In 1998 it seemed clear that a pair of ‘sister species’ of tropical butterflies, Heliconius melpomene and Heliconius cydno persisted in sympatry in spite of occasional although regular hybridization. They speciated and today can coexist as a result of ecological divergence. An important mechanism in their speciation was the switch in colour pattern between different Müllerian mimicry rings, together with microhabitat and host-plant shifts, and assortative mating produced as a side effect of the colour pattern differences. An international consortium of Heliconius geneticists has recently been investigating members of the cydno superspecies, which are in a sense the ‘sisters’ of one of the original ‘sister species’, cydno. Several of these locally endemic forms are now recognized as separate species in the eastern slopes of the Andes. These forms are probably most closely related to cydno, but in several cases bear virtually identical colour patterns to the local race of melpomene, very likely resulting from gene transfer from that species; they therefore can and sometimes do join the local mimicry ring with melpomene and its more distantly related co-mimic Heliconius erato. I detail how recent genetic studies, together with ecological and behavioural observations, suggest that the shared colour patterns are indeed due to hybridization and transfer of mimicry adaptations between Heliconius species. These findings may have general applicability: rapidly diversifying lineages of both plants and animals may frequently share and exchange adaptive genetic variation.