Punctuated equilibrium

An evolutionary model in which species show long periods of relative stability interrupted by comparatively brief episodes of rapid change, often associated with speciation.

Allopatric speciation: The formation of new species when populations become geographically isolated and evolve separately. Geographic isolation can concentrate evolutionary change in small populations that later appear in the fossil record.
Niles Eldredge: An American paleontologist who co-developed punctuated equilibrium with Stephen Jay Gould. His study of fossil trilobites helped prompt the model’s formulation.
Gradualism: The view that evolutionary change accumulates through small changes over long periods. Punctuated equilibrium disputes gradualism as a universal pattern, not the possibility of gradual change.
Phacops: An extinct genus of trilobites known from Devonian rocks and studied in debates about morphological stasis. Eldredge’s analysis of Phacops populations contributed to the model’s early fossil evidence.
Peripatric speciation: The formation of a new species from a small population isolated at the edge of a larger population’s range. This speciation model offers one route to rapid divergence in a geographically restricted population.
Stephen Jay Gould: An American paleontologist and evolutionary biologist who co-developed punctuated equilibrium with Niles Eldredge. He helped articulate the model and defend its implications for evolutionary theory.
Uniformitarianism: The principle that geological processes observable today also operated in the past, though their rates may vary. The model’s rapid episodes are sometimes mistaken for rejecting uniformity in geological processes.
Bryozoa: A phylum of aquatic invertebrates that typically form colonies of small, filter-feeding zooids. Fossil bryozoan lineages became prominent in Gould’s discussions of stasis and evolutionary change.
Founder effect: A change in allele frequencies when a new population is established by a small sample of a source population. Small founding populations can diverge quickly, a proposed contributor to change during speciation.
Punctuated Equilibria: An Alternative to Phyletic Gradualism: A 1972 paper by Niles Eldredge and Stephen Jay Gould that introduced punctuated equilibria as an alternative evolutionary model. This paper set out the model’s central claims and connected them to speciation.

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