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Phenetics and Numerical Taxonomy Applied to Systematic Nematology T Phenetics and Numerical Taxonomy Applied to Systematic Nematology t W. WAYNE Moss AND W. A. WEBSTER ~ Numerical taxonomy and the phenetic modified by speculation on their evolutionary approach to classification have aroused history. Essentially, a phenetic study at- considerable interest and debate among tax- tempts to recognize groups based on data, onomists in recent years. It will not be rather than on inferences from data. The possible here to examine the controversy in desired product of a phenetic study is a any depth, but a few comments on certain "natural" classification (16), a set of group- aspects of theory and methodology may be ings based on a maximum of shared char- helpful in considering their relevance to sys- acters. Such classifications are generally tematic hematology. References to pertinent assumed to possess both a maximum in- articles dealing with these and additional formation content and a higher predictability aspects are included in the bibliography (8, than classifications based on fewer, heavily- 12, 13, 14, 16, 19, 23, 27, 33, 50, 51, 52, weighted characters deemed to have evolu- 53). tionary significance, although there are PHENETICS reasons for suggesting that this assumption The phenetic approach to the classifica- need not always hold true, as discussed tion of a set of organisms or objects involves below. the establishment of inter-object similarities The phenetic approach is nothing new; based on all available data from the objects, taxonomists have been processing their ma- with resultant groupings purposely left un- terial phenetically for centuries, by intuitively grouping together individuals sharing a ma- Received for publication 5 September 1969. jority of characteristics in common. The 1 Project initiated jointly in the Department of Entomology congruence between many of the newer classi- of The University of Kansas, Lawrence, and the Animal Diseases Research Institute, Hull, Canada; continued at fications obtained by computerized methods the Academy of Natural Sciences, Philadelphia with help with earlier classifications obtained conven- from NSF Research Grant GB-6851; and completed in the Division of Entomology, University of California, Berke- tionally is therefore not surprising, as phe- ley. Special thanks are due the Miller Institute for Basic Research in Science of the University of California, netics is a major component of most orthodox Berkeley, for support in the form of a postdoctoral re- classifications. Since the time of Darwin, search fellowship to W. W. Moss. Computer time was provided by the Computation Center of the University of however, it has been considered desirable to Kansas (Project 0729), and by the Computer Centers of the University of Pennsylvania (Projects 086446 and describe the results of biological classifica- 4025) and the University of California (Projects 8639 and tions in an evolutionary context. 708"/). Photographs were taken by the Bin-Graphic Unit, Research Branch, Canada Department of Agriculture. In contrast to cladistic methods that at- The manuscript benefitted greatly from a critical reading by J. H. Camin, Department of Entomology, University tempt to classify on the basis of presumed o[ Kansas, Lawrence. u Postdoctoral Research Fellow, Division of Entomology, common ancestry (regardless of phenetic University of California, Berkeley 94"/20; on leave from similarity or dissimilarity) (9, 24, 26), and Academy of Natural Sciences, 19th and Parkway, Phila- delphia, Pa. 19103 and Animal Diseases Research Institute, phyletic ("evolutionary") approaches that Canada Department of Agriculture, P. O. Box 1400, Hull, Quebec. combine a vaguely-defined mixture of phe- 16 SYMPOSIUM: PHENETICS AND NUMERICAL TAXONOMY • Moss, Webster 17 netics and cladistics (32), phenetics is a groupings recognized are natural, phenetic relatively straightforward, "here-and-now" assemblages. sort of approach aimed at determining To a person engaged in a phenetic study, present-day similarities, regardless of how the fact that a character may be present in they might have come about. The fact that three recognizable character states, A, B the organisms to be classified are the prod- and C, is certainly of interest, and his aim ucts of a process of organic evolution is will be to group together those objects with certainly of interest, but the /act of evolu- identical character states. The fact that state tion's occurrence need be neither pertinent A might be considered "primitive" or "an- nor useful in the determination of phenetic cestral" or "plesiomorphic" by a phyletic similarities (2). Evolution is without argu- worker (and the same state very likely ment a central, unifying concept of biology. "specialized" or "derived" or "apomorphic" However, although it is true that a knowl- by another) is a consideration outside the edge of evolutionary processes and pathways defined scope of a phenetic taxonomy. De- can be gained through analysis of phenetic spite the fact that many phenetic workers data, it is also true that the actual course are interested in aspects of evolution as they followed by evolution will be known with pertain to areas outside of taxonomy, it has certainty for a vanishingly small number of been suggested that pheneticists are opposed biological groups to be classified; and this to inferences on evolutionary processes be- statement is particularly true for the Nema- cause of their refusal to attempt to classify toda. The unravelling of possible or probable in an evolutionary context (30). This criti- evolutionary pathways in this context is a cism is analogous to saying that a man is stimulating intellectual exercise, but it can be anti-water because he drinks water, but re- little more than that. A classification that fuses to try to walk on it! reflects presumed evolutionary pathways is a theory (31 ), but a theory that is not testable NUMERICAL TAXONOMY other than in terms of itself (2). There is Numerical taxonomic studies attempt to no observer of the entire phylogeny of a recognize similarities and establish classi- group to whom we can turn and say, "Does fications by means of operationally-defined, our classification correctly express the phy- numerically-oriented procedures (56). Most logeny of this group?". Accordingly, even numerical studies are purely phenetic, al- with a moderately complete fossil record, any though some attempts have been made to evolutionary classification is based on spec- carry out phyletic studies operationally and ulation from phenetic data, and cannot be numerically (9, 10, 22, 29, 58, 60, 61). objectively confirmed ( 13 ). We may be able Some of the most bitter disagreements in to predict that additional, as yet undis- the history of systematic biology have accom- covered, taxa will fit into the categories that panied the introduction and application of we have erected, and that additional char- acters will be found to show trends con- numerical taxonomy (NT), as recent issues of cordant with characters used previously. pertinent journals such as Systematic Zoology However, the fact that such (phenetically) and Taxon will witness. Heralded as a agreeable taxa and characters may indeed be panacea by its proponents, NT has been found proves nothing about the congruence condemned as an abomination by its critics. of the classification with the actual phy- It appears likely that both sides have an logeny, although it may indicate that the element of truth. There is no doubt that 18 Journal of Nematology, Vol. 2, No. 1, January 1970 many existing, conventionally-derived taxon- conventional taxonomy. As an example of omies have a significant content of valuable a double-edged criticism, NT has been con- information, and will not be swept away by demned (32) for not being able to separate the application of numerical approaches. At phena (sample[s] of phenotypically similar the same time, there are ways in which NT specimens; or, more usually, groups recog- can considerably clarify taxonomic situations nized in a phenetic study) from taxa ("tax- muddled by conflicting phyletic opinions onomic group[s] sufficiently distinct to be based on different, heavily-weighted char- worthy of being distinguished by name and acter complexes, as well as by variations in to be ranked in a definite category"); and descriptive technique. In addition, NT offers this is quite true. The fact that several a variety of stimulating, new ways of attack- individuals form a distinctive, phenetic group ing systematic problems and of visualizing in a numerical study provides few clues as their results. to whether or not the group is an intraspecific In view of the controversy surrounding population, a species, a genus, etc. Such a NT, its admitted drawbacks in some areas, decision must be left to the taxonomist carry- and the fact that the field is still in a con- ing out the study. This weakness would seem siderable state of flux, a reasonable question a formidable deficit of numerical phenetics; for a nematode taxonomist to ask with ref- yet is the conventional taxonomist in any erence to NT is: "Why bother?"; and there better position? In fact, he is not. In recog- is no easy answer to this question. There nizing groups above and below the species are both theoretical and practical objections, level, the conventional taxonomist must rely as well as advantages, to the undertaking of predominantly (or entirely) on phenetic a phenetic, numerical study, and the potential data, usually morphologic; at the species worker should be aware of these before com- level the conventional taxonomist may have mitting himself. recourse to the biological species concept, if THEORETICAL CONSIDERATIONS: Oil the he can apply it to the preserved material theoretical level, NT has been roundly that serves as the basis of classificatory con- criticized by respected workers with consider- clusions in most groups of organisms (e.g., able taxonomic experience (6, 30, 45, 47, Nematoda), but in most cases he finds him- 48), and a prospective numerical worker self in the same position as a pheneticist. In should be aware that he is risking "an excur- a way he may be even worse off, since he is sion into futility" (46).
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