SPECIES AND SPECIATION IN PHYLOGENETICSYSTEMATICS, WITH EXAMPLES FROM THE NORTH AMERICAN FISH FAUNA' E. O. WILEY AND RICHARD L. MAYDEN2 Phylogenetic systematics as developed byHennig (1950, 1966) is a system ofmethods anda view of the world designed to integrate system-atics with the evolutionary "paradigm." By evo-lutionary "paradigm" we mean the world viewthat organisms always have parents and that or-ganismic diversity is a byproduct ofdescent withmodification. Ifour view ofphylogenetic system-atics is correct, or even partly so, the results de-rived from Hennig's methods should be of gen-eral utility to the evolutionary community. Weshall attempt to show this utility by consideringhow the results of phylogenetic analysis can beused to infer the mode of speciation involved inthe origin of species. Before doing so, however,we need to discuss what constitutes a speciesbecause our concepts will influence our choiceof units to be analyzed. SPECIES IN PHYLOGENETIC SYSTEMATICS There is no consensus concerning either thenature of species-as-taxa or the importance ofspecies among those who align themselves withHennig's (1966) basic philosophy. This shouldcome as no surprise because no consensus oneither question has been reached by the rest ofthe biological community. At the risk of over-simplifying, we will define two basic attitudes.The first is what might be called the heuristicattitude. Rosen (1979: 277) defined species as "apopulation or group of populations defined byone or more apomorphous features, it is also thesmallest natural aggregation of individuals witha specifiable geographic integrity that can be de-fined by any current set ofanalytical techniques."Nelson and Platnick (1981: 11) stated "speciesare simply the smallest detected samples of self-perpetuating organisms that have a unique set ofcharacters." Rosen's statement is somewhat morerestrictive, both in its inclusion of a geographiccriterion and in its criterion that the species musthave at least one apomorphous character. Both, 'This study was funded by a grant from the Nationalof course, are not "process free" concepts sinceone (Rosen's) contains a statement based oncharacter evolution while the other (Nelson andPlatnick's) contains a statement based on repro-duction. Nevertheless, both concepts are asheuristic as the authors can make them. The second attitude might be termed the "pro-cess" attitude. The concept applied is not re-stricted to purely heuristic considerations but also the evolutionary nature of the entities. Of all phylogeneticists, Hennig (1966) himself was the most radical proponent of this attitude, a fact that is not always appreciated. Hennig (1966) spent a substantial part ofhis book arguing against heuristic concepts because he rejected ideal mor-phology and the "morphological system." To Hennig (1966: 79-80), it was phylogeny that was important, morphology being a way ofgetting at the phylogeny: The categories [= taxa] of phylogenetic sys-tematics are not constructed by abstraction. They are not defined as bearers of a complex ofcharacters that remains if, starting with the individuals, we subtract more and more char-acters that are specific to the individuals and then to progressively more inclusive groups of individuals. In the phylogenetic system the categories [= taxa] at ail levels are determined by genetic [= genealogical] relations that exist among their subcategories [= taxa included within them]. Knowledge of these relations is a prerequisite for constructing these categories [= taxa], but the relations exist whether they are recognized or not. Consequently here the morphological characters have a completely different significance than in the logical and morphological systems. They are not them-selves ingredients ofthe definition ofthe higher categories [= taxa] but aids used to apprehend the genetic [= genealogical] criteria that lie be-hind them. [Brackets and emphases ours.]I Science Foundation (DEB-8103532) and two grantsfrom the University of Kansas General Research Fund (Nos. 3338 and 3591). 2 Museum of Natural History and Department of Systematics and Ecology, The University of Kansas, Law-rence, Kansas 66045.ANN. MISSOURI BOT. GARD. 72: 596-635. 1985.