PHILOSOPHICAL FOUNDATIONS OF CLASSICAL EVOLUTIONARY CLASSIFICATION WALTER J. BOCK Abstract Bock, W. J. (Department of Biological Sciences, Columbia University, New York, New York, 10027) 1974. Philosophical foundations of classical evolutionary classification. Syst. Zool. 22:375-392.—The primary objective of biological classification is to provide the foundation of all comparative studies in biology. These studies require consideration of Downloaded from two factors, namely: (a) the degree of genetic similarity between organisms; and (b) the phylogenetic sequence of events in their history. Classical evolutionary classification provides the best approach to classification, based on Popper's criterion of content, because it attempts to maximize simultaneously both of these semi-independent variables. Evo- lutionary classification is based on the evolution of organisms, not just their phylogeny. Application of the Popperian philosophy of the demarcation and methodology of science to classification suggests that a major task is the development of severe tests of falsification by which classifications can be tried. Although phenetic methods appear to be the best http://sysbio.oxfordjournals.org means of recognizing taxa and deducing classifications, cladistic methods appear to pro- vide the best tests in attempts to disprove these statements of relationships. The best approach to definitions in this historical science is a theoretical one in which the defining criteria and the recognizing criteria may differ. Comparisons, which are closely akin to definitions, must be based on evolutionary theory. Homology is the primary principle in comparative biology with the possibility that all other principles of comparison are re- ducible to homology. The defining criterion of homology is phylogeny and the only recognizing criterion of homology is similarities of all sorts between features. Any approach to classification that excludes homology and its recognizing criterion of similarity as a primary step in deducing relationships is invalid. Use of falsifying tests suggests that relative weighting of characters may have greatly reduced importance in taxonomic methods. Lastly, the formal classification and the phylogenetic diagram of a group of organisms are not different, but redundant images of each other. Both are essential parts of the conclusions of any study of biological relationships. [Philosophy; Evolutionary at Stanford Medical Center on June 7, 2010 classifications; Definitions; Comparisons; Homology.] INTRODUCTION accepted classifications that accomodated Classification of living organisms dates readily the numerous new species of living back into antiquity, having its roots in the and fossil organisms being described daily, development of language and the first at- The maior deficiency in this early nineteenth tempts to convey information from one century systematics was a satisfying philo- person to another. The earliest organized sophical and scientific foundation. Biology efforts to classify animals and plants pre- was bf nS *?<& alon§ f™1/ in ^ *an- sition from e an ient idea dates the beginnings of oral and written * £ <r that everything histories. Scientific classification is among *f sta*c and fixed t0 &e neweJ «»<*Pte the oldest of biological disciplines and has that chan§e is occurring everywhere in the developed slowly under a variety of philo- universe and that the observed world sophical foundations, all of which share around us is a result of this ever continuing the central theme of comprehending order modification. A search for mechanisms among the vast diversity of biological underlying biological change and for an organisms. A well-established field of explanation of the particular type of order biological systematics existed in the first among organisms discovered by systematists half of the last century, complete with a were among the major motivations behind definite philosophy, a firm methodology the formulation of the theory of organic and excellent results in the form of widely evolution by Darwin. With the rapid, wide- 375 376 SYSTEMATIC ZOOLOGY spread acceptance of the Darwinian theory Various phenetic methodologies, such as of evolution by biologists, this new concept numerical taxonomy (Sokal and Sneath, became the philosophical basis for classifi- 1963), are largely concerned with the cation. The notion that a group of organ- development of techniques by which the isms are related because of their evolution degree of similarity (or difference) be- from a common ancestor became the funda- tween organisms can best be measured. mental criterion underlying biological com- Arguments as to whether phenetists parison and classification. All major modern measure genetical or phenotypical similarity approaches to biological classification are or whether they compare "overall" or "over- Downloaded from evolutionary because they are based on the some" similarity are of little significance. Darwinian theory of evolution and accept Degree of similarity between organisms the criterion of descent from a common is an important factor in ascertaining ancestor as the basis for relationship. relationships and in establishing classifica- Simple as this latter statement may be, it tions; phenetists are correct in stressing its is subject to several interpretations which importance. Most significant is that in pure http://sysbio.oxfordjournals.org have given rise to widely differing ap- phenetics, the only variable used in con- proaches to classification. Analysis of these structing classifications is similarity between varying interpretations of common descent organisms. No phyletic information is used as the hallmark of relationship has been a (Colless, 1967), this being considered by central theme of classificatory theory. phenetists as "the phylogenetic fallacy." The approaches to biological classifi- Although I believe that this fallacy is in- cation arising from the varied interpreta- valid (Bock, 1969 b), I agree with phenetists tions of common descent can be grouped that phyletic information must be excluded under three headings which characterize from their approach to classification. As the major modern schools of systematics. soon as any phyletic information is ad- The purpose of this essay is to appraise the mitted into phenetics, then no basis exists at Stanford Medical Center on June 7, 2010 philosophical foundation of only one of to exclude any phyletic information, with these approaches—that of classical evo- the result that the philosophical foundation lutionary classification—not to contrast of phenetics has been modified radically, critically or evaluate all three. In view of and little would separate phenetics from their common origins in Darwinian evo- classical evolutionary classification. Such lution, brief demarcation of each school is problems exist with the development of essential to insure clarity of discussion and techniques such as phenetic cladistics meaning. These schools of systematic which have emerged from pure phenetics theory are: without a careful restatement of its philo- A) Phenetics: This approach is often, sophical base. but erroneously, synonymized with numeri- B) Cladistics: The term "cladistics," cal taxonomy which is a methodology often referring to branching, is less ambiguous used in phenetics but has broader useful- and indeed more descriptive than the term ness. Phenetics is based on the concept that "phylogenetic systematics" (Hennig, 1966) relationship between organisms is ascer- which is preferred by most proponents of tained by their overall degree of similarity. this approach to classification. Relationship Greater similarity indicates closer relation- among organisms, in cladistics, is based ship and a smaller amount of evolutionary upon joint possession of derived features change from the common ancestor. Implicit ( = apomorphous or advanced characters). in this approach is the assumption that the Species which share derived features are degree of difference between organisms is related, and hierarchies of taxonomic proportional to the amount of evolutionary groups are established on hierarchies of change since their common ancestor and derived characters. Thus relationships be- hence to their degree of relationship. tween organisms are strictly determined by I.C.S.E.B.—SYSTEMATIC PHILOSOPHIES 377 phylogenetic branching and may be re- The name for this third approach to garded as genealogical or kinship relation- biological classification is awkward and ships. somewhat misleading in that it carries the Cladistics is based strictly upon joint implication that this is the only approach possession of derived features and excludes based on organic evolution. This is not the absolutely any evaluation of similarity from case, and once again it should be empha- the determination of relationships and con- sized that all major approaches to classifi- struction of classification. Thus, Hennig cation are based on Darwinian evolution. states clearly (1966:10) that "Genealogic Evolutionary classification is an eclectic Downloaded from relationships are, however, something en- approach in that it combines the important tirely different from 'similarity,'" and (p. elements from phenetics and cladistics, but 12) "The second error in the idea of a "eclecticism" is a poor name because it is logical and historical primacy of nonphylo- not descriptive of the basic conceptualiza- genetic (e.g., typological) systematics stems tion of this approach to classification. from the assumption