Ultraviolet in swallowtail butterflies: contrasted dorso-ventral evolution highlights a trade-off between natural and sexual selection on visual cues.

The evolutionary dynamics of color pattern diversification in animals is strongly influenced by visual interactions within and among species. While much attention has been given to color pattern variation in the human-visible range, perception outside this range is observed in a wide array of species and is poised to influence color pattern evolution. Butterfly species often show sensitivity to ultraviolet (UV) light, impacting wing color pattern diversification as their evolution is influenced by both predator vision and sexual selection. Here, we explore UV color pattern diversification in Papilionidae within a comparative phylogenetic framework by quantifying variation from UV photographs of museum specimens using a machine-learning-based method. We find decoupled dorsal and ventral UV color pattern evolution, with brighter and more rapidly evolving ventral sides, especially in males. Conversely, we find a smaller dorso-ventral difference in visible-light color patterns. Moreover, we find divergence in male ventral UV patterns in closely related sympatric species, even after accounting for variation due to visible-light patterns. These results suggest an influence of sexual selection on UV ventral pattern diversification. These findings highlight how the trade-off between sexual and natural selection may lead to contrasted evolution of ventral vs. dorsal sides of the same organ.