Cladistics and Generic Concepts in the Compositae

Introduction Taxonomy classes are often taught that we as systematists are concerned with grouping and ranking. We are actually concerned with two levels of grouping: the grouping of individuals (into species), and the grouping of these "groups of individuals." There are, therefore, only two categories, species and groups of species (Nelson and Platnick, 1981). If we strive to make our species represent the units of evolution, then all higher ranking categories are really just monophyletic groups of species, no matter what we choose to call them. The importance of this concept lies in a logical extension of it: as long as they are natural, the various levels at which the "groups of species" are recognized are not of major concern to cladists. However, as is evidenced by my use of the qualifier "natural," we are concerned with the quality of the groups. It is not my intention to discuss cladistic methodology in this paper because many publications are available that accomplish this task (e.g., Eldredge and Cracraft, 1980; Hennig, 1966; Humphries and Funk, 1984; Nelson and Platnick, 1981; Wiley, 1981). However, it is necessary to explain a few terms. First, the term "apomorphy" has been variously defined as a unique character (Platnick, 1979) and as an evolutionary novelty (Wiley, 1981). What these definitions attempt to convey is the concept of a derived character that is shared by one or more taxa because of common descent. A "synapomorphy" is an apomorphy shared by more than one terminal taxon and an "autapomorphy" is characteristic of a single terminal taxon. For instance, in a cladogram of Tetragonotheca L. (Fig. 1; Seaman and Funk, 1983) there is a synapomorphy that unites the terminal taxa T. texana Gray & Engelm. and T. repanda Small, and a second synapomorphy that unites T. ludoviciana Gray and T. helianthoides L. Second, the terms "clade," "monophyletic group," and "natural group" all mean the same thing: a group that includes all taxa above a synapomorphy on the cladogram. These may also be defined as all descendents of a common ancestor. Such groups are regarded as natural because they represent a unit that has a common evolutionary history. In Fig. 2, the angiosperms and gymnosperms form a monophyletic group because they are defined by at least one synapomorphy. Likewise, the angiosperms, gymnosperms and ginkgos form a monophyletic group. The third monophyletic group on this cladogram contains all four taxa, the angiosperms, gymnosperms, ginkgos and cycads. Groups that do not fulfill the requirement for being clades are either paraphyletic or polyphyletic. Paraphyletic taxa are those that are defined only by plesiomorphies (characters that are not unique or novel at