Behavioral isolation can catalyze speciation and permit the slow accumulation of additional reproductive barriers between co-occurring organisms. We illustrate how this process occurs by examining the genomic and behavioral bases of pre-mating isolation between two bird species (Sporophila hypoxantha and the recently discovered S. iberaensis) that belong to the capuchino seedeaters, a recent, rapid radiation characterized by variation in male plumage coloration and song. Though these two species co-occur without ecological barriers to reproduction, we document strong assortative mating and behaviorally-mediated species recognition associated with genomic regions underlying male plumage patterning. Plumage differentiation originated through the reassembly of standing genetic variation, indicating how novel sexual signals can quickly arise and maintain species boundaries.

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