Reproductive isolation caused by colour pattern mimicry

Speciation is facilitated if ecological adaptation directly causes assortative mating, but few natural examples are known. Here we show that a shift in colour pattern mimicry was crucial in the origin of two butterfly species. The sister species <i>Heliconius melpomene</i> and <i>Heliconius cydno</i> recently diverged to mimic different model taxa, and our experiments show that their mimetic coloration is also important in choosing mates. Assortative mating between the sister species means that hybridization is rare in nature, and the few hybrids that are produced are non-mimetic, poorly adapted intermediates. Thus, the mimetic shift has caused both pre-mating and post-mating isolation. In addition, individuals from a population of <i>H. melpomene</i> allopatric to <i>H. cydno</i> court and mate with <i>H. cydno</i> more readily than those from a sympatric population. This suggests that assortative mating has been enhanced in sympatry.