Punctuated Equilibria: The Tempo and Mode of Evolution ReconsideredStephen Jay Gould; Niles EldredgePaleobiology, Vol. 3, No. 2. (Spring, 1977), pp. 115-151.Stable URL:http://links.jstor.org/sici?sici=0094-8373%28197721%293%3A2%3C115%3APETTAM%3E2.0.CO%3B2-HPaleobiology is currently published by Paleontological Society.Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available athttp://www.jstor.org/about/terms.html. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtainedprior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content inthe JSTOR archive only for your personal, non-commercial use.Please contact the publisher regarding any further use of this work. 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Punctuated equilibria: the tempo and mode of evolution reconsidered Stephen Jay Gould and Niles Eldredge Abstract.-We believe that punctuational change dominates the history of life: evolution is concentrated in very rapid events of speciation (geologically instantaneous, even if tolerably continuous in ecological time). Most species, during their geological history, either do not change in any appreciable way, or else they fluctuate mildly in morphology, with no apparent direction. Phyletic gradualism is very rare and too slow, in any case, to produce the major events of evolution. Evolutionary trends are not the product of slow, directional transforma- tion within lineages; they represent the differential success of certain species within a clade- speciation may be random with respect to the direction of a trend (Wright's rule). As an a priori bias, phyletic gradualism has precluded any fair assessment of evolutionary tempos and modes. It could not be refuted by empirical catalogues constructed in its light because it excluded contrary information as the artificial result of an imperfect fossil record. With the model of punctuated equilibria, an unbiased distribution of evolutionary tempos can be established by treating stasis as data and by recording the pattern of change for all species in an assemblage. This distribution of tempos can lead to strong inferences about modes. If, as we predict, the punctuational tempo is prevalent, then speciation-not phyletic evolution-must be the dominant mode of evolution. We argue that virtually none of the examples brought forward to refute our model can stand as support for phyletic gradualism; many are so weak and ambiguous that they only reflect the persistent bias for gradualism still deeply embedded in paleontological thought. Of the few stronger cases, we concentrate on Gingerich's data for Hyopsodus and argue that it provides an excellent example of species selection under our model. We then review the data of several studies that have supported our model since we published it five years ago. The record of human evolution seems to provide a particularly good example: no gradualism has been detected within any hominid taxon, and many are long-rangirig; the trend to larger brains arises from differential success of essentially static taxa. The data of molecular genetics support our assumption that large genetic changes often accompany the process of speciation. Phyletic gradualism was an a priori assertion from the start-it was never "seen" in the rocks; it expressed the cultural and political biases of 19th century liberalism. Huxley advised Darwin to eschew it as an "unnecessary difficulty." We think that it has now become an empirical fallacy. A punctuational view of change may have wide validity at all levels of evolutionary processes. At the very least, it deserves consideration as an alternate way of interpreting the history of life. Stephen Jay Gould. Museum of Comnparative Zoology, Harvard University, Cambridge, Mass. 02138 Niles Eldredge. Department of Fossil Inuertebrates. American Museum of Natz~ral History, Central Park West at 79th Street, New York, N.Y. 10024 Accepted: January 5, 1977 You have loaded yourself with an un-I. Gradualism and Stasis necessary difficulty in adopting Natura In 1944, G. G. Simpson published a booknon facit saltum so unreservedly. that brought paleontology within the modern Huxley to Darwin, Nov. 23,1859, the synthesis of evolutionary theory. He used his duy before publication of the Origin. title to identify the principaI topics that pale- I see you are inclined to advocate the pos- ontology might pursue to enlighten evolution- sibility of considerable 'saltus' on the part ary theory-tempo and mode. But tempo and of Dame Nature in her variations. I altoays mode do not share an equivalent status as took the same view, much to Mr. Darwin's subjects for study in the fossil record. Tempos disgust. can be observed and measured: modes must Huxley to Bateson, Feb. 20, 1894 be inferred, usually from empirical distribu- Copyr. @ 1977 The Paleontological SocietyAll rights reserved US ISSN 0094-8373116 COULD & ELDREDGE tions of tempos. Such inferences, if they are to be made properly, require a random sam- ple of tempos-or at least a sample not hope- lessly biased by a priori assumptions about evolutionary rates. Paleontologists have never been able even to approach such a random sample. Our model of punctuated equilibria