The Out-Group Comparison Method of Character Analysis

Watrous, Larry E. , and Quentin D. Wheeler (Division of Insects, Field Museum of Natural History, Chicago, Illinois 60605, and Department of Entomology, Cornell University, Ithaca, New York 14853) 1981. The out-group comparison method of character analysis. Syst. Zool. , 30:1-11. -An operational rule for analyzing character polarity with out-group comparison is presented and a series of observations, including potential problems in applying the rule, are discussed. The "commonality principle" (="frequency of occurrence," "common equals primitive") for determining character polarity is reviewed and dismissed as a reliable alternative to out-group comparison. Based on the rule and observations, a general method for character analysis is synthesized. [Cladistics; character analysis; out-group comparison; polarity. ] Although many authors have discussed criteria for distinguishing plesiomorphic (primitive) and apomorphic (derived) character states (see Maslin, 1952; Hennig, 1965, 1966; Kluge and Farris, 1969; Marx and Rabb, 1970, 1972; Lundberg, 1972; Ross, 1974; Munroe, 1974; Ekis, 1977; Crisci and Stuessy, 1980; and references therein), there has been little discussion of the deficiencies of some of these criteria, and in some instances there is considerable misunderstanding of what we consider to be the most reliable criterion, that of out-group comparison. Our goals here are to discuss some problems with one criterion in particular, the so-called "commonality principle" (Eldredge, 1979), and to develop a general method of character analysis based on out-group comparison. Many authors have fully understood out-group comparisons (e.g., Kluge and Farris, 1969; Lundberg, 1972; Ross, 1974), but others have confused the concept with that of commonality. Because out-group comparison can often be invoked where commality is arbitrarily used, and because the resultant hypotheses are subject to rigorous testing, its central position in character analysis is assured. It is because of the importance of out-group comparison and because of its confusion with commonality (e.g., Crisci and Stuessy, 1980), that we attempt to formalize the criterion and synthesize the method. PROBLEMS WITH COMMONALITY From the following quotations, abstracted from a cursory review of some recent literature, it is apparent that the ''commonality principle" for determining in-group character polarity receives wide support in both botanical and zoological circles. At least some authors (Crisci and Stuessy, 1980; Eldredge, 1979) consider this to be the most widely accepted and applied of all criteria. "The character state that consistently appears in many members of each of the taxa being studied is deemed most likely to be primitive for the group of taxa. "-H. B. Boudreaux (1979:8). "Criterion of Frequency of Occurrence . The objective is to determine how extensively a character state is distributed within the taxon under study . " -G. Ekis (1977:117). "A primitive state is more likely to be widespread within a group than is any one advanced state. "-A. G. Kluge and J. S. Farris (1969:5). ". the relatively more primitive states are likely to be distributed more generally throughout the group under study. "-G. Estabrook (1972: 439). "The criterion of common is primitive is not only the oldest, but also the most widely used of all the criteria of primitiveness . the concept is a good one to employ in most groups. "-J. V. Crisci and T. F. Stuessy (1980:119, 130).