Song dialects, spatial boundaries, and the speciation process in coastal white-crowned sparrows (Zonotrichia leucophrys)

Learned bird songs are key mechanisms in speciation, rapidly diversifying into reproductive barriers. Females use song to identify conspecific males, and males use song to mediate intrasexual competition. Given their role in maintaining species boundaries, researchers have long sought to understand when and how intraspecific song divergence reduces gene flow between populations. However, the mixed evidence linking song variation to reduced gene flow has left this question unresolved. One complicating factor is the cultural transmission of song. Learned mating signals (e.g., songs) can either impede or facilitate gene flow, highlighting the need to study their intraspecific diversification, in addition to interspecific patterns. In my dissertation, I explored the role of intraspecific song variation in two subspecies of white-crowned sparrow (Zonotrichia leucophrys), a model for song learning and cultural evolution. I examined song as a reproductive barrier at multiple stages of speciation: between populations of the Nuttall’s subspecies (Z. l. nuttalli) (Chapter 1 and 2) and between the Nuttall’s and Puget Sound (Z. l. pugetensis) subspecies (Chapter 3). In Chapter 1, I found some song differences without associated genetic differences; however, acoustic divergence aligned with a genetic break. To assess this alignment statistically, I implemented boundary statistics in a new R package in Chapter 2, which supported the link between song and genetic divergence in Z. l. nuttalli. I posit tcategorical distinctions between dialects are insufficient to reduce gene flow, but high acoustic divergence can act as a reproductive barrier. In Chapter 3, I found that song divergence also reduced gene flow between subspecies, with differences in whistle length—an innate song trait—aligning with a genetic boundary. My dissertation suggests that quantitative acoustic divergence, rather than categorical dialects, and genetically heritable differences in song may determine when intraspecific song variation leads to reproductive barriers.