Punctuated_Equilibrium

A version of evolution where changes such as speciation occur very quickly, with long periods of little change (equilibria) in between. This does a good job explaining 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 threshold 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.

Punctuated equilibrium is to be contrasted with gradual evolution, which is what Darwin proposed. Probably both play a role; their relative importance is still debated.