Punctuated Equilibrium and Phyletic Gradualism

Abstract

Since Eldredge and Gould (1972) first wrote about “punctuated equilibrium” as a pattern for the history of life, three factions have appeared among evolutionary theorists. One faction favors the picture of punctuated equilibrium (Eldredge and Gould, 1972; Gould and Eldredge, 1977; Gould, 1982, 1985; Stanley, 1975, 1979, 1982). Another favors the traditional alternative of phyletic gradualism or at least gradual change (Gingerich, 1976, 1984, 1985; Charlesworth et al., 1982; Rose and Bown, 1984; Malmgren and Kennett, 1981, 1983; Ozawa, 1975). And the third faction suggests that the major differences between punctuated equilibrium and phyletic gradualism largely are semantic because evolution can and does produce both patterns, and moreover both were envisioned by the formulators of the modern evolutionary synthesis and even by Darwin (for example, Levinton and Simon, 1980; Stebbins and Ayala, 1981; Mayr, 1982; Hoffman, 1982; Johnson, 1982; Charlesworth and Lande, 1982; Ginzburg and Rost, 1982; Rhodes, 1983; Chaline, 1984; Newman et al., 1985). The differences in opinion persist largely because of a relative paucity of facts—facts about evolutionary relationships within lineages, about the rate of morphological change through time, about the distribution of the changes in space, and about the relationship between morphological change and speciation.

Many breaks in the fossil record are real; they express the way in which evolution occurs, not the fragments of an imperfect record.

Eldredge and Gould, 1972, p. 96

Absence of evidence is absence of evidence, not evidence of absence.... with respect to gaps in the evolutionary record.

Gingerich, 1984, p. 338