What is the difference between prezygotic and postzygotic reproductive isolating mechanisms? A. Prezygotic isolation always prevents copulation. B. Prezygotic isolation occurs before speciation takes place; postzygotic isolation occurs within a pair of newly defined species. C. Prezygotic isolation prevents fusion of gametes; postzygotic isolation occurs after fertilization. D. Postzygotic isolation prevents the formation of hybrids. E. Postzygotic isolation usually evolves after previously isolated populations have come back into contact.