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Palaeontologia Electronica PUNCTUATED EQUILIBRIUM Reviewed by Philip D. Gingerich Palaeontologia Electronica http://palaeo-electronica.org PUNCTUATED EQUILIBRIUM Reviewed by Philip D. Gingerich Stephen Jay Gould Harvard University Press 2007 ISBN 0-674-02444-3, $18.95 Punctuated Equilibrium is a theory about how Development of the the evolutionary process works, based on patterns theory of punctuated of first appearances and subsequent histories of equilibrium was initiated species in the fossil record. The theory holds that by Niles Eldredge in a species originate too rapidly to enable their origins study titled ‘The allopat- to be traced by paleontologists (punctuation), and ric model and phylogeny then persist unchanged through geological time in in Paleozoic inverte- stasis (equilibrium). All is due to a mysterious brates,’ which appeared shared homeostasis that is postulated to regulate in Evolution in 1971. the collective morphology of individuals. When Here Eldredge described species-level homeostasis is working, species per- the appearances and subsequent histories of three sist unchanged; when species-level homeostasis subspecies of Phacops rana trilobites that charac- breaks down, speciation results. It is difficult to terize successive geological stages of the middle imagine a construct more antithetical to Darwinian Devonian. As described, each subspecies had a natural selection. fixed number of dorsoventral columns of eye The late Stephen Jay Gould was obsessed lenses, 18, 17, and 16-15, respectively. These did with punctuated equilibrium from the time it was not change through Cazenovia, Tioghnioga, and invented, and the obsession comes through so Taghanic stages spanning 6-7 million years of Mid- clearly in Punctuated Equilibrium that a reader dle Devonian time. Eldredge interpreted this would think he invented it all. The book itself is a stepped change to be the pattern expected from an 396 page extract from Gould’s 1433 page Structure allopatric model of speciation where morphological of Evolutionary Theory (Harvard University Press, change is postulated to occur rapidly in peripheral 2002). The introduction here (pp. 1-13) reprints isolates. pages 37 to 48 in chapter 1 of Structure of Evolu- The same year, Gould and Eldredge pub- tionary Theory, and the remainder of the text (pp. lished a meeting abstract titled ‘Speciation and 14-361) reprints Chapter 9 of Structure of Evolu- punctuated equilibria: an alternative to phyletic tionary Theory. If you succeeded in reading the gradualism’ (Geological Society of America 2002 book you probably do not need to read this Abstracts with Programs, 1971: 584-585). Here again. If you never made it through all 1433 pages they proposed that evolution is not a process of of the original Structure of Evolutionary Theory, slow transformation, but a process, in their words, then you might want to buy this 396 page version of ‘homeostatic equilibria, rarely disturbed by rapid and try again. events of speciation.’ Homeostasis is a property of organisms with cells linked by nerves, blood, and Ginatedgerich, Philip D., 2007. [Review of Punctuated Equilibrium]. Palaeontologia Electronica Vol. 10, Issue 3, R6; 2pp. 129KB; http://palaeo-electronica.org/paleo/2007_3/books/equal.htm GINGERICH: Punctuated Equilibrium endocrine signaling that enables them to regulate in time. This is the world we live in, and rapid evo- their physiology and activity to maintain a stable lution leading to stasis is the essence of an impor- internal environment. Gould and Eldredge’s postu- tant article on coevolution written by Stenseth and lation was that the average morphology of species Maynard Smith and published in Evolution in 1984. is regulated similarly to the average physiology of Surprisingly the article is not discussed or even organisms, and by implication species are function- cited here. It explains punctuated equilibria with ally organisms at some higher scale. Homeostasis, natural selection—no homeostasis required. I have equilibrium, punctuation, and even allopatric speci- written about this too, based on quantification of ation are theories to explain what cannot be seen rates of evolution, again not cited here. and studied. The fossil record is only as good as the strati- This view was more fully articulated in a book graphic record, which often has gaps in local sec- chapter by Eldredge and Gould titled ‘Punctuated tions. An alternative to glorifying the gaps, by equilibria: an alternative to phyletic gradualism’ (in basing a theory of speciation on such absences of Models in Paleobiology edited by T. J. M. Schopf; evidence, is to search more widely for places Freeman Cooper, 1972). Introducing the chapter, where the stratigraphic record is complete for an Schopf wrote: interval of interest. My sense as a paleontologist is The significance of ‘gaps’ in the fossil that we could learn much about species origins record has been a recurrent ‘difficulty,’ from the fossil record if we studied better sections used on the one hand to show that spon- more intensively and if we measured more. taneous generation is a ‘fact,’ and on the Punctuated equilibrum developed hand-in- other hand to illustrate the incomplete- hand with cladistics as an approach to systematics. ness of the fossil record ... The interpre- In cladistics, time and variation are ignored, and tation supported by Eldredge and Gould morphology is viewed as something that can be is that allopatric speciation in small, discretely coded. Punctuated equilibrium is a the- peripheral populations automatically ory used to justify coding, but time and variation results in ‘gaps’ in the fossil record. won’t go away. It is a point in Darwin’s favor that Curious company, spontaneous generation, and the Origin of Species and natural selection both punctuated equilibria. featured time and variation. What are paleontology An innovation in the 1972 Eldredge and Gould and evolution without them? chapter is the claim that speciation takes place ran- I once won a bet with Steve Gould concerning domly through time, and randomly in terms of the number of figures in the Origin of Species direction of change. Hence trends, if there are any, (there is one; Gould said two), and in a day when must result from differential survival of species more words are published than a person can read, much as a Darwinian would view selection in a I am increasingly drawn to figures as a way to population of individual organisms. The idea that absorb information efficiently. My recommendation speciation takes place randomly through time has to readers of Punctuated Equilibrium is to start with been tested by Vrba, Brett, Ivany, and others, who the figures: what do they show, how are they found pulses of turnover and coordinated stasis scaled, and are they scaled appropriately for what when faunas were studied through time. Rather they claim to show? How do you recognize ‘punc- than being random, speciations, like gaps in the tuation’? How do you recognize stasis? Can these fossil record and boundaries of geological forma- be quantified in terms of rate for comparison to tions, are often coincident, and further correlate change on the generation-to-generation scale of with times of environmental change. the evolutionary process? Has this been done? If Punctuated equilibria [sic] as patterns of sta- not, why not? sis and episodic turnover are important generaliza- The publisher, promoting Punctuated Equilib- tions about the fossil record that have been rium, suggests “we may now be living within a represented in the lexicon of paleontology over the punctuation, and our awareness of what this years in many different ways. In contrast, Gould’s means may be the enduring legacy of one of Amer- punctuated equilibrium [sic] as a process explain- ica's best-loved scientists.” When you finish the ing punctuated equilibria has serious competition. book, ask yourself whether we are living in a punc- A pattern of rapid evolution leading to stasis is the tuation? One of America’s best-loved scientists— logical expectation of Darwinian gradualism when yes. Living in a punctuation—who knows what this evolution is fast, geological time is long, and major might mean? events of environmental change are widely spaced 2.
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