Answer
Speciation can occur rapidly once the conditions exist to isolate one population from another either due to differential natural selection or to the high effectiveness of mechanisms such as habitat isolation. However, such driving forces may only arise rarely, allowing species to be stable for long periods in between.
Work Step by Step
This question is asking you to see the difference between the conditions which lead to speciation events (maybe rare) and the mechanisms which actually produce the separation (often quite quick).
