Evolution__3rd_Edition

476 PART 4 / Evolution and Diversity
Phylogenetic inference can be
uncertain
Phenetic characters can be
quantified
classificatory system has some compelling justification, external to the method it uses
and the practitioners who practice it, for classifying in the way it does.
Objective classifications are preferable to subjective ones. If classification is objective,
then different, rational people, working independently, should be able to agree that
it is the way to classify. The results should be relatively stable and repeatable.
We can now look at how well the three classificatory schools meet the objectivity
criterion. To do so, we need to look in more detail at how the three schools actually
operate.
16.5 Phenetic classification uses distance measures and
cluster statistics
The modern forms of phenetic classification are numerical and multivariate, and they
were developed in reaction to the uncertainties and imprecision of evolutionary
classification. Evolutionary classification, whether of the pure cladistic kind or the
mixed evolutionary taxonomy of Mayr and others, requires a knowledge of phylogeny.
Chapter 15 described how phylogenies can be inferred. Here, all we need to know is
that, although the phylogenetic relations between species can often be inferred, the
inferences are sometimes uncertain. Phylogenetic knowledge is subject to change, as
improved evidence comes in, and a classification of a group based on its phylogeny is
liable to be unstable a not because the phylogeny itself is unstable but because our
knowledge of it is. For many groups of living things, hardly anything is known about
phylogeny, and a “phylogenetic” classification of such a group will inevitably be poorly
supported by evidence. Numerical phenetics aimed to avoid all the evolutionary uncertainty
by classifying only by phenetic relations, and by using quantitative techniques to
measure them. The classification would follow automatically, and therefore (it was
thought) objectively, from the phenetic measurements. Let us consider the methods in
some more detail, and see how well these aims can be achieved.
The simplest kind of phenetic classification is defined by only one or two characters.
We might classify the vertebrates, for example, by the number of their legs, to form
groups with 0, 2, or 4 legs. The trouble with this procedure is that it is likely to be
subjective in the same way as classifying species by their order of discovery. Different
individual characters show different distributions among species and therefore tend to
produce different classifications. Consider the birds and some reptilian groups, such as
crocodiles, lizards, and turtles. (We met this example in Chapter 15, and the conflicting
character distributions are illustrated in Figure 15.2, p. 427. Figure 16.1 in this chapter
also partly illustrates this example.)
Crocodiles are more similar to lizards and turtles than to birds if we look at their
external surfaces, number of legs, and cold-blooded physiology. But crocodiles are
more similar to lizards and turtles than to birds if we look at the anatomy of their skulls.
The characters conflict. This is a universal problem, not just a peculiar problem in this
example. A taxonomist working with one sample of characters will often produce a
different classification from another taxonomist working with a different set of characters.
As long as we stay with the principle of classifying according to a small number
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