Summary of the talk that Charissa Varma (Darwin Correspondence Project) gave on Monday 11th November, 2013
From diversity in anatomical designs, ecological specializations and life history strategies, to successfully populating all continents and almost all habitats, including open oceans, species in the order Diptera (which include flies, mosquitoes and gnats) have challenged systematists, Willi Hennig (1913-1976) spent most of his career trying to organize this order. While entomologists celebrate his efforts on Diptera, the greater systematics community remembers Hennig for his landmark book Phylogenetic Systematics (1950/1966), a book that transformed systematics methodology. One controversial feature of Hennig’s methodological reform was his treatment of branching diagrams in systematics, and in this talk Iexplore the philosophical underpinnings of his account of hierarchies. There have been debates in the biological and philosophical literature around the question Hennig’s account of hierarchies: did Hennig conflate the Linnaean hierarchy, a set-theoretic hierarchy, with a “divisional hierarchy”, a mereological hierarchy? Philosopher David Hull explains the concern in the following way:
The problem is how to impose the traditional inclusion hierarchy set out by the ancient Greeks on these branching hierarchies. Daughter species are not included in their mother species. All three may be included in a more inclusive taxon. Split, split, split is not the same relation as subsume, subsume, subsume. These relations form hierarchies, but these hierarchies are quite different.
What did Hennig have in mind with his account of hierarchies? In this talk I suggest Hennig introduced the concept of a division hierarchy, a concept he borrowed from biologist J.H. Woodger who belonged to a movement called “Organicism”. When looking at the papers Hennig published around the same time as the first edition of Phylogenetic Systematics, it becomes clear that he felt he needed philosophical concepts that emphasized the relational nature of his system, concepts that would enable him to assign ontological status to what he would deem individuals (complex wholes that were homeostatically stabilized, self-regulating individuals). Hennig used at least two terms to express the kind of hierarchy he had in mind: division hierarchy and enkaptic system. Division hierarchies did similar philosophical work as the concept enkaptic system, in terms of unifying his account of a phylogenetic system. In the second edition of Phylogenetic Systematics, Hennig’s decision to include J. R. Gregg to the discussion of hierarchies, as well as discussion of the ontological status of species, shows that he knew that there was a difference between the type of hierarchy Gregg proposed and Woodger proposed. Hennig included both because he felt they provided the clearest discussion on hierarchies, but a careful reading of the text shows that he understood the differences between them.
Rieppel, Olivier “Hennig’s enkaptic system” Cladistics 25 (2009): 311–317. P. 311