Beyond Cladistics: The Branching of a Paradigm

9. Monophyly and the Two Hierarchies

147 NI NE Olivier Rieppel MONOPHYLY AND THE TWO HIERARCHIES The school of biological systematics known as cladistics is notorious for drawing a distinction between pattern and process (Nelson and Platnick 1981; Beatty 1982). The pattern is one of relative degrees of relationships , the process is one of species lineages splitting and splitting again. A cladogram potentially has two interpretations (Platnick 1977). If the nodes in the cladogram are taken to be speciation events, and the internodes to be stem species, then the cladogram is isomorphic with a phylogenetic tree. If the cladogram is taken to represent a hierarchy of relative degrees of relationships that can also be represented in a Venn diagram (i.e., as a system of sets within sets), then the cladogram can be taken as a basis for classification, Linnaean or otherwise. Hull (1988: 101) argued that a branching diagram and a corresponding classification could indicate common ancestry or phylogenetic relationship, but not both. Phylogenetic relationships are expressed as a phylogenetic tree, which is a system of species lineages splitting and splitting again. The result is a “division hierarchy” that does not specify relations of inclusiveness . Relative recency of common ancestry is indicated by an inclusive hierarchy of groups within groups, the more inclusive groups sharing a more distant ancestry than the included, less inclusive groups. Beyond Cladistics: The Branching of a Paradigm, edited by David M. Williams and Sandra Knapp.Copyright byTheRegentsoftheUniversityofCalifornia.Allrightsofreproduction in any form reserved. 148 / CLADI ST I CS In 1992, P. A. Williams “suggested that some major problems found in cladism stem from a confusion inherent in its philosophy.” Specifically , she (Williams 1992: 151) claims to “have demonstrated that the divisional ‘hierarchy’ and the Linnaean hierarchy are different types of ‘hierarchy,’” that “Hennig conflated the two types of ‘hierarchy’ and, in so doing, made the school of biological taxonomy known as ‘cladism’ philosophically confused.” Cladists naturally responded that the confusion is located in philosophy, not in cladism (D. M. Williams et al. 1996: 134): “there is little excuse for philosophers, when it is their job to enlighten their readers rather than to obfuscate the issues. At the very least, philosophers should attempt to understand the arguments of cladists rather than depend on the views of their critics.” One of the authors whom cladists (D. M. Williams et al. 1996) found to be critical of their program is Hull (1988), who in turn was acknowledged by P.A.Williams (1992: 152) for having clarified some of the issues she was dealing with when writing her paper. Indeed, Hull (1988: 101) stated, “Although diagrams representing phylogenetic trees and the arrangements of taxonomic names in classifications might appear to be related to each other in a systematic and straightforward way, this appearance is extremely deceptive.”The question thus arises whether P. A. Williams’s (1992: 151) claim that “the only way to eliminate the confusion and problems from cladism is to eliminate cladism as a school of biological taxonomy” is based on a mere misunderstanding, or whether some deeper issue is at stake in this debate. THE TWO HIERARCHIES P. A. Williams (1992) located the confusion in cladistics in Hennig’s (1966) allegedly mistaken interpretation of Gregg’s (1954) analysis of the Linnaean hierarchy in the light of Woodger’s (1952) work. Indeed, referring to Woodger’s (1952) and Gregg’s (1954) analysis of the nature of a hierarchy, Hennig (1966: 16–17) asserted, “We consider the investigations of Woodger and Gregg extraordinarily important because they clarify, with methods that exclude all confusion and contradiction, the peculiarities of the hierarchic system.” From the lasting success of a hierarchical representation of biodiversity, Hennig (1966: 20) concluded to some “deeper significance”—namely, that “the phylogenetic system, which corresponds exactly to this hierarchic type, is . . . indeed the general reference system of biology that we have been seeking.” Hennig here MONOPHYLY AND T HE T WO HI ERARCHI ES / 149 invokes a “hierarchic type,” which presumably implies that there are different types of hierarchies, of which one type is best suited to represent genealogy. In his 1950 book, at a time before he was acquainted with the work of Woodger (1952) and Gregg (1954; see Hennig 1957), Hennig (1950: 20) raised the question whether “the fact that similarity relations which exist between two discrete [numerically different] natural things . . . are best represented by a hierarchical system of groups, is in itself enough already to prove the historical...