COLOR PATTERN EVOLUTION, ASSORTATIVE MATING, AND GENETIC DIFFERENTIATION IN BRIGHTLY COLORED BUTTERFLYFISHES...

In butterflyfishes (Chaetodontidae), color pattern evolves rapidly and is often the only morphological trait separating closely related species. Vivid coloration is frequently assumed to provide critical signals for mate recognition and mate choice, but few direct experimental tests are available. Here we analyze the relationship between color pattern change, mate choice, and genetic differentiation in a group of three very closely related allopatric butterflyfishes. We found that in only one member of this group, Chaetodon multicinctus, is color pattern evolution associated with mate preference and genetic divergence. For its two sister species, C. punctatofasciatus and C. pelewensis, color pattern change has not resulted in assortative mating (based on laboratory pairing experiments and field observations) or in significant mtDNA or allozyme differentiation. In a contact zone on reefs in the Solomon Islands and Papua New Guinea, hybridization between the two forms 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 distributed widely within C. punctatofasciatus and C. pelewensis, which suggests few constraints to mitochondrial or nuclear gene flow across the color pattern boundary. These contrasting patterns strongly suggest that selection is maintaining color pattern differences in allopatry in the face of potentially 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 fish. In this case, divergence in mating preference can follow color pattern evolution, but is not contemporaneous with it.