punctuated equilibrium

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The theory that the evolution of life on earth typically follows a pattern in which long periods of little morphological change are punctuated by relatively short periods of significant change, when speciation occurs.
• noun Evolutionary development of this kind.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A theory of evolution holding that evolutionary change tends to be characterized by long periods of stability, or equilibrium, punctuated by episodes of very fast development.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a theory of evolution holding that evolutionary change in the fossil record came in fits and starts rather than in a steady process of slow change