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Date: Fri, 16 Feb 2001 09:46:55 -0600
From: "Bryant, Harold MACH" <HBryant@mach.gov.sk.ca>
To: "'PhyloCode@ouvaxa.cats.ohiou.edu'" <PhyloCode@ouvaxa.cats.ohiou.edu>
Subject: RE: On the Other Phylogenetic Systematics, Nixon and Carpenter
As one of the "disciples" referred to in Nixon and Carpenter's paper, I thought I would comment on their paper, and especially their "paleoherb" example. It also provides a context for some comments on qualifying clauses. I had planned to comment on JW's earlier comments on the latter issue, but until today I hadn't found the time. N&C's paleoherb example certainly illustrates the potential for marked changes in the taxonomic content associated with a name given changes to phylogenetic hypotheses. This is not a new revelation; it has been evident for some time (e.g., Schander and Thollesson [1995], Zool. Scripta 24:263-268) and was the stimulus for the development of various devices, including qualifying clauses, that can markedly increase the stability in the content of taxa with particular names with changes, or differences of opinion, regarding phylogenetic pattern. I find N&C's paper, and use of this example, misleading because they do not consider these options in the coining of a phylogenetic definition for this name, or in their discussion of the content issue. Also, they do not address the issue of whether it would have been appropriate to even define the name in the first place. The PhyloCode (Rec. 9B) recommends extreme caution in defining names that refer to poorly supported clades. If someone wanted to provide a phylogenetic definition for paleoherbs, the clade conceptualized by Donoghue and Doyle in their 1989 paper would be best reflected in a definition that included a qualifying clause that identified the phylogenetic situations in which the name would not apply to ANY clade. Such a definition could be phrased in various ways but one form would be: "the least inclusive clade that contains species A and B (+ others as required), but does not include species C (+ others as required)." (the reference to the exclusion of C is the qualifying clause) Given this definition, the name would not apply to any clade on cladograms where C is included in the least inclusive ancestry circumscribed by A and B. I find this approach very appropriate. Most names are proposed within a particular phylogenetic context (see Bryant 1997 Biol. J Linn. Soc. 62:495-503). On cladograms that lack that context (because of either changes in our understanding of phylogeny due to new information, or simply differences of option in the choice of a reference phylogeny), I feel that best approach is to simply not use that clade name in referring to that cladogram. Certainly this approach avoids the marked changes in content evident in N&C's paleoherb example. N&C's discussion is also misleading in that they totally ignore the fact that with the current codes content associated with a taxonomic name can change or differ simply because of subjective differences of option regarding the boundaries between taxa, or the rank in the Linnean hierarchy where they should be placed. This source of instability is eliminated with PN, where changes in content are associated solely with changes or differences of option regarding phylogenetic pattern. I feel that the jury is still out regarding the relative degree of stability in the content of named taxa due to differences in phylogenetic pattern (assuming that devices such as qualifying clauses are used, and that care is exercised in the construction of phylogenetic definitions). When the additional sources of instability inherent to the traditional system are also taken into account, I think there is a very good chance that the OVERALL level of stability may even be higher with PN. (my discussion assumes that content stability is a good thing - that is a point that could also be debated (my view: sometimes yes, sometimes no) - however, there is no question that it is an important issue if for no other reason than this is the more frequent criticism of PN.) Harold ---------------------------------------------------------------------------- ------------------------------------------------ Harold Bryant Royal Saskatchewan Museum 2340 Albert Street Regina, Saskatchewan S4P 3V7 Canada 306-787-2826 FAX 306-787-2645