SPECIATION AND FEATHER ORNAMENTATION IN BIRDS

The hypothesis that sexual selection promotes speciation has rarely be en tested. We identified 70 evolutionarily independent events of feath er ornaments in birds. For each focal species we noted the number of o rnamented and nonornamented species belonging to its genus and its num ber of subspecies, as well as its mating system and the extent of its geographic range. For purposes of comparison, we randomly chose a seco nd, nonornamented species for which we obtained information on the num ber of subspecies, and in cases in which the nonornamented species was in the same genus, we chose a third, nonornamented species in a relat ed genus and obtained the same information. We then noted the number o f species in each genus and the difference in numbers of species, or s pecies richness, between paired genera. For the genera of the focal or namented species, we regressed number of ornamented species on number of nonornamented species and found a positive relationship. As number of species per genus rose, number of ornamented species per genus rose more rapidly, indicating that more speciose genera have a higher prop ortion of ornamented species than less speciose genera. We then took t he deviations from this regression, the residual number of species, an d regressed them on the differences in species richness between the pa ired genera. This relationship was positive indicating that ornamented genera with more than the expected number of ornamented species were more speciose with respect to their paired genera than were genera wit h fewer than the expected number of ornamented species. Finally, we co mpared the deviations from this regression, the residual number of orn amented species, with species' mating system and found a greater resid ual number of ornamented species among species whose mating system is associated with greater skew in male mating success and thus more inte nse sexual selection. Ornamented species had more subspecies than nono rnamented species, even when controlling for geographic range, suggest ing an association between subspeciation and ornaments.