Punctuated Equilibrium is a theory of evolution where changes such as speciation occur very quickly, with long periods of little change (equilibria) in between. This accords well with what some perceive as the relative lack of intermediate forms in the fossil record.
Simulations provide some insight into how this might work: the equilibrium periods show a gradual accumulation of neutral mutations, and the jump occurs when some combination of them reaches a certain threshhold percentage (1/e2). On the other hand, speciation could also be triggered by classical means like separation of populations.
A recent study on some trout that had been separated, in fact, showed that after only a few generations the two populations tended not to interbreed due to minor behavioral differences. Thus, even if they were remixed, the two groups would probably diverge genetically. This is much faster than anyone expected separation to occur.
External links: Talk Origins Archive