Definitions in phylogenetic taxonomy: critique and rationale.

A general rationale for the formulation and placement of taxonomic definitions in phylogenetic taxonomy is proposed, and commonly used terms such as "crown taxon" or "node-based definition" are more precisely defined. In the formulation of phylogenetic definitions, nested reference taxa stabilize taxonomic content. A definitional configuration termed a node-stem triplet also stabilizes the relationship between the trio of taxa at a branchpoint, in the face of local change in phylogenetic relationships or addition/deletion of taxa. Crown-total taxonomies use survivorship as a criterion for placement of node-stem triplets within a taxonomic hierarchy. Diversity, morphology, and tradition also constitute heuristic criteria for placement of node-stem triplets.