Paraphyletic and Polyphyletic Groups

Platnick, N. I. (Department of Entomology, The American Museum of Natural History, New York, New York 10024) 1977. Paraphyletic and polyphyletic groups. Syst. Zool. 26: 195-200.-Previously proposed definitions of the terms monophyletic, paraphyletic, and polyphyletic are examined. The definitions provided by Hennig and Ashlock are internally flawed because they do not prevent a single group from being simultaneously paraphyletic and polyphyletic. The definitions provided by Nelson are internally consistent but have undesirable consequences by either precluding the existence of monotypic genera or allowing coordinate taxa to both correspond and not correspond to monophyletic groups. The definitions provided by Farris are internally consistent and lack the drawbacks of Nelson's views; it is suggested that they are successful because they identify necessary relationships between the concepts of polyphyly and parallelism and the concepts of paraphyly and character reversal. [Phylogenetic systematics; monophyly; paraphyly; polyphyly.] One major disagreement between evolutionary and phylogenetic systematists seems to center at present around the question of whether paraphyletic groups should be permitted in classifications. Although I personally reject the use of such groups, the purpose of this paper is not to argue for one side or the other on this issue, but to point out that such arguments are unlikely to be productive unless proponents of both sides use the terms monophyly, paraphyly, and polyphyly in the same way. At least four different sets of definitions of these terms, those of Hennig (1965, 1966, 1975), Ashlock (1971, 1972), Nelson (1971, 1973), and Farris (1974), have been proposed. I will argue that the definitions advocated by Hennig and Ashlock suffer from fatal intrinsic flaws, that those provided by Nelson are internally sound but have undesirable consequences, and that only those proposed by Farris are fully acceptable. Hennig (1966) recognized these three types of groups and provided (p. 73) the following definition: "A monophyletic group is a group of species descended from a single ('stem') species, and which includes all species descended from this stem species." Subsequently (1975:247), Hennig pointed out that "A categorical distinction exists between monophyletic groups (Ashlock's holophyletic), whose members possess a stem species common only to themselves, and non-monophyletic groups (paraphyletic and polyphyletic), whose members always possess a common stem species but not one common only to themselves." It is important to note that Hennig's distinction between monophyletic and nonmonophyletic groups is based on the genealogical relationships existing within groups and not on our ability to recognize those relationships; thus, he has not defined a monophyletic group as one whose members possess a uniquely derived character state. Hennig (1965:103) provided diagrams (Fig. 1) that unambiguously distinguish his concepts of paraphyletic groups (those based on symplesiomorphy) and polyphyletic groups (those based on convergence). The diagrams (Fig. 2) provided by Hennig (1966:148) are not, however, equally unambiguous; as was indicated by Nelson (1971, 1973), if the cladogram of a polyphyletic group in Fig. 2 is redrawn with its phenetic component eliminated (as in Fig. 3), no genealogical differences exist between the paraphyletic and polyphyletic groups. Hennig (1975:248) nonetheless endorsed his ambiguous concept, arguing that the distinction of paraphyly and polyphyly "is possible only at the methodological level." With reference to a new diagram (Fig. 4), Hennig (1975) asserted that "Both types of groups, those based on convergence (what I consider polyphyletic