Visual mating behaviors in *Heliconius* butterflies are controlled by *regucalcin1*, which has been shared through hybridization. The study combines behavioral, population genomic, and expression analyses to show that two *Heliconius* species have evolved the same preferences for red patterns by exchanging genetic material through hybridization. Neural expression of *regucalcin1* correlates with visual preference across populations, and disruption of *regucalcin1* with CRISPR/Cas9 impairs courtship towards conspecific females, providing a direct link between gene and behavior. The results support a role for hybridization in the evolution of behavioral traits and show how visually-guided behaviors contributing to adaptation and speciation are encoded within the genome.Visual mating behaviors in *Heliconius* butterflies are controlled by *regucalcin1*, which has been shared through hybridization. The study combines behavioral, population genomic, and expression analyses to show that two *Heliconius* species have evolved the same preferences for red patterns by exchanging genetic material through hybridization. Neural expression of *regucalcin1* correlates with visual preference across populations, and disruption of *regucalcin1* with CRISPR/Cas9 impairs courtship towards conspecific females, providing a direct link between gene and behavior. The results support a role for hybridization in the evolution of behavioral traits and show how visually-guided behaviors contributing to adaptation and speciation are encoded within the genome.