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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 kmwide
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. | |
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