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Biological Classification: Toward a Synthesis of Opposing Methodologies

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Biological Classification: Toward a Synthesis of Opposing Methodologies ly, applying it to any approach making use of the "similarity" of species and other taxa, while to the strict numerical pheneticists the term phenetic means the "theory-free" use of unweighted charac- Biological Classification: Toward a ters. Cladistics (or cladism). This method of classification (4), the first comprehen- Synthesis of Opposing Methodologies sive statement of which was published in 1950 by Hennig (5), bases classifications Ernst Mayr exclusively on genealogy, that is, on the branching pattern of phylogeny. For the cladist phylogeny consists of a sequence of dichotomies (6), each representing the For nearly a century after the publica- or unspoken starting point of virtually all splitting of a parental species into two tion of Darwin's Origin (1) no well-de- systems of classification. Any classifica- daughter species; the ancestral species fined schools of classifiers were recog- tion incorporating the method of group- ceases to exist at the time of the dichoto- nizable. There were no competing meth- ing taxa by similarity is, to that extent, my; sister groups must be given the same odologies. Taxonomists were unanimous phenetic. categorical rank; and the ancestral spe- in their endeavor to establish classifica- In the 1950's to 1960's several investi- cies together with all of its descendants tions that would reflect "degree of rela- gators went one step further and suggest- must be included in a single "holophy- tionship." What differences there were ed that classifications be based exclu- letic" taxon. among competing classifications con- sively on "overall similarity." They also Evolutionary classification. Phenetics cerned the number and kinds of charac- proposed, in order to make the method and cladistics were proposed in the en- ters that were used, whether or not an more objective, that every character be deavor to replace the methodology of author accepted the principle of reca- given equal weight, even though this classification that had prevailed ever pitulation, whether he attempted to would require the use of large numbers since Darwin and that was variously "base his classification on phylogeny," of characters (preferably well over a designated as the "traditional" or the and to what extent he used the fossil hundred). In order to reduce the values "evolutionary" school, which bases its record (2). As a result of a lack of of so many characters to a single mea- classifications on observed similarities and differences among groups of organ- on November 1, 2008 isms, evaluated in the light of their in- Summary. Currently a controversy is raging as to which of three competing ferred evolutionary history (7). The evo- methodologies of biological classification is the best: phenetics, cladistics, or evolu- lutionary school includes in the analysis tionary classification. The merits and seeming deficiencies of the three approaches all available attributes of these organ- are analyzed. Since classifying is a multiple-step procedure, it is suggested that the isms, their correlations, ecological sta- best components of the three methods be used at each step. By such a synthetic tions, and patterns of distributions and approach, classifications can be constructed that are equally suited as the basis of attempts to reflect both of the major generalizations and as an index to information storage and retrieval systems. evolutionary processes, branching and www.sciencemag.org the subsequent diverging of the branches (clades). This school follows Darwin methodology, radically different classifi- sure of "overall similarity," each char- (and agrees in this point with the cla- cations were sometimes proposed for the acter is to be recorded in numerical dists) that classification must be based same group of organisms; also new clas- form. Finally, the clustering of species on genealogy and also agrees with Dar- sifications were introduced without any and their taxonomic distance from each win (in contrast to the cladists) "that adequate justification except for the other is to be calculated by the use of genealogy by itself does not give classifi- claim that they were "better." Dissatis- algorithms that operationally manipulate cation" (8). Downloaded from faction with such arbitrariness and seem- characters in certain ways, usually with The results of the evolutionary analy- ing absence of any carefully thought out the help of computers. The resulting sis are incorporated in a diagram, called methodology, led in the 1950's and diagram of relationship is called a pheno- a phylogram, which records both the 1960's to the establishment of two new gram. The calculated phenetic distances branching points and the degrees of sub- schools of taxonomy, numerical phenet- can be converted directly into a classifi- sequent divergence. The method of in- ics and cladistics, and to a more explicit cation. ferring genealogical relationship with the articulation of Darwin's methodology, The fullest statement of this method- help of taxonomic characters, as it was now referred to as evolutionary classifi- ology and its underlying conceptualiza- first carried out by Darwin, is an applica- cation. tion was provided by Sokal and Sneath tion of the hypothetico-deductive ap- (3). They called their approach "numeri- proach. Presumed relationships have to cal taxonomy," a somewhat misleading be tested again and again with the help of The Major Schools of Taxonomy designation, since numerical methods, new characters, and the new evidence including numerical weighting, can be frequently leads to a revision of the Numerical phenetics. From the earli- and have been applied to entirely differ- inferences on relationship. This method est preliterary days, organisms were ent approaches to classification. The is not circular (9) as has sometimes been grouped into classes by their outward term numerical phenetics is now usually suggested. appearance, into grasses, birds, butter- applied to this school. This has intro- and others. duced The author is Alexander Agassiz Professor Emeri- flies, snails, Such grouping some ambiguity since some au- tus, Museum of Comparative Zoology, Harvard "by inspection" is the expressly stated thors have used the term phenetic broad- University, Cambridge, Massachusetts 02138. 510 0036-8075/81/1030-0510$01.00/0 Copyright 1981 AAAS SCIENCE, VOL. 214, 30 OCTOBER 1981 Is There a Best Way to Classify? cific observations can be generalized by nomic units" (OTU's); and, most impor- testing them against other taxa in the tantly, the method insisted on the equal Each of the three approaches to classi- system or against other kinds of charac- weighting of all characters. fication-phenetics, cladistics, and evo- ters (10-12). It is now evident that no computing lutionary classification-has virtues and Pheneticists, as well as cladists, have method exists that can determine "true weaknesses. The ideal classification claimed that their methods of construct- similarity" from a set of arbitrarily cho- would be one that would meet best as ing classifications are nonarbitrary, auto- sen characters. So-called similarity is a many as possible of the generally ac- matic, and repeatable. The criticisms of complex phenomenon that is not neces- knowledged objectives of a classifica- these methods over the last 15 years (13) sarily closely correlated with common tion. have shown, however, that these claims descent, since similarity is often due to A biological classification, like any cannot be substantiated. It is becoming convergence. Most major improvements other, must serve as the basis of a conve- increasingly evident that a one-sided in plant and animal classifications have nient information storage and retrieval methodology cannot achieve all the been due to the discovery of such con- system. Since all three theories produce above-listed objectives of a good classifi- vergence (14). hierarchical systems, containing nested cation. Different types of characters-mor- sets of subordinated taxa, they permit The silent assumption in the method- phological characters, chromosomal dif- the following of information up and ologies of phenetics and cladistics is that ferences, enzyme genes, regulatory down the phyletic tree. But this is where classification is essentially a single-step genes, and DNA matching-may lead the agreement among the three methods procedure: clustering by similarity in to rather different grouping. Different ends. Purely phenetic systems, derived phenetics, and establishment of branch- stages in the life cycle may result in from a single set of arbitrarily chosen ing patterns in cladistics. Actually a clas- different groupings. characters, sometimes provide only low sification follows a sequence of steps, The ideal ofphenetics has always been retrieval capacity as soon as other sets of and different methods and concepts are to discover a measure of total (overall) characters are used. The effectiveness of pertinent at each of the consecutive similarity. Since it is now evident that the phenetic method could be improved steps. It seems to me that we might this cannot be achieved on the basis of a by careful choice of selected characters. arrive at a less vulnerable methodology set of arbitrarily chosen characters, the However, the method would then no by developing the best method for each question has been asked whether there is longer be "automatic," because any se- step consecutively. Perhaps the steps not a method to measure degrees of lection of characters amounts to weight- could eventually be combined in a single difference of the genotype as a whole. ing. algorithm. In the meantime, their sepa- Improvements in the method of DNA Cladists use only as much information rate discussion contributes to the clarifi- hybridization offer hope that this method on November 1, 2008 for the construction of the classification cation of the various aspects of the clas- might give realistic classifications on a as is contained in the cladogram. They sifying process. phenetic basis, at least up to the level of convert cladograms, quite unaltered, orders (15). The larger the fraction of the into classifications, only when the clado- nonhybridizing DNA, the less reliabie grams are strictly dichotomous.
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