Color pattern evolution, assortative mating, and genetic differentiation in brightly colored butterflyfishes (Chaetodontidae)

In butterflyfishes (Chaetodontidae), color pattern evolves rapidly and is o ften the only morphological trait separating closely related species. Vivid coloration is frequently assumed to provide critical signals for mate reco gnition and mate choice, but few direct experimental tests are available. H ere we analyze the relationship between color pattern change, mate choice, and genetic differentiation in a group of three very closely related allopa tric butterflyfishes. We found that in only one member of this group, Chaet odon multicinctus, is color pattern evolution associated with mate preferen ce and genetic divergence. For its two sister species, C. punctatofasciatus and C. pelewensis, color pattern change has not resulted in assortative ma ting (based on laboratory pairing experiments and held observations) or in significant mtDNA or allozyme differentiation. In a contact zone on reefs i n the Solomon Islands and Papua New Guinea, hybridization between the two f orms has nearly homogenized color pattern differences. Outside these areas, however, color pattern remains distinct. Genetic variation is homogeneous over a much larger geographic scale. Sequence variation in the tRNA-proline end of the mitochondrial control region and allozyme variation was distrib uted widely within C. punctatofasciatus and C. pelewensis, which suggests f ew constraints to mitochondrial or nuclear gene flow across the color patte rn boundary. These contrasting patterns strongly suggest that selection is maintaining color pattern differences in allopatry in the face of potential ly homogenizing levels of gene flow. The mating pattern data show that this selection is not operating on mate recognition in the strictest sense, but probably on some other aspect of the social system of these territorial fi sh. In this case, divergence in mating preference can follow color pattern evolution, but is not contemporaneous with it.