A Critical Assessment of Punctuated Equilibria. I. Duration of Taxa

• Published in: Evolution 1982-11, Vol.36 (6), p.1144-1157
• Main Author: Schopf, Thomas J. M.
• Format: Article
• Language: English
• Subjects: Biological taxonomies; Evolution; Fossils; Genera; Geology; Gradualism; Paleontology; Sibling species; Species; Taxa
• ISSN: 0014-3820; 1558-5646
• DOI: 10.1111/j.1558-5646.1982.tb05485.x

At least four assumptions must be made in order to perform rigorous paleontological tests of punctuated equilibria. These are the presence of population samples, geographic coverage, statistical testing of trends, and the assumption that one is dealing with biological species. In the vast majority of cases these criteria are extremely difficult to meet, and hence the issue has been very difficult to test directly. Two major predictions of punctuated equilibria are that mean species durations are long (often said to be on the order of 10 m.y.) and that most genomic and morphologic change occurs at speciation "events." This paper explores the first of these predictions concerning species durations, and calls attention to four prominent biases that act to lengthen durations of taxa as usually reported from the fossil record. The combined effect of these four biases appears to lengthen measured paleontological species durations by a factor of 10 to 100. Thus (if we use a factor of 50), mean species durations are not 10 m.y., but 200,000 years, and many morphologic species in the fossil record are simply not distinguishable into biological species given standard paleontological procedures. Finally, the two major attempts to test statistically for the presence of punctuated equilibria (Bookstein et al., 1978; Raup and Crick, 1981) either argue that it is an uncommon event, or that no conclusion can be drawn. A view of hierarchy in the biological world is quite reasonable, but the hierarchical view is not dependent on punctuated equilibria, whose existence is doubted.