The theory of punctuated equilibrium was put forward towards the end of the last century by Stephen Jay Gould and Niles Eldredge. This holds that most sexually reproducing species persist with little change for most of their geological history ("stasis"). When change does occur it is relatively rapid, with splitting of a species into two different species rather than the gradual transformation of one species into a new one. The theory has been the source of much contention among evolutionary scientists and has been seized on by opponents of evolution as casting doubt on the fundamentals of evolution. I have posted reviews of two books on this by Eldredge: Reinventing Darwin and The Pattern of Evolution. Both are worth reading but the first is the one to choose if you just want to find out what the argument has been about.