A Critique of the Punctuated Equilibria Model and Implications for the Detection of Speciation in the Fossil Record

Abstract

Levinton, J. S., and C. M. Simon (Department of Ecology and Evolution, State University of New York, Stony Brook, New York 11794) 1980. A critique of the punctuated equilibria model and implications for the detection of speciation in the fossil record. Syst. Zool., 29: 130-142.-The evolutionary models of punctuated equilibria and species selection: 1. rely upon a model of species origin from peripheral isolates, and 2. interpret trends as the net result of selection among daughter species, whose morphologies are random with respect to the trend. The punctuational model asserts that gradual morphological change is less important than the sudden and rapid morphological change which occurs at speciation, and that morphological stasis is the rule through most of the duration of a species' history. Several considerations suggest limitations of these models. 1. Peripheral population model-Evolutionary biologists acknowledge a diversity of speciation models. The extinctionof-intermediates allopatric model and the parapatric model do not require peripheral isolates, and suggest that morphological differentiation may likely arise from typical within-species geographic variation. 2. Anagenesis versus speciation-The punctuated equilibria model, by establishing a dichotomy between evolution at speciation and evolution before and after speciation, obscures more about the tempo and mode of evolution than it clarifies. 3. StasisThe nature of paleontological species level taxonomy requires the identification of speciesspecific characters which are invariant with time. This confounds the identification of stasis with species identification. Perhaps a comparison of within- versus among-species character variation might suggest whether within-species variation is the stuff' of larger scale evolution. 4. Species selection-This requires that morphological characteristics of daughter species be random with respect to a long term trend. Ontogenetic and functional morphological interpretations of phylogeny suggest that trends within species are not necessarily random with respect to trends among species. We describe an example (scallops) where the range of possible daughter species is very restricted. We finally describe a test for the effect of average species duration on rate of anagenesis. Comparing species longevities does not adequately predict the extent of morphological divergence. [Punctuated equilibria; speciation; anagenesis; gradualism; species selection.]