CLADISTIC CHARACTERS AND CLADOGRAM STABILITY

Phylogenetic stability is the tendency for monophyletic groups that are resolved by an analysis to continue to be resolved when either the data or the analytical method is altered. Bootstrapping and similar procedures assess the stability of monophyletic groups, but provide little information to indicate which characters are crucial in the support of particular groups. We present a method that does so. Sequential Character Removal consists of a series of cladistic analyses, each conducted with a different subset of a complete data set; the subsets are obtained by the sequential and exhaustive removal of characters, individually and in all combinations of two or more. A character or combination of characters is critical to the resolution of a monophyletic group when the removal of that character or set of characters results in the loss of resolution of the group. Thus, for any monophyletic group, Sequential Character Removal generates a list of characters and character combinations that are critical to the resolution of the group, in the context of an otherwise complete data set. An analysis of a real data set, representing chloroplast DNA restriction site variation among 27 species of Poaceae, demonstrates that characters critical to the recognition of a monophyletic group need not be synapomorphies of that group, and that a clade that is resolved by a complete data set, and fails to be resolved when a particular set of characters is removed, may reappear when additional characters are removed.