Paraphyletic and Polyphyletic Groups

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.