THE PUZZLES OF EVOLUTIONARY STASIS, FIFTY YEARS AFTER PUNCTUATED EQUILIBRIA

Eldredge and Gould (1972) famously argued that morphological stasis in the fossil record should be taken at face value, as evidence of stasis in evolutionary lineages. This move set up both empirical and explanatory challenges for evolutionary biology and paleontology. With fifty years of hindsight, we can see that one of the most remarkable things about punctuated equilibria was the theory’s fruitfulness, or generativity. Punctuated equilibria was even, in a sense, a victim of its own fruitfulness, as the research tradition that it inspired – with a major focus on evolutionary stasis – would soon move beyond Eldredge and Gould’s original bipolar framing of the clash between PE and phyletic gradualism. The fruitfulness of PE can be attributed to the following three features: (1) It motivated empirical work to document patterns of stasis in the fossil record; (2) It set up, but did not fully resolve, a new explanatory challenge for evolutionary theory. Why are patterns of stasis so common? Aside from some unsatisfying speculations about species as homeostatic systems, Eldredge and Gould’s 1972 paper did not offer much in the way of an explanation for stasis. (3) Indirectly, though it did not itself include an explanation for stasis, PE indirectly helped motivate a hierarchical expansion of evolutionary theory. That hierarchical expansion broadened scientists’ explanatory resources for tackling the puzzles of evolutionary stasis. Understanding these three features of PE can point the way to a more general account of what makes theories (or models) particularly fruitful.