If variation in mate preferences is present, selection should favor reproductive isolation. Hybrid zones are common. Evidence for reinforcement is less so. Both allopatric and paratric speciation are well supported by theory. Sympatric speciation has long been more controversial.
Sympatric speciation is theoretically possible.
Polymorphism can be supported by assortative mating. If polymorphism resulting from assortative mating is advantageous, selection can reinforce the assorting until reproductive isolation is complete. As in other modes of speciation, reinforcement depends on availability of variation affecting mate preference. Compelling cases where sister species that can only be explained sympatrically are difficult to provide. The text provides a couple of examples.