A method for recognizing morphological stasis
Deborah K. Anderson · Cambridge University Press eBooks · 1993
“Stasis is a real phenomenon” (Eldredge, 1989, p. 72). Although this opinion is rarely stated so explicitly, most discussions of evolutionary rate or change in morphology over time include the assumption that stasis does occur (Futuyma, 1986; Lande, 1986; Bookstein, 1987; Stanley and Yang, 1987; Levinton, 1988; Mayr, 1988; Eldredge, 1989). Stanley and Yang (1987) present a specific example of stasis in fossil and Recent bivalve shape. Barnosky (1987) presents several examples of stasis in small fossil mammals based on an analysis of molar size. Futuyma (1986) recognizes examples of stasis in the extant genus Plethodon. Generally, the questions that need to be addressed are not Does stasis occur? but Why does it occur? and How can we recognize it? This chapter concentrates on the latter question. “Stasis” has almost as many different definitions as “species” (Eldredge, 1971, 1986, 1989; Stanley, 1982; Lande, 1986; Rose and Bown, 1986; Levinton, 1988; Mayr, 1988). Because “stasis” is used to describe an absence of change in a population of a particular species, the two concepts are frequently used together. Currently, then, the interpretation of stasis may be as unique as the species definition applied to the organism(s) or referred to in discussion. Without direct evidence of reproductive isolation between populations, paleobiologists evoke the morphospecies concept for identifying species. If a morphospecies is a group of individuals that are not significantly different morphologically, then the general criterion for recognizing stasis is identical with the criterion for identifying a particular species and determining species longevity. In this case, character stasis is actually what is recognizable; species stasis is inferred.