Taxic revisions

Parsimony analysis provides a straightforward way of assessing homology on a tree: a state shared by two terminals comprises homologous similarity if optimization attributes that state to all the stem species lying between th ose terminals. Three-taxon statements (3ts), although seemingly "exact" in that each either fits a tree or does not, do not provide a satisfactory ass essment of homology, because that assessment can be internally contradictor y and because 3ts systematically exclude homologous resemblance in reversed states. Modified 3ts analysis (m3ta), a method in which both plesiomorphic and apomorphic states of "paired homologue" (PH) characters (those other t han presence/absence data) are regarded as "informative" (able to distingui sh groups), can (obviously) group by symplesiomorphy and so form paraphylet ic groups unless data are clocklike enough. Patterson's pattern analysis (p pa) has the same shortcoming, to which it adds the drawback that only chara cters fitting the tree perfectly are used, a restriction that can easily le ad to discarding most of the structure in the data. Revised m3ta (rm3ta), a method in which plesiomorphic states are not taken as informative, can als o form paraphyletic groups, because it cannot apply reversals as apomorphie s. The idea that knowledge of phylogeny has been derived from classificatio ns does not imply that nonevolutionary methods should be employed for class ification, but instead means that systematic methods must be logically capa ble of phylogenetic interpretation. Neither m3ta nor rm3ta satisfies that r equirement because of their contradictory assessments of homology. (C) zool The Willi Hennig Society.