tion through intermediate populations in this ring species are analogous to the gradual changes that might occur over time in geographically isolated populations diverging in a similar manner (Irwin et al., 2001a).
Songs may diverge as a direct result of habitat-dependent selection or indirectly as a consequence of morphological adaptations, such as those related to foraging. It has long been recognized that different types of vocalizations vary in their quality as signals in different habitats, and recent studies suggest a role for habitat-dependent selection in population divergence and reproductive isolation. Two subspecies of song sparrows (Melospiza melodia) differ both in song characteristics and preferred habitat, with Melospiza melodia hermanii occupying more dense vegetation than Melospiza melodia fallax and singing a lower frequency song with more widely spaced elements, a pattern consistent with acoustic adaptation (Patten et al., 2004). Playback experiments further indicate that both males and females show greater response to homotypic songs, suggesting a role for song in reproductive isolation and the consequent development of significant genetic differentiation between the subspecies (Patten et al., 2004). Similar habitat-dependent vocal divergence accompanies morphological differentiation in the little greenbul (Andropodus virens) and may promote population differentiation across ecological gradients (Slabbekoorn and Smith, 2002b).
Ecological selection on other characters also may result in correlated vocal evolution that contributes to prezygotic reproductive isolation. In Darwin’s finches, bill morphology and vocal characteristics are correlated because of physical constraints on sound production, perhaps contributing to the diversification of these species (Podos, 2001). In contrast, the black-bellied seedcracker (Pyrenestes ostrinus), which shows a similar pattern of divergent selection on bill morphology, shows no effect of bill size on vocal performance, contributing to the conclusion that bill size variation in this species reflects only intraspecific niche polymorphism and not incipient speciation (Slabbekoorn and Smith, 2000; Smith, 1993).
As a learned behavior in many birds, song is subject to rapid cultural evolution in which stochastic innovations or errors in copying are spread as individuals learn songs from their parents and/or neighbors (Grant and Grant, 1996; Payne, 1996). Song learning may also increase the rate at which genetic predispositions to learn or prefer certain songs evolve in allopatry (Lachlan and Servedio, 2004). Song learning, however, may sometimes inhibit reproductive isolation upon secondary contact if there has been only minimal divergence in the capacity to learn particular songs and/or the morphological structures affecting sound production (Slabbekoorn and Smith, 2002a). A well known example of reproductive character displacement involves the Ficedula flycatchers in which a sexually selected male plumage trait shows greater divergence in sympatry