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Date: Thu, 03 May 2001 12:25:01 -0400
From: Philip Cantino <cantino@ohiou.edu>
To: PhyloCode@ouvaxa.cats.ohiou.edu
Subject: conflict between monophyletic taxonomy and rank-based classification
Dick Olmstead wrote: >There is nothing in the neo-Linnaean hierarchy of ranks that is >incompatible with a strictly monophyletic classification. Many who oppose >the PhyloCode think we can do just fine within that framework. I happen to >disagree. The first sentence is correct but may be misunderstood by some readers. Indeed, after reading the first sentence, people may reasonably ask why we can't "do just fine within [the] framework [of the neo-Linnaean hierachy of ranks]." Although strictly monophyletic taxonomy is not incompatible with the neo-Linnaean hierarchy (i.e., there is no problem with classifying some clades as families, some as genera, etc.), it is incompatible with the current system in which some ranks are either mandatory or treated as though they were mandatory. The incompatibility arises in at least two situations: the classification of ancestors, and the classification of well supported clades within a group that is otherwise poorly resolved. As has been pointed out many times through the years, the immediate common ancestor of any pair of genera cannot be assigned to a monophyletic genus. However, every species must be assigned to a genus in our current system. The fact that the genus category is mandatory in binomial nomenclature therefore conflicts with the goal of strictly monophyletic taxonomy. The same conflict occurs at other ranks, such as family, that are treated by convention as though they were mandatory. Many people who have pointed out this conflict have used it to argue that paraphyletic taxa are inevitable because these people presuppose the maintenance of the current system of classification. I draw a different conclusion--that mandatory ranks are unacceptable--because I give primacy to monophyletic taxonomy. The classification of ancestors is a theoretical problem that rarely if ever arises in practice, but there is another very common situation in which a conflict arises between mandatory ranks and monophyletic taxonomy. Suppose that within a subfamily (for example), some clades are well supported and could thus be recognized as genera, but there are residual species whose relationships remain unresolved--they do not form well supported clades within the subfamily. In the current system, each species must be assigned to a genus, but our present state of knowledge is inadequate to place some of them in monophyletic genera. One could place each such species in a monotypic genus, but if some of the species are very similar to each other, placing them in marginally distinct monotypic genera is not a very practical approach. Dick Olmstead, Steve Wagstaff and I documented an example of this situation in a 1999 paper in Systematic Botany (23: 369-386). When the taxa in question are species, as in this example, we are required by the current system to assign every one to a genus even if the genera are paraphyletic or questionably monophyletic (thus conflicting with the goal of strictly monophyletic taxonomy). The same sort of problem can arise at other levels; for example, it may be the case that some genera belong to clearly monophyletic tribes but others do not. Because tribe is not a mandatory rank, but it is the convention to place every genus of a family in a tribe if any of them are placed in a tribe, a frequent outcome is that the taxonomist decides not to recognize a well supported clade that happens to fall at the tribal rank because doing so would necessitate placing all the other genera in tribes and the evidence is inadequate to place them in monophyletic tribes. The problem described here is primarily taxonomic (not nomenclatural) but it can be avoided by adopting phylogenetic nomenclature, because the use of ranks (or any particular rank) is not mandatory. There is also a nomenclatural problem with the current system that Kevin alluded to in his May 1 message--rank assignment affects the spelling and application of names, leading to name changes when clades shift in rank. This source of nomenclatural instability is eliminated by the PhyloCode, where the rank assignment (if ranks are used) does not affect the name. Phil Philip D. Cantino Professor and Chair Department of Environmental and Plant Biology Ohio University Athens, OH 45701-2979 U.S.A. Phone: (740) 593-1128; 593-1126 Fax: (740) 593-1130 e-mail: cantino@ohio.edu