1. The Galton Laboratory, University College London, 4 Stephenson Way, London NW1 2HE, UK
2. Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Panama
3. Downing College, The University of Cambridge, Cambridge CB2 1DQ, UK

Correspondence to: Chris D. Jiggins^1,2 Correspondence and requests for materials should be addressed to C.D.J. (e-mail: Email: jigginsc@naos.si.edu).

Abstract

Speciation is facilitated if ecological adaptation directly causes assortative mating¹, 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 Heliconius melpomene and Heliconius cydno 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 H. melpomene allopatric to H. cydno court and mate with H. cydno more readily than those from a sympatric population. This suggests that assortative mating has been enhanced in sympatry.