Geographic contrasts between pre- and postzygotic barriers are consistent with reinforcement in Heliconius butterflies.

Identifying the traits causing reproductive isolation and the order in which they evolve isfundamental to understanding speciation. Here, we quantify prezygotic and intrinsicpostzygotic isolation between allopatric, parapatric and sympatric populations of thebutterflies Heliconius elevatus and Heliconius pardalinus. Sympatric populations from theAmazon (H. elevatus and H. p. butleri) exhibit strong prezygotic isolation and rarely mate incaptivity; however, hybrids are fertile. Allopatric populations from the Amazon(H. p. butleri) and Andes (H. p. sergestus) mate freely when brought together in captivity, butthe female F1 hybrids are sterile. Parapatric populations (H. elevatus and H. p. sergestus)exhibit both assortative mating and sterility of female F1s. Assortative mating in sympatricpopulations is consistent with reinforcement in the face of gene flow, where the driving force,selection against hybrids, is due to disruption of mimicry and other ecological traits ratherthan hybrid sterility. In contrast, the lack of assortative mating and hybrid sterility observedin allopatric populations suggests that geographic isolation enables the evolution of intrinsicpostzygotic reproductive isolation. Our results show how the types of reproductive barriersthat evolve between species may depend on geography.