Evolution proceeds at a steady, gradual pace. In other words, “phyletic gradualism” was thought to be the evolutionary norm for organismal phenotypes, both within and across organismal lineages.
stasis; phenotypic change
Evolution proceeds at a steady, gradual pace. In other words, “phyletic gradualism” was thought to be the evolutionary norm for organismal phenotypes, both within and across organismal lineages.
According to paleontologists Niles Eldredge and Stephen J. Gould, evolution is better characterized by long periods of stasis punctuated by bursts of phenotypic change typically associated with speciation. This much-discussed model was termed punctuated equilibrium or “rectangular” evolution (the latter coming from the fact that nodes in phylogenetic trees would look squared-off rather than V-shaped if most evolutionary change took place during speciation events). The model grew from the frequent observation in the fossil record of apparent phenotypic discontinuities from sequential geological strata. Eldredge and Gould interpreted these discontinuities as being literal footprints of rapid bursts of phenotypic change associated with speciation.
Notwithstanding its considerable conceptual impact in the field of paleontology, I give this purported paradigm shift a relatively low ranking for several reasons. First, in my opinion the authors overstated their case that the traditional evolutionary view entailed strict phyletic gradualism. For example, the paleontologist George Gaylord Simpson – one of the main architects of the modern evolutionary synthesis (see Chapter 21) – had championed the notion that evolution has multiple tempos and modes. Second, the punctuated equilibrium scenario itself morphed through time (beginning as a simple extension of well-established models of allopatric speciation but later shifting to an emphasis on long-term stasis within an evolutionary lineage). Thus it has proved to be a moving target for its critics. Third, the proposal gave the misleading impression that microevolution (population genetics) and macroevolution were somehow uncoupled, which cannot ultimately be true. Finally, empirical evidence in favor of rectangular evolution (and its insistence that nearly all evolutionary change is compacted into speciation events) remains contentious, at the levels of both genotypes and organismal phenotypes. Thus, overall, this scientific revolution appears to have been rather heavy on hyperbole but somewhat light on substance.
1. Simpson GG. Tempo and Mode in Evolution New York, NY: Columbia University Press; 1944.
2. Eldredge N, Gould SJ. Punctuated equilibria: an alternative to phyletic gradualism. In: Schopf TJM, ed. Models in Paleobiology. San Francisco, CA: Freeman; 1972;:82–115.
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5. Gould SJ. The Structure of Evolutionary Theory Cambridge, MA: Harvard University Press; 2002.