“REPRODUCTIVE ISOLATION in ecological speciation is due to prezygotic mechanisms (mate choice) and extrinsic (ecology-dependent) postzygotic mechanisms in the absence of intrinsic postzygotic hybrid dysfunction. Better studied examples include insect host races, and adaptive radiations of birds and fish, but ecological speciation is widespread and common.”
http://www.sciencedirect.com/science/article/pii/S0960982206014138