ctuated Equilibrium:The Missing Link in Evolution Education BrianJ. Alters WilliamF. McComas Accordingto the theoryof naturalselection an interminable ogy, geology and history of science at Harvard, numberof intermediateforms must have existed, linking formulated the theory of punctuated equilibrium. In togetherall speciesin eachgroup by gradations as fineas our existingvarieties . (Darwin 1975,p. 182). their now classic article, "Punctuated Equilibria:An Alternative to Phyletic Gradualism," Eldredge and WhenSteven Stanley, professor of paleontologywas asked: Gould (1972) define punctuated equilibrium as evo- "If you couldgo backin time and sit down with Charles lutionary lineages that are characterized by brief Darwin, . what do you think would be important to tell Downloaded from http://online.ucpress.edu/abt/article-pdf/56/6/334/46818/4449848.pdf by guest on 27 September 2021 him about what's happenedto his theory?"Stanley an- episodes of rapid speciation separated by long inter- swered: ". I'd like to tell him about the punctuational vals of stasis of the species thus produced (Avers scheme.. ." (Campbell1986, p. 727). 1989). In 1977, the two refined their earlierpaper and responded to criticismsof the new concept (Gould & T nHE scheme referredto by Stanley has been called punctuated equilibriumby its developers Eldredge1977). Myriadjournal papers, news articles, Niles Eldredge and Stephen Jay Gould. The and even books were written about punctuated equi- idea has astonished the scientificcommunity in much librium, primarilyin response to these two papers. the same way that Darwin did a century earlierwhen In 1980, a conference on macroevolutiontook place he proposed natural selection as the mechanism to at Chicago's Field Museum of Natural History. Due explain how living organisms have evolved through to the considerationgiven to punctuated equilibrium, an unbroken chain extending back to the first primor- many scientists at the conference suggested that the dial cells. Central to Darwin's theory of evolution by meeting was a turning point in the history of evolu- natural selection is the view that evolution is both tionary theory and was the most important confer- smooth and gradual. What Eldredge and Gould ence on evolutionary biology in 30 years (Lewin (1972) proposed is the revolutionary view that or- 1980). The majority of the world's top paleontolo- ganic evolution is not steady and regular, but epi- gists, anatomists, evolutionarygeneticists, and devel- sodic and jerky with long periods of minute change opmental biologists present at the meeting supported interspersed with rapid bursts of large-scaletransfor- some form of punctuated equilibrium(Adler & Carey mation of species. Bakker (1991), himself a revolu- 1980) as the most accurate view of the fossil record tionary by championing the notion of "hot blooded" and hence of the history of life itself. dinosaurs, considers punctuated equilibriumthe big- Punctuated equilibrium does not overturn the es- gest bomb in evolutionary biology dropped this cen- sential notions of organic evolution, despite such tury (Bakker1991). Asimov (1989) listed punctuated claims by creationists, but rather provides a more equilibrium as one of the seven most significant accurateview of the history of life. Natural selection, events of world science for the year in which it was put forward by Darwin, is still the primary mecha- proposed, and both evolutionary biologists, Scully nism for the development of the new species. Only (1987) and Futuyma (1985) have stated that no other the rate and pattern of such speciation has been concept in modern evolutionary theory has drawn reformulated by punctuated equilibrium. What El- more attention in both scientific journals and the dredge and Gould have done is to bring the debate mass media. regardingthe tempo and mode of evolution to a new In 1971, Niles Eldredge, Chairmanand Curatorof generation of scientists. Even Darwin's staunchest Invertebrates at the American Museum of Natural contemporarysupporter, T. H. Huxley, criticizedhim History, and Stephen Jay Gould, who teaches biol- for insisting on a gradualistic pattern for evolution despite the lack of fossil evidence. The words spoken by Huxley (1900, p. 27), "You have loaded yourself BrianJ. Altersis a Ph.D.candidate in science education at the Universityof SouthernCalifornia. He can be reached at 3419Via with an unnecessary difficultyin adopting naturanon Lido,Newport Beach, CA 92663. WilliamF. McComas, Ph.D.,is facit saltum [nature makes no leaps], so unreserv- Directorof Science EducationPrograms and AssistantProfessor edly," could serve as the credo for today's punctua- at the Universityof SouthernCalifornia, School of EducationWPH 1001E,Los Angeles, CA 90089-0031. tionalists. Despite the importance of punctuated equilibrium 334 THEAMERICAN BIOLOGY TEACHER, VOLUME 56, NO. 6, SEPTEMBER1994 A comparison of the tenets of phyletic gradualism and punctuated equilibrium. Name:Phyletic Gradualism Name:Punctuated Equilibrium PrincipalProponent: Darwin PrincipalProponents: Eldredge and Gould * New species develop gradually and slowly with little * New species develop rapidly and then experience long evidence of stasis (no significant change). periods of stasis. * The fossil record should contain numerous * The fossil record should contain few transitionalforms transitionalforms within the lineage of any one type with the maintenanceof given forms for long periods of organism. of time. * New species arise via the transformationof an 0 New species arise as lineages are split. ancestralpopulation. * The entire ancestral form usually transformsinto the 0 A small subpopulation of the ancestralform gives rise new species. to the new species. * Speciation usually involves the entire geographic 0 The subpopulation is in an isolated area at the range of the species (called sympatry). periphery of the range (called allopatry). Adapted from Eldredge 1989;Futuyma 1986;Rhodes 1983;and Gould & Eldredge 1977. Downloaded from http://online.ucpress.edu/abt/article-pdf/56/6/334/46818/4449848.pdf by guest on 27 September 2021 to evolution, biology and science in general, it has not answer may lie in the confusion engendered in the yet been given appropriatespace in textbooks, if it is debate between those who support phyletic gradual- mentioned at all. This is unfortunate, since students ism and those who support punctuated equilibrium. of all ages should be provided with a complete understanding of modern evolutionary thought and should be given an opportunity to discuss its impor- What Is Punctuated Equilibrium? tance. In addition, punctuated equilibriumhas great value in its use as a tool for generating higher-order To understand punctuated equilibrium, one must thinking skills. examine it point by point with the Darwinianview of A survey of punctuated equilibrium'streatment in phyletic gradualism.Table 1 compares the two views a variety of middle and high school life science and of the evolution of life. biology texts (24 books, 9 publishers, published from The debate between proponents of phyletic gradu- 1990-1993) revealed that only 67% contained any alism and punctuated equilibriumis one of the most mention of punctuated equilibrium (Alters 1993). Of spirited in modern evolutionary thought. First, phy- those texts mentioning punctuated equilibrium, the letic gradualists contend that most evolution occurs average number of paragraphs was approximately within established species (Stanley 1982). Speciation 3.5. These paragraphs contained conflicting classifi- occurs via the conversion of an ancestral population cations, misnomers concerning the name "punctuat- into its modified descendents (Rhodes 1983) (see ed equilibrium,"incorrect definitions, and many mis- Figure 1). In contrast, proponents of punctuated leading and/or incorrectstatements. Such spotty and equilibriumhold that most speciation does not occur incomplete treatment is unfortunate in resources via the conversion of entire ancestralpopulations but upon which teachers rely to present the latest con- instead by cladogenesis (branching of a lineage into cepts of biology as a new foundation (or cofounda- two populations) (Gould 1991b) (see Figure 2). tion) for interpreting life's history. Kurten (1981 in Levinton 1982) takes issue with this It is interesting, as Gould (1991a) himself pointed punctuational cladogenic idea, specifically concern- out in an articleunrelated to punctuated equilibrium, ing mammals, stating that, within the last 3 million that virtually every life biology text in print contains years, muskrats constitute an evolving lineage with a detailed discussions of the long-discredited ideas of smooth transition from one genus-level taxon to the Lamarck's inheritance of acquired characteristics. next. Conversely, Stanley feels that there is no way to While the historicalperspective provided by a discus- explain the evolution of the age of mammals without sion of Lamarck'sideas is somewhat useful, should punctuated equilibrium. "I don't see any way that such notions be included at the expense of other, one can explain how entire species [without punctu- more cutting edge, ideas in biology? After all, there ated equilibrium]could become wholly transformed are only so many pages available in a textbook. So so rapidly . ." (Campbell 1986, p. 725). perhaps it is time to dispense with the old traditions Second, speciation in a gradualistic model is in biology text writing in favor of materialthat reveals thought to occur over all or