A Reconsideration of the Coding of Inapplicable Characters: Assumptions and Problems

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Abstract

Character coding entails assumptions that may be problematic within the context of parsimony analysis using current computer algorithms. The example discussed here involves a character-variable (e.g., tail color) that is inapplicable in some taxa in the analysis because the part (e.g., tail) with which it is associated is lacking in those taxa. The part and character-variable can be coded as separate characters, or they can be fused into a single character. If the part and character-variable are coded as separate characters there is transformational independence between the part and the character-variable, but the logical dependence inherent to the hierarchical relationship between the part and its character-variable is only partly accounted for. Fusing the part and character-variable into one multistate character fully accounts for the logical dependence, but it is equivocal regarding the transformational independence. Separate coding is consistent with the primary homology statement that the part is homologous in all taxa possessing it, whereas fused coding is equivocal regarding this hypothesis of primary homology. As a result fused coding involves a loss of phylogenetic information. Use of a stepmatrix or other mechanisms associated with fused coding that preserve this phylogenetic information involves weighting schemes or ordered characters that have other assumptions that may also be difficult to justify.

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