Reply to Henderson: On Delimiting Species for Taxonomic Analyses

In a recent series of papers, Henderson (2004, 2005a,b) has described and employed a method for de limiting species for use in taxonomic analyses. Because his work has focused on phylogenetic analyses based on herbarium specimens, Henderson (2005b) has ar gued that some species concepts (e.g., the biological [BSC] and monophyletic species concepts) are simply not informative at this stage of research and that other concepts (e.g., the morphological [MSC] and taxonomic [TSC] species concepts) are unscientific. Having dis missed these and other species concepts as inappro priate, Henderson then adopts a modification of the phylogenetic species concept (PSC; Mayden's [1997] PSQ) as the only justifiable approach for delimiting species in the context of herbarium-based revisionary or monographic studies. Henderson (2005b) cites Luckow (1995) and Mayden (1997) to provide support for his rejection of various species concepts. What Henderson fails to do is to put these comments in context or verify exactly what was presented in those papers. I have no quarrel with Luc kow's (1995) comments: her intent was clearly stated ("Instead, I will review species concepts used in phy logenetics ... ") and she provided a thoughtful review. However, Luckow's discussion of the BSC, coupled with her failure to address the similarity of the PSC and the phenetic (PhSC) species concept, does invite examination with respect to what follows. Mayden (1997), on the other hand, is careless with at least one species concept. He quotes Sneath (1976) to provide a definition of the PhSC: "... the species level is that at which distinct phenetic clusters can be observed/7 First, this is not a definition of a species and, second, what's more pertinent is what precedes the ellipsis in the Sneath (1976) quote: "For the present discussion we may consider... " It is clear, at least to me, that Sneath was making no attempt to provide a phenetic definition of species, although his ideas were mirrored by Andersson (1990): "... the species is de fined as the category of clusters that are inwardly con tinuous and outwardly discrete,... ". Later, Mayden (1997) implies that the phenetic species concept, along with several others, is based on morphology alone, and identifies both the MSC and TSC as synonyms of the PhSC. May den (1997) is not alone in perpetuating this misrepresentation. For example, Claridge et al. (1997), commenting on Sokal and Crovello (1970; see below), state that critics of the BSC (i.e., Sokal and Crovello, among others) "... preferred either an overtly morpho species approach [my emphasis] or some sort of phenetic system/7 This view was stressed earlier by Doyen and Slobodchikoff (1974) who claimed that "... phenetic data are restricted to morphological and physiological characteristics'' as opposed to their approach which, like that of Sokal and Crovello (1970) begins with phe netic analyses, but then introduces both reproductive and ecological data to clarify putatively misleading phenetic results. Had Mayden (1997), Claridge et al. (1997), and Doy en and Slobodchikoff (1974) read Sokal (1973) and Sneath and Sokal (1973) more carefully, they would have realized that this is true of the "classical phenetic species concept/' which Sokal (1973) described as that based on morphological differences. The "phenetic species concept," as developed most fully by Sokal and Crovello (1970), who viewed it as a surrogate for the BSC, was dealt with by Sokal (1973) as the "phenetic species concept advocated by the numerical taxono mists," a concept that is, to quote Sokal (1973), "only a quantification and refinement of the phenetic species of the orthodox taxonomist." But, what a refinement? numerical taxonomists advocated the use of as many characters as possible, from as many data sources as possible (including chemical, behavioral, ecological and distributional characters; see Sneath and Sokal [1973], pages 90-96), for delimiting groups that could be equated with species and higher level taxa. There is nothing in Sokal and Crovello (1970), Sokal (1973), Sneath and Sokal (1973), or Sneath (1976) to suggest that the refined version of the phenetic species concept, which Sokal (1973) referred to as the "numerical phe netic species," is limited to morphological data. I have no real quarrel with much of what Henderson proposes. Additionally, I do not intend to engage in the debate over which species concept is to be pre ferred. Mayden (1997) presents a general overview of species concepts and any reader who has followed this literature knows that some of these concepts, such as