Punctuated equilibrium - (n.)
Proposition regarding evolution of life introduced by paleontologists Nils Eldredge and Stephen Jay Gould in 1972. It views evolution as a series of episodically branching speciations superimposed on long intervals of species stability (called a "stasis"). The idea is reminiscent of earlier ideas on the alternation of times of slow evolution with times of rapid species "radiation." All such concepts depend on whether the changes in morphology of fossils in presumed sequences of ancestors and descendants correctly reflects the rate of genetic change. Although this cannot be proven, it seems reasonable that the rate of change in a given lineage of organisms should not be constant, but should vary between slow and fast, presumably in response to changes in climate.