Stasis is the situation in which evolutionary lineages persist for long periods without change. In the fossil record, stasis is common but it has recently taken on a new importance with the punctuated equilibrium debate.
Living fossils such as lungfish are lineages which have experienced a long period of stasis.
The reasons for stasis are controversial in biology: Simon Conway Morris gives his own view.
An animation illustrates the well documented case of stasis occuring in the horseshoe crab lineage.
The coelecanth pictured opposite is a living fossil which has undergone stasis for the past three hundred million years.
Why does stasis exist?