Punctuated equilibrium is a hypothesis suggested as a variation of the Theory of Evolution. It suggests that, instead of a steady rate of change over long time periods as the original model suggested, change happens quickly in short time periods (geologic time scale) with long periods of static conditions.
This hypothesis attempts to explain the abrupt appearance of many new forms and the lack of fossil evidence for transitional forms. It was proposed in 1972 by Niles Eldredge and Stephen Jay Gould, who were both experienced in the fields of paleontology and evolutionary biology.
A key element in the proposal is the observation from evolutionary biology that larger populations are more stable against genetic changes. On the fringes of the geographical range of a species are often environments that are more or less shielded from the main area, for example by mountains or oceans. The populations in these niches are small and therefore able to evolve more rapidly. If they evolve far enough to prevent interbreeding with the original population and in such a way as to have a reproductive advantage, then they may proceed to replace the original species when they re-enter the original area. It is less likely that the small, isolated populations leave behind fossils during development, so the displacement may appear in the fossil record as the sudden appearance of a new and distinct species.