page_title: Stephen Jay Gould
section_title: Punctuated equilibrium
hierarchical_section_title: Stephen Jay Gould / Scientific career / Punctuated equilibrium
caption_attribution_description: English: The model of punctuated equilibrium contrasted against the phyletic gradualism model of speciation.
context_page_description: Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read authors of popular science of his generation. Gould spent most of his career teaching at Harvard University and working at the American Museum of Natural History in New York. In 1996, Gould was hired as the Vincent Astor Visiting Research Professor of Biology at New York University, where he divided his time teaching there and at Harvard.
  Gould's most significant contribution to evolutionary biology was the theory of punctuated equilibrium developed with Niles Eldredge in 1972. The theory proposes that most evolution is characterized by long periods of evolutionary stability, infrequently punctuated by swift periods of branching speciation. The theory was contrasted against phyletic gradualism, the popular idea that evolutionary change is marked by a pattern of smooth and continuous change in the fossil record.
  Most of Gould's empirical research was based on the land snail genera Poecilozonites and Cerion.
context_section_description: Early in his career, Gould and his colleague Niles Eldredge developed the theory of punctuated equilibrium, which describes the rate of speciation in the fossil record as occurring relatively rapidly, which then alternates to a longer period of evolutionary stability. It was Gould who coined the term "punctuated equilibria" though the theory was originally presented by Eldredge in his doctoral dissertation on Devonian trilobites and his article published the previous year on allopatric speciation.
  According to Gould, punctuated equilibrium revised a key pillar "in the central logic of Darwinian theory." Some evolutionary biologists have argued that while punctuated equilibrium was "of great interest to biology generally," it merely modified neo-Darwinism in a manner that was fully compatible with what had been known before.
  Other biologists emphasize the theoretical novelty of punctuated equilibrium, and argued that evolutionary stasis had been "unexpected by most evolutionary biologists" and "had a major impact on paleontology and evolutionary biology."
  Comparisons were made to George Gaylord Simpson's work in Tempo and Mode in Evolution (1941). However Simpson describes the paleontological record as being characterized by predominantly gradual change (which he termed horotely), though he also documented examples of slow (bradytely), and rapid (tachytely) rates of evolution. Punctuated equilibrium and phyletic gradualism are not mutually exclusive, and examples of each have been documented in different lineages. The debate between these two models is often misunderstood by non-scientists, and according to Richard Dawkins has been oversold by the media.
  Some critics jokingly referred to the theory of punctuated equilibrium as "evolution by jerks",
  which prompted Gould to describe phyletic gradualism as "evolution by creeps."
caption: The punctuated equilibrium model (above) consists of morphological stability followed by episodic bursts of evolutionary change via rapid cladogenesis. It is contrasted (below) to phyletic gradualism, a more gradual, continuous model of evolution.
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