CHARACTER WEIGHTING AND CLADISTIC ANALYSIS1

Abstract.—Explicit, ad hoc character weighting, widely dismissed by cladists as too subjective for practical application, can be useful in resolving difficult instances of homoplasy. Such weighting should be avoided to the extent possible, and should only be tried when critical reexamination of the evidence and the parsimony criterion have failed. So long as such uses of weighting are fully explicit and their results are viewed as tentative solutions, they support formulations of hypotheses where none might otherwise be possible. [Character weighting; cladistics; phylogenetics; convergence; homoplasy.] For centuries taxonomists have used some characters at the exclusion of others in constructing classifications. Although both pheneticists and cladists have advo-cated large sets of evidence be considered in making classifications, the idea persists that some kinds of characters are inher-ently more informative about phylogenet-ic relations than others. For example, Far-ris (1982) recently observed that: No one supposes, however, that characters in gen-eral all deserve the same weight—that they all yield equally strong evidence.... Drawing con-clusions despite conflicting evidence requires that some evidence be dismissed as homoplasy. It is surely preferable to dismiss weaker evidence in deference to stronger. This appears, at face value, to contradict statements by other authors indicating that all comparative evidence is of potential value to biologists in studying cladistic re-lations (e.g., Nelson and Platnick, 1981). But this contradiction is illusory. As Plat-nick (1979) suggested, the real difficulty lies in correctly recognizing "characters" (which he defined as evolutionary novel-ties, in a sense equivalent to Hennig's term "apomorph"). Were we to correctly delim-it all of the characters in the data set there would be no conflict and a single, uncon-tested hypothesis of relationships (clado-gram) would emerge from the analysis