Evolution__3rd_Edition

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Evolutionary classification admits
paraphyletic and monophyletic
groups
CHAPTER 16 / Classification and Evolution 485
reasonable response to the problem of how to represent our phylogenetic knowledge in
a formal classification. While only tens or hundreds of thousands of species had been
named, and phylogenetics was a backwater of biological research, Hennig’s cladistic
principles posed little challenge to the Linnaean system of ranks. Now we know about
millions of species, and molecular systematics is a major research program. Some
adjustments to the Linnaean system may yet be needed to accommodate our expanding
phylogenetic knowledge, but meanwhile, it remains useful to classify biological species
into the Linnaean higher taxa. But decisions about which levels of the Linnaean hierarchy
to apply to which nodes in a phylogeny are arbitrary and subjective.
16.7 Evolutionary classification is a synthesis of phenetic
and phylogenetic principles
Finally, we should look at evolutionary classification. Evolutionary classification
incorporates both phenetic and phylogenetic elements, and both paraphyletic and
monophyletic groups. In terms of the kinds of taxonomic groups (see Figure 16.4),
evolutionary classification recognizes paraphyletic and monophyletic but not polyphyletic
groups. In terms of the kinds of characters used to infer phylogeny (see
Figure 16.3), it forms groups by homologies rather than homoplasies, but does not distinguish
ancestral from derived homologies. Referring back to Figure 16.1, evolutionary
classification picks the phenetic classification in the reptilian case (Figure 16.1c) but
the phylogenetic where there is convergence (Figure 16.1b).
How can evolutionary taxonomy be justified? The evolutionary school predates
both numerical phenetics and cladism and the main original discussions of the school
therefore did not meet the phenetic and cladistic arguments head on. Moreover, no
complete modern evolutionary taxonomic defense against numerical and cladistic
taxonomy exists. As such, the school differs from the other two, which were conceived
partly in opposition to evolutionary taxonomy and made their objections to it clear.
Nevertheless, a case can be made.
Evolutionary taxonomists disagree with phenetic classification for much the same
reasons we discussed above, though they express the argument differently. They
criticized phenetic systems for being idealistic, that is for supposing that a phenetic
classification represents some “ideal” phenetic relationship between species. The
ideal relationship would be some “idea” or “plan” in nature. An example is the pre-
Darwinian theory that classifications represent the thoughts of God. An idealist would
then aim to classify species according to an idea that exists in the mind of God. That
idea, arguably, manifests itself in (and is inferrable from) living nature. It is difficult for
a modern scientist to make much sense of these old arguments, but “divine” taxonomy
at least provides a concrete example of what idealism means. Other versions of idealism
supposed that it was possible to deduce the existence of fundamental forms or plans of
nature from a purely scientific analysis of species’ morphology. 1
1 Section 13.5, p. 363, about “typological thinking,” is concerned with much the same point. Idealism is an
example of typological thinking.