Evolutionary stasis, constraint and other terminology describing evolutionary patterns

Current use of terms to describe evolutionary patterns is vague and inconsistent. In this paper, logical definitions of terms that describe specific evolutionary patterns are proposed. Evolutionary inertia is defined in a manner analogous to inertia in physics. A character in a static state of evolutionary inertia represents evolutionary stasis while a character showing consistent directional evolutionary change represents evolutionary thrust. I argue that evolutionary stasis should serve as the null hypothesis in all character evolution studies. Deviations from this null model consistent with alternative hypotheses (e.g. random drift, adaptation) can then give us insight into evolutionary processes. Failure to reject a null hypothesis of evolutionary stasis should not be used as a serious explanation of data. The term evolutionary constraint is appropriate only when a selective advantage for a character state transition is established but this transition is prevented by specific, identified factors. One type of evolutionary constraint discussed is evolutionary momentum. A final pattern of evolutionary change discussed is closely related to evolutionary thrust and is referred to as evolutionary acceleration. I provide examples of how this set of definitions can improve our ability to communicate interpretations of evolutionary patterns. (C) 2001 The Linnean Society of London.