Boolean logic and character state identity: Pitfalls of character coding in metazoan cladistics

Abstract

A critical study of the morphological data sets used for the most recent analyses of metazoan cladistics exposes a rather cavalier attitude towards character coding. Binary absence/presence coding is ubiquitous, but without any explicit justification. This uncompromising application of Boolean logic in character coding is remarkable since several recent investigations have nominated absence/presence coding as the most problematic coding method available for standard cladistic analysis. Moreover, the prevalence of unspecified "absence" character states in the published data sets introduces a discrepancy between the theoretical foundations of phylogenetic parsimony and current practices in metazoan cladistics. Because phylogenetic parsimony assumes transformation of character states, its effective operation breaks down when not all character states are carefully delimited. Examples of resulting meaningless character state transformations are discussed in two categories: 1) when unspecified "absence" states are plesiomorphic; and 2) when unspecified "absence" states are apomorphic (character reversals). To facilitate future progress in metazoan cladistics, the mandatory link between comparative morphology and character coding needs to be reestablished through a more explicit study of morphological variation prior to character coding, and through a more explicitly experimental approach to character coding.