WHAT INITIATES SPECIATION IN PASSION-VINE BUTTERFLIES

Studies of tile continuum between geographic races and species provide the clearest insights into the causes of speciation, Here we report o n mate choice and hybrid viability experiments in a pair of warningly colored butterflies, Heliconius erato and Heliconius himera, that main tain their genetic integrity in the face of hybridization. Hybrid ster ility and inviability have been unimportant in the early stages of spe ciation of these two Heliconius. We find no evidence of reduced fecund ity, egg hatch, or larval survival nor increases in developmental time in three generations of hybrid crosses, Instead, speciation in this p air appears to have been catalyzed by the association of strong mating preferences with divergence in warning coloration and ecology, In mat e choice experiments, matings between the two species are a tenth as l ikely as matings within species. FI hybrids of both sexes mate frequen tly with both pure forms, However, male F1 progeny from crosses betwee n H. himera mothers and N. erato fathers have somewhat reduced mating success, The strong barrier to gene flow provided by divergence in mat e preference is probably enhanced by frequency-dependent predation aga inst hybrids similar to the type known to occur across interracial hyb rid zones of H. erato, In addition, the transition between this pair f alls at the boundary between wet and dry forest, and I are hybrids may also be selected against because they are poorly adapted to either bi otope, These results add to a growing body of evidence that challenge the importance of genomic incompatibilities in the earliest stages of speciation.