The role of hybridization in the evolution of Heliconius butterflies: Species diversification, the evolution of reproductive isolation, and interspecific gene flow

In this thesis I explore the ways hybridization has influenced speciation and evolution in one clade of Heliconius butterflies (Lepidoptera: Nymphalidae). First, I use DNA sequence variation at 16 loci to measure the impact natural selection and gene flow have played in the evolutionary histories of H. melpomene, H. cydno, H. pachinus, and H. hecale . Various population genetic and phylogenetic analyses indicate a few examples of recent selection and gene flow among species, and the evolutionary relationships at one locus, Distal-less, are consistent with the hypothesis that H. pachinus arose due to introgressive hybridization between H. cydno and H. melpomene. Second, I use patterns of courtship and mating among H. cydno, H. pachinus, and H. melpomene to address the evolution and genetic basis of assortative mate preference. H. cydno and H. pachinus mate assortatively based on divergence in male preference for wing patterns, and geographical variation in mate preference in H. cydno and H. melpomene suggests that reinforcement, or the strengthening of prezygotic isolation in response to selection against hybrids, has played an important role in the divergence of mate preference. The specific cue used by H. cydno and H. pachinus males to recognize conspecific females is identified as the color (yellow or white) of wing patches as opposed to wing pattern itself or characteristics such as pheromones. The segregation of preference in hybrid males indicates that a majority of the divergence in mate preference between H. cydno and H. pachinus may be controlled by two alleles at a single locus which interact additively. Finally, I use allele frequency variation at almost 900 nuclear amplified fragment length polymorphism loci to measure the extent and direction of interspecific gene flow among H. melpomene, H. cydno, and H. pachinus . Gene flow is extensive, spatially wide-spread, and significantly asymmetrical, with greater introgression from H. pachinus into H. cydno, and greater introgression from H. cydno and H. pachinus into H. melpomene . These asymmetries match those which are expected based on the genetics of wing patterning and the probability that hybrids of various phenotypes will survive and reproduce in different mimetic environments.