Much ado about nothing: inapplicable data as insertion–deletion events

The treatment of inapplicable characters has proved especially vexing to systematists. Investigators have wrestled with alternative coding scenarios to capture both the presence and absence of a feature, and its variation when present, in a reasonable manner. Three basic issues have presented themselves: (i) impossible states at internal nodes; (ii) action at a distance among disparate parts of the tree; and (iii) “secondary” (i.e. aspect variation) characters overwhelming “primary” (i.e. character presence/absence) patterns in grouping taxa. Multiple methods have been proposed to deal with these issues in the context of standard character coding with varying levels of complexity. Here, I show that these issues can be dealt with in a direct fashion by treating presence/absence not as a character, but as insertion/deletion of a character with all its potential variation. This approach removes these three problems in simple, straightforward manner.