ON PUNCTUATED EQUILIBRIA (Invited Presentation)

‘Punctuated equilibria’ (“PE”—Eldredge, 1971; named and further developed in Eldredge & Gould, 1972) is simply ‘allopatric speciation’ applied to evolutionary patterns of stasis + change in the fossil record.

Dobzhansky (1937) and Mayr (1942) developed allopatric speciation—where new species are seen to arise in geographic isolation from ancestral species. Their overall picture of evolution is the “rabbit ear antenna” of old TV sets: gradual divergence after a splitting event. Simpson (1944) emphasized the importance of the fossil record demonstrating evolutionary patterns in geological time; he downplayed the importance of speciation per se.

Darwin (Ntbk B, 1837-8) posited two models where adaptive change could accrue: (1) passage of time and (2) geographic isolation. Re (1) he was aware of stasis, but followed Lamarck on the notion of gradual transformism. Re (2) Darwin had field data. In Ntbk E (1838-9) Darwin wrote “If separation in horizontal direction (i.e. allopatric speciation) is far more efficient in making species, than time (as cause of change) which can hardly be believed, then uniformity in geological formation (i.e. stasis) intelligible.” He had PE in his grasp—but rejected it, arguably because he could not see how isolation occurs over vast homogeneous stretches, such as the pampas/Patagonian scrub where he did much of his work. Climate change as a scientific subject had not yet been developed in the 1830s. Darwin’s early thoughts were not known to evolutionary biologists until the last decade of the 20th century.

The recognition of stasis as a common pattern within the history of species is the empirical linchpin of PE—which is otherwise developed as a fusion of the insights of Simpson, Mayr and Dobzhansky. That stasis has also become commonly recognized in somatic and cultural evolutionary domains was in part enabled by its status as a foundational component of PE.