Stasis

Stasis (, "a standing still", "stability" ) is a period of little or no evolutionary change in a species for most of their geological history. In other words, the fact that species in the fossil record remain largely unchanged for long periods in the fossil record rather than showing transitions. Instead of gradually transformed into another taxon, the only change presented is a variation and diversity within the boundaries of the original taxon. The term was dubbed by Niles Eldredge and Stephen Jay Gould in a 1972 paper elaborating the notion of "punctuated equilibria".

Punctuated equilibrium
Stasis is an important topic in the punctuated equilibrium model of evolutionary biology. Eldredge and Gould proposed that the degree of gradualism commonly attributed to Charles Darwin is virtually nonexistent in the fossil record, and that stasis dominates the history of most fossil species. The long period of stasis is the portion of the process described by punctualists referred to as the period of equilibrium.