sokal (1966) is a clear introduction. Hennig (1966) is the classic work on
cladism a but he is not an easy read! Wiley et al. (1991) and Kitching et al. (1998) are
two texts on cladism. Mayr (1976, 1981, and Mayr & Ashlock 1991), Dobzhansky
(1970), and Simpson (1961b) are key works by key evolutionary taxonomists. Mayr &
Diamond (2001) classify the birds of Melanesia according to traditional “Mayrian”
principles.
Bryant & Cantino (2002) review criticisms of phylogenetic nomenclature. Science
March 23, 2001 contains a newspiece about phylocode. Benton (2000b) defends the
traditional (cladistic) practice. Ereshefsky (2001) is more critical. References to the
primary sources can be traced through these sources. This controversial topic can be
followed in occasional papers in the journals Systematic Biology and Cladistics. See
also the website www.ohio.edu/phylocode.
On the principle of divergence, Darwin (1859) is the obvious source. Bolnick (2001)
did an experiment showing how evolutionary divergence, and adaptation to exploit
new resources, is more likely in the presence of competition.
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