Three-butterfly system provides a field test of mullerian mimicry

In 1879, Muller proposed that two brightly coloured distasteful butterfly species (co-models) that share a single warning-colour pattern would benefit by spreading the selective burden of educating predators(1-5). The mutual benefit of sharing warning signals among distasteful species, so-called mullerian mimicry, is supported by comparative evidence(2,3), theoretical studies(5,6) and laboratory simulations(7); however, to date, this key exemplar of adaptive evolution has not been experimentally tested in the field. To measure natural selection generated by mullerian mimicry, I exploited the unusual polymorphism of Heliconius cydno (Lepidoptera: Nymphalidae)(8). Here I show increased survival of H. cydno morphs that match locally abundant monomorphic co-model species. This study demonstrates mullerian mimicry in the field. It also shows that mullerian mimicry with several co-models generates geographically divergent selection, which explains the existence of polymorphism in distasteful species with warning coloration(9).