Punctuated Equilibrium and Phyletic Gradualism

Abstract

Punctuated equilibrium and phyletic gradualism are contrasting patterns of evolution among a spectrum of patterns found in the fossil record. In punctuated equilibrium, species tend to show morphological stasis between abrupt speciation events, whereas in phyletic gradualism species undergo more continuous change.

Keywords: punctuated equilibrium; phyletic gradualism; stasis; speciation; microevolution

Figure 1.

The contrasting idealized evolutionary patterns of phyletic gradualism (a) and punctuated equilibrium (b). The top of each evolutionary tree represents the most recent time depicted, such as the present day. The horizontal distance between lineages (lines of descent) indicates in a general way the degree of morphological difference. In punctuated equilibrium (b), all morphological change is concentrated in branching events, with stasis thereafter. Note that, for comparative purposes, the two patterns have been drawn with a similar number of branching points. In addition, if the patterns are superimposed on each other, corresponding lineages have the same morphology at the latest occurrence (i.e. at their extinction or at the top of the diagram), but the morphological routes by which lineages reach those points are very different in each pattern. Fine‐scale zig‐zags in morphology will occur in both phyletic gradualism and punctuated equilibrium, but are not shown in these simplified depictions.

Figure 2.

The Plus ça change model, which combines aspects of punctuated equilibrium and phyletic gradualism. It proposes that, over geological time scales (e.g. a million years), gradualism (right) is characteristic of narrowly fluctuating, relatively stable environments such as a tropical rainforest and the deep sea. By contrast, punctuated equilibrium (left) is expected to prevail in the physically more unstable environments that dominate the fossil record, especially shallow seas. In a widely fluctuating environment (left), a threshold might occur not only when some environmental variable exceeds wide reflecting boundaries, but also when it contracts to become narrowly fluctuating (dotted line). ‘Environment’ indicates some long‐term physical aspect of environment, e.g. mean temperature, sea level or type of sediment forming the sea floor. ‘Morphology’ indicates some aspect, or aspects, of morphology. High‐frequency environmental oscillations such as annual cycles are not shown. From Sheldon , modified after Sheldon.

close

References

Carroll RL (1997) Patterns and Processes of Vertebrate Evolution. Cambridge, UK: Cambridge University Press.

Eldredge N (1995) Reinventing Darwin. New York: Wiley.

Eldredge N and Gould SJ (1972) Punctuated equilibria: an alternative to phyletic gradualism. In: Schopf TJM (ed.) Models in Paleobiology, pp. 82–115. San Francisco: Freeman and Cooper.

Erwin DH and Anstey RL (eds) (1995) New Approaches to Speciation in the Fossil Record. New York: Columbia University Press.

Futuyma DJ (1998) Evolutionary Biology, 3rd edn. Sunderland, MA: Sinauer.

Gould SJ and Eldredge N (1993) Punctuated equilibrium comes of age. Nature 366: 223–227.

Prothero DR and Heaton TH (1996) Faunal stability during the Early Oligocene climatic crash. Palaeogeography, Palaeoclimatology, Palaeoecology 127: 257–283.

Sheldon PR (1990) Shaking up evolutionary patterns. Nature 345: 772.

Sheldon PR (1996) Plus ça change – a model for stasis and evolution in different environments. Palaeogeography, Palaeoclimatology, Palaeoecology 127: 209–227.

Stanley SM (1998; originally published 1979; with new introduction) Macroevolution – Pattern and Process. Baltimore, MA: Johns Hopkins University Press.

Stanley SM and Yang X (1987) Approximate evolutionary stasis for bivalve morphology over millions of years: a multivariate, multilineage study. Paleobiology 13: 113–139.

Further Reading

Clarkson ENK (1998) Invertebrate Palaeontology and Evolution, 4th edn. Oxford: Blackwell Science.

Kemp TS (1999) Fossils and Evolution. Oxford: Oxford University Press.

Ridley M (1996) Evolution, 2nd edn. Oxford: Blackwell Science.

Skelton PW (ed.) (1993) Evolution – A Biological and Palaeontological Approach. Wokingham: Addison Wesley.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Sheldon, Peter R(Apr 2001) Punctuated Equilibrium and Phyletic Gradualism. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001774]