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.

Science and storytelling

“…The only reason we conceive/believe in/tell stories about ghostly contours and unseen worlds, other than supernatural faith or delusion, is because we have empirical evidence from systematics. Darwin said as much, as did Willi Hennig and Ernst Mayr (Brower, 2023). That foundation is what lends a scientific imprimatur to the stories that get told.…”

“…However, the next section, “Reifying archetypes”, begins to meander somewhat confusingly between epistemological (explanans–explanandum) and ontological (cause and effect) issues—a challenge to the reader that persists through most of the book. Jenner says that Darwin “substituted fact for fantasy by collapsing Owen's ‘ ideal pattern ’ into a concrete ancestor” (p. 35); “characters now reflected and were explained by common ancestry” (p. 36; see Brower, 2023 on “reflections”); and “systematic and morphological relationships between collateral relatives are expressions of a more fundamental type of relationship—namely that between ancestors and descendants” (p. 38). These statements suggest a commitment to a realist perspective.…”