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The extent of range overlap of incipient and recent species depends on the type and magnitude of phenotypic divergence that separates them, and the consequences of phenotypic divergence on their interactions. Signal divergence by social selection likely initiates many speciation events, but may yield niche-conserved lineages predisposed to limit each others' ranges via ecological competition. Here, we examine this neglected aspect of social selection speciation theory in relation to the discovery of a nonecotonal species border between sunbirds. We find that Nectarinia moreaui and Nectarinia fuelleborni meet in a ∼6 km wide contact zone, as estimated by molecular cline analysis. These species exploit similar bioclimatic niches, but sing highly divergent learned songs, consistent with divergence by social selection. Cline analyses suggest that within-species stabilizing social selection on song-learning predispositions maintains species differences in song despite both hybridization and cultural transmission. We conclude that ecological competition between moreaui and fuelleborni contributes to the stabilization of the species border, but that ecological competition acts in conjunction with reproductive interference. The evolutionary maintenance of learned song differences in a hybrid zone recommend this study system for future studies on the mechanisms of learned song divergence and its role in speciation. [ABSTRACT FROM AUTHOR]

Hybrid zones provide a window into the evolutionary processes governing species divergence. Yet, the contribution of mate choice to the temporal and spatial stability of hybrid zones remains poorly explored. Here, we investigate the effects of assortative mating on hybrid‐zone dynamics by means of a mathematical model parameterized with phenotype and genotype data from the hybrid zone between all‐black carrion and gray‐coated hooded crows. In the best‐fit model, narrow clines of the two mating‐trait loci were maintained by a moderate degree of assortative mating inducing pre‐ and postzygotic isolation via positive frequency‐dependent selection. Epistasis between the two loci induced hybrid‐zone movement in favor of alleles conveying dark plumage followed by a shift in the opposite direction favoring gray‐coated phenotypes ∼1200 generations after secondary contact. Unlinked neutral loci diffused near‐unimpeded across the zone. These results were generally robust to the choice of matching rule (self‐referencing or parental imprinting) and effects of genetic drift. Overall, this study illustrates under which conditions assortative mating can maintain steep clines in mating‐trait loci without generalizing to genome‐wide reproductive isolation. It further emphasizes the importance of the genetic mating‐trait architecture for spatio‐temporal hybrid‐zone dynamics. [ABSTRACT FROM AUTHOR]