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Abstract In song-learning birds, vocalizations are species recognition signals and may act as premating reproductive barriers; for allopatric taxa, testing how the signals can influence the speciation processes is quite challenging. This study aims to understand genomic divergence and species recognition via songs in 2 allopatric taxa, eastern and western Nashville warblers (Leiothlypis ruficapilla ruficapilla vs. Leiothlypis ruficapilla ridgwayi). We performed playback experiments to assess their reciprocal behavioral responses, which suggests an asymmetric barrier: the eastern L. r. ruficapilla discriminates between the 2 songs, but the western L. r. ridgwayi does not. Using whole-genome sequencing, we also examined the extent of the taxa’s genomic divergence and estimated their demographic history. We identified dozens of highly differentiated genomic regions, as well as fluctuations in historical effective population sizes that indicate independent demographic trajectories during the Pleistocene. To contextualize the magnitude of divergence between L. ruficapilla subspecies, we applied the same genomic analyses to 2 additional eastern-western pairs of parulid warblers, Setophaga virens vs. Setophaga townsendi and Setophaga coronata coronata vs. Setophaga coronata auduboni, which have existing behavior studies but are not in strict allopatry. Our findings provide insights into the role of vocalizations in defining within-pair relationship and the important legacy of isolation during the Pleistocene. 

Divergent natural selection should lead to adaptive radiation—that is, the rapid evolution of phenotypic and ecological diversity originating from a single clade. The drivers of adaptive radiation have often been conceptualized through the concept of “adaptive landscapes,” yet formal empirical estimates of adaptive landscapes for natural adaptive radiations have proven elusive. Here, we use a 17-year dataset of Darwin’s ground finches (Geospiza spp.) at an intensively studied site on Santa Cruz (Galápagos) to estimate individual apparent lifespan in relation to beak traits. We use these estimates to model a multi-species fitness landscape, which we also convert to a formal adaptive landscape. We then assess the correspondence between estimated fitness peaks and observed phenotypes for each of five phenotypic modes (G. fuliginosa, G. fortis [small and large morphotypes], G. magnirostris, and G. scandens). The fitness and adaptive landscapes show 5 and 4 peaks, respectively, and, as expected, the adaptive landscape was smoother than the fitness landscape. Each of the five phenotypic modes appeared reasonably close to the corresponding fitness peak, yet interesting deviations were also documented and examined. By estimating adaptive landscapes in an ongoing adaptive radiation, our study demonstrates their utility as a quantitative tool for exploring and predicting adaptive radiation.