Evolution__3rd_Edition

486 PART 4 / Evolution and Diversity
Evolutionary taxonomists criticized
phenetic classification
Classification with paraphyletic
groups is ambiguous
Notice that idealism could in principle solve the problem of subjectivity in phenetic
classification. The plan of nature would provide an objective, external reference point
for the phenetic classification to aim at. The only snag is that no plan of nature, in the
idealist sense, exists. That is why modern numerical phenetics dropped the idealist
philosophy of earlier phenetic classifications.
Section 16.5 argued that, in the absence of any natural hierarchy of phenetic similarity,
phenetic classification lapses into subjectivity. The idealist error is the other side
of the same coin. Idealists believe that a phenetic hierarchy exists “out there,” but offer
no good reason for their belief. When evolutionary taxonomists criticized phenetic
classification for committing the error of idealism, they meant that no real phenetic
hierarchy exists in nature, and a system which assumes such a hierarchy will be
fundamentally subjective. Phenetic classifications try to group species according to a
relationship a the ideal morphological system a that evolution does not produce.
Evolution does not produce one particular privileged phenetic hierarchy that is more
real than all other phenetic hierarchies.
Phenetic idealism can be avoided if taxonomists represent evolution, not phenetic
similarity. But what does “evolution” mean? Evolutionary taxonomists exclude polyphyletic,
but not paraphyletic, groups from classification. To see why, we must look at
what evolutionary taxonomists say about cladism. Evolutionary taxonomists criticize
cladism for its unnecessary puritanism. Cladism, as we have seen, leads to what at first
sight can appear bizarre conclusions, such as the destruction of the Reptilia. It does so
because of its distinction between paraphyletic and monophyletic groups. If both kinds
of group are allowed, most of the long-recognized groups such as reptiles and fish can
be retained.
That is how evolutionary classification sees itself. But cladists, and pheneticists, see it
rather differently. According to a cladist, the argument that evolutionary taxonomists
accept against the pheneticist’s polyphyletic groups works just as well against paraphyletic
groups. If you accept paraphyletic groups, you must accept polyphyletic ones
too, or be inconsistent. Paraphyletic groups are defined phenetically, just like polyphyletic
groups. If some of the descendants of a common ancestor are to be excluded
from a group, it has to be decided how many such descendants are to be left out, and the
decision is phenetic and arbitrary (Figure 16.7); paraphyletic groups are not formed by
phylogenetic relations a as Figure 16.4 illustrated. The standard phenetic problem
then re-enters: according to some measures of phenetic similarity, one paraphyletic
group will seem appropriate; according to another, another will. The choice between
them is subjective a or idealist. When paraphyletic groups are admitted, the argument
against phenetic classification is lost. If phenetic criteria can be used in the case of
paraphyletic groups, why not for polyphyletic ones? Paraphyletic groups presuppose
idealism just as much as polyphyletic groups.
Evolutionary taxonomy mixes phenetic and phylogenetic methods, but in a consistent
and principled manner. It defines groups by homologies and excludes homoplasies;
it therefore does not recognize polyphyletic groups. It allows phenetic groups
such as reptiles and fish, but classifications in biology have a strong practical purpose
and there is a case not to disband these long-established groups if a convincing reason
can be found for keeping them. However, it is questionable whether the evolutionary
taxonomist’s reason is convincing enough.
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