Punctuated Equilibrium

Punctuated equilibrium is the theory that most species, once they arise, change relatively little for most of their existence, a state called stasis, and that the evolutionary change that does occur is concentrated in comparatively brief, geologically rapid bursts tied to speciation events, rather than accumulating steadily and gradually across a lineage's whole history. Niles Eldredge and Stephen Jay Gould proposed it in a 1972 paper, building on the biologist Ernst Mayr's existing model in which new species most often arise in small, geographically isolated populations at the edge of a parent species' range, where change can spread through a small gene pool quickly, then persist largely unchanged once the new population is established and widespread. The theory does not reject natural selection or gradual change within a population; it argues about the pace and distribution of that change over geological time, a distinction its own proponents and its critics, including Richard Dawkins, have disputed the size of ever since.