Polymorphism at a mimicry supergene maintained by opposing frequency-dependent selection pressures

Significance Although biological diversity is being lost at an alarming rate, a mimetic butterfly from the Amazon reveals a selective mechanism that can lead to rich adaptive diversity. In these toxic and vividly colored butterflies, several forms are maintained within populations, accurately mimicking distinct local butterflies, and controlled by differentiated alleles of a supergene. Such diversity is not expected because selection by predators promotes the fixation of a single morph. We show that the mating behavior favors pairings of dissimilar forms and boosts allelic diversity at the mimicry supergene. Therefore, sexual selection maintains a phenotypic diversity that predators channel and refine into a diversity of warning phenotypes, which correspond to the signals used by other defended species of butterflies in the local community.