The Logic and Limitations of the Outgroup Substitution Approach to Cladistic Analysis

Abstract

The outgroup substitution approach can be used when a well-corroborated hypothesis of the more inclusive cladistic relationships of a study group is unavailable. It is particularly appropriate when the set of outgroups, each of which could plausibly be the sister group, includes some that may be only distantly related to others. All plausible sister groups are used as outgroups, alone and in various arrangements, to assess character polarities. In each case an ingroup cladogram is constructed and these are searched for areas of congruence. Through this approach, robust phylogenetic hypotheses can be generated in spite of uncertain outgroup relationships. Caution must be exercised because polarity assessment is sensitive to the exact arrangement of outgroups. Even if completely congruent cladograms are obtained, it may not be possible to specify character support for particular clades. The approach may be most useful when one''s primary objective is to establish an outgroup hypothesis for a subsequent cladistic analysis at a lower taxonomic level.