There has been a recent revival of interest in the role of ecology in speciation. The wing patterns of Heliconius butterflies are signals to predators as well as mates, and can cause strong reproductive isolation between populations. Reproductive isolation has been studied in some detail between the sympatric species Heliconius melpomene and Heliconius cydno, and in reviewing this work I show that habitat isolation and color pattern preference are by far the most important factors causing speciation. The surprising observation that genes for mate preference and color pattern are genetically associated implies divergence in sympatry or resulting from sexual selection. Color pattern is therefore an example of an ecological trait that contributes to speciation through pleiotropic effects on mate choice, although phylogenetic evidence shows that it is only one of many factors responsible for speciation in mimetic butterflies.