Evolution__3rd_Edition

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Phylogenetic classification faces
several problems
A full phylogeny has many more
levels than the Linnaean hierarchy
CHAPTER 16 / Classification and Evolution 483
and paraphyletic groups, the evolutionary information in the classification becomes
muddled. The beauty of a purely phylogenetic classification is that there can be no
doubt what the branching relations of the classificatory groups are (Figure 16.3). But if
taxonomists define some relations phenetically and others phylogenetically, it is no
longer possible to say what any particular relation means. The branching relations are
obscured and lost.
16.6.3 A knowledge of phylogeny does not simply tell us the rank levels
in Linnaean classification
The main advantage of phylogenetic classification is theoretical. It can run into many
problems in practice. One is ignorance. We do not know the phylogenetic relations
of many living creatures, and cannot classify them phylogenetically. Another problem
is instability. Scientific knowledge can change when new evidence comes in, and
our knowledge of phylogeny is no exception. When our knowledge of a phylogeny
changes, a change will also be required in the classification of that group. A third problem
has arisen recently, as the number of levels in the known phylogenetic hierarchy
has increased beyond the ability of the Linnaean hierarchy to represent it. Chapters 15
and 18 look at our knowledge of phylogeny. Here we concentrate on the other two
problems. (Other problems can arise when evolution is non-hierarchical, for instance
because of hybrid speciation (Section 14.7, p. 405) or horizontal gene transfer (Figure
15.19, p. 456).)
We saw in Figure 16.3 how a phylogeny of a group of species can be converted into
a Linnaean classification. However, the phylogenetic hierarchy only determines the
pattern of groups within groups, not the ranks of the groups. For example, if we know
that humans and chimpanzees share a more recent common ancestor with each other
than either does with gorillas, then we know that humans and chimpanzees should be
grouped together within the larger group of great apes. But the phylogenetic knowledge
alone does not tell us whether the group of humans and chimpanzees should be ranked
as a genus, subfamily, family, or something else.
The Linnaean hierarchy has perhaps seven main levels (kingdom, phylum, class,
order, family, genus, species); these can be multiplied by adding in super-, sub-, and
infra-levels (e.g., superfamily, suborder, and so on) and other extra levels, such as the
tribe (between genus and family). But even an expanded Linnaean hierarchy could not
have much more than about 25 levels. This raises a problem of how to combine levels in
the phylogenetic hierarchy to accommodate it in the Linnaean system. We cannot
naively say that each successive branching point (or node) in the phylogeny can have its
own Linnaean rank. There are just too many nodes.
For instance, Figure 15.27 (p. 465) showed the phylogeny of the Hawaiian picturewing
fruitflies. A cladistic classification of one part of it a such as the Drosophila
adiostola species group a would need us to invent five new levels in the Linnaean
hierarchy between the levels of genus and species (Figure 16.6). Historically, the problem
has not been acute because we have been relatively ignorant of phylogenetic relations
and the number of described species has not been too large. But we should now
be planning for a complete phylogenetic knowledge of about 10–100 million species.