Conclusions

Phylogenetic taxonomy has added an extremely useful refinement to the process of classifying organisms in emphasizing the need to associate taxo-nomic names with a pattern of relationships, and thus an ancestor, rather than with lists of mutable characters. When dealing with the fossil record it is inevitable that the closer to a branching point we get (and therefore to the answers that we are most interested in), the fewer and more subtle the characters differentiating groups will become. This is why classifications based only on modern forms work poorly when applied to the > 99% of life on Earth that has gone extinct (Raup, 1992; Schopf, 1982). Following this reasoning Primates is defined here according to a node-based approach to include plesiadapiforms, even though all known plesiadapiforms lack some features seen in modern Primates. No other classificatory position available for plesiadapiforms both conforms to modern taxonomic practice (i.e., in rejecting non-monophyletic groups) and emphasizes their importance as the key taxa to the study of the sequence of adaptations leading up to the origination point of Euprimates.

In spite of my enthusiasm for some of the tenets of phylogenetic taxonomy (i.e., its methods for defining groups), I feel that by aiming to replace the ranked Linnean system of taxonomy with a rankless system, the Phylocode has gone too far. For biologists whose central interest is the understanding and interpretation of evolution, using a ranked system offers the benefit of communicating details of the preferred phylogeny that are simply not outweighed by the metaphysical stability offered by the Phylocode. Also, historical stability is a consideration that should not be overlooked. By endeavoring to start from scratch, the adoption of the Phylocode would lead to a long period of flux, in which the central goal of taxonomy, communication, will not be met (Benton, 2000).

The basic premise upon which the Phylocode is based seems to be that we have, or will have very soon, a complete understanding of the phylogeny of all organisms. If this were true it would be a relatively simple matter to apply their guidelines to point at the nodes that we wish to name (Nixon and Carpenter, 2000, call it the "node-pointing" system to recognize this). In light of the fact that to date we have likely uncovered only a tiny fraction of the species that have lived on this planet, and considering the disagreements that still surround details of the branching pattern of known organisms, such a view seems extremely naive. The Linnean system, in spite of all its faults, offers flexibility in that the taxonomist can make choices about how names should be applied. Particularly, if the Phylocode were enacted, the term Primates would have to be established using one of the allowed definition types, and then registered, before it could be utilized. Once that procedure was executed, the decision made by whoever performed this conversion would stand for the rest of time. If this definition were found to be based on an incorrect cladogram, the meaning of Primates as understood by most workers could easily come into conflict with the technical definition, which seems a problematic situation for a system intended first and foremost for clear communication. Although the end result of the flexibility allowed under the Linnean system is some lack of consistency, and a good deal of arguing, this flexibility seems essential in a world where there is so much variation in what we know for different groups, and in how widely accepted patterns of relationships are.

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