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Most definitions of the term monophyly are ambiguous because they do not limit the common ancestor sufficiently. Simpson's (1961) definition does not define monophyly; rather, it attempts to define a nameable taxon. Hennig's (1966a) definition, which requires a group to contain all the descendants of the common ancestor, is precise and useful, but it is narrower than the classic concept of monophyly and is here renamed holophyly. One way to limit the common ancestor is to require that it be a member of the group of which it is the stem, but this would seem to require a macroevolutionary appearance of groups. A precise definition of monophyly may be formulated if the common ancestor need be only cladistically a member. If a group can be shown to have one or more unique evolutionary innovations, it may be inferred that the most recent common ancestor also had these innovations; and it is, then, cladistically a member of the group. The ancestor need not have reached the grade of, or phenetically be, a member of the group. Holophyly, with Hennig's contrasting term paraphyly, then become two aspects of monophyly, while polyphyly describes a group whose common ancestor is not cladistically a member of the group.
Systematic Zoology © 1971 Oxford University Press