The antecedents of punctuated equilibria

Eldredge, N. 2015. Eternal ephemera: Adaptation and the origin of species from the nineteenth century through punctuated equilibria and beyond. Columbia Univ. Press, New York, NY. 376 pp. ISBN-13: 9780231153164; $35.00 HB. In the early 1970s, Niles Eldredge, along with his collaborator Stephen J. Gould, proposed a radical and controversial model of evolution, “punctuated equilibria,” that galvanized scientific discourse in evolutionary biology for nearly two decades (Eldredge and Gould 1972). As its name suggests, the model had two key components: punctuation and equilibrium. Drawing on the theory of allopatric speciation, Eldredge and Gould argued that most evolutionary change occurs in a short interval around the time of speciation when small peripheral founder populations become rapidly differentiated from their parent species through a combination of founder effects, genetic drift, directional adaptation to their marginal environments, and reinforcement as the new species expand back into the geographic ranges of their parent. Rapid evolution in founder populations was not in itself controversial, but Eldredge and Gould also argued that these bursts of change were but brief punctuation marks between long intervening phases of equilibrium during which species experienced little or no evolutionary change. Again drawing on principles of peripheral isolate speciation, they argued that founder populations quickly reach the adaptive optima of their new environments, by which time they have acquired the characteristics that differentiate them from their parents. Although they considered new species to be capable of geographic expansion, Eldredge and Gould maintained that the inertia of increasingly widespread gene pools combined with stabilizing selection, some of it driven by Red Queen like interspecies competition, dampened phenotypic change after speciation. The punctuated equilibria model was controversial in paleontology because of its implications for how evolution should be studied in the fossil record. The model’s logical consequences, according to Eldredge and Gould, were that adaptive change occurs dominantly at the time of speciation; that the process of speciation lasts only a few generations, too few to be recorded completely in the fossil record; and that offspring species arise outside their parents’ geographic ranges. A paleontologist studying a stratigraphic succession of fossils at a single location should therefore expect new species to appear de novo as founder events from a parent species whose range was elsewhere and to persist essentially unchanged until they become extinct. If new speciation events occurred during that interval, Eldredge and Gould said that one should expect to see them in another stratigraphic column, not in the same one as the parent species. From this they argued that the fossil record is not full of gaps, as is often asserted, but that the apparent absence of continuous transitions between species is the natural result of the evolutionary process. More controversially, they asserted that examples of putatively gradual evolutionary change in the fossil record are series of different closely related species that have been mistaken to be temporally successive populations of the same species lineage. Embraced by some paleontologists, Eldredge and Gould’s idea was rejected by others. The vexed question of whether stratigraphically successive fossil samples can be interpreted as ancestor-descendant sequences received impassioned attention with little methodological consensus (see review in Wagner and Marcot 2010). Conversely, the equally important statistical question of whether a stratigraphic sequence of phenotypes represent directional evolution, random change, or stasis received less verbiage, but inspired groundbreaking methodological advances, including statistical tests based on random walk null models and sophisticated algorithms for fitting evolutionary models, such as Brownian motion or Ornstein-Uhlenbeck processes (see recent review by Hunt and Rabosky 2014). These new techniques have since been used to systematically survey fossil species lineages to determine whether equilibrium is as common as was