Hierarchies, classifications, cladograms and phylogeny

Abstract

Figure 18 of Hennig's Phylogenetic Systematics (University of Illinois Press, Urbana, IL, 1966) shows a phylogenetic tree (a generative hierarchy) and what appear to be nested sets (an inclusive hierarchy) that he stated were two representations of the same pattern of relationships. This essay questions whether this is correct or not, explores the meanings of different hierarchical patterns, reviews various interpretations of Hennig's figure, and discusses the conceptual path from systematic evidence to phylogenetic explanation. The crux of the argument is that systematic hierarchies as we know them scientifically are nested sets that group theoretical entities based on patterns of synapomorphy. The notions of phylogeny and common ancestry reflect this hierarchical pattern.