Evolution__3rd_Edition

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CHAPTER 16 / Classification and Evolution 475
Table 16.1
Phenetic, cladistic, and evolutionary classifications can be distinguished by the characters they use to define groups, and the kinds of
group they recognize.
Characters used
Groups recognized Homologies
Classification Monophyletic Paraphyletic Polyphyletic Homoplasies Ancestral Derived
Phenetic Yes Yes Yes Yes Yes Yes
Cladistic Yes No No No No Yes
Evolutionary Yes Yes No No Yes Yes
We distinguish objective from
subjective classificatory principles
The third school to be discussed uses a synthesis, or mixture, of phenetic and
phylogenetic methods and is often called evolutionary taxonomy. In the reptilian example
(Figure 16.1c) evolutionary taxonomy prefers the phenetic classification; in the
barnacle example (Figure 16.1b), the phylogenetic. This school’s best known advocates
include Mayr (1981), Simpson (1961b), and Dobzhansky (1970).
16.4 A method is needed to judge the merit of a school of
classification
How should we decide which school of classification, if any, is the best? To do so, we
need a criterion to judge them against, and many biologists use the objectivity criterion
for this purpose. An objective classification is one that represents a real, unambiguous
property of nature. Objective classification can be contrasted with subjective
classification, in which the classification represents some property arbitrarily chosen by
the taxonomist.
For instance, I might arbitrarily choose to classify species into one group if I discovered
them on a Monday or Tuesday and another group if I discovered them between
Wednesday and Friday. The classification would then be subjective because I should
have no method of justifying the choice a except my personal whim or convenience. If
challenged about why I did not instead have one group for all days beginning with the
letter “T” and another group for all other days, I should have no principled argument to
defend my classification. The underlying classificatory principle a time of discovery a
is ambiguous because it could be applied in a number of equally valid ways that would
give differing classifications. It is also unreal because there is no inherent property
shared by the organisms discovered on Monday and Tuesday and not shared by those
discovered on Tuesday and Thursday. The objectivity test, therefore, is to ask whether a