Day versus night and the evolution of sexual dimorphism in Lepidoptera

Many species exhibit consistent morphological differences between males and females. This sexual dimorphism sparked debate between Charles Darwin, who attributed it to sexual selection favoring male traits that appeal to females, and Alfred Russell Wallace, who suggested that natural selection favored cryptic traits in females to evade predators. Here, using multispectral imaging of wing reflectance and patterning across 274 butterfly and moth species sampled from recent phylogenetic frameworks, we demonstrate that Darwinian and Wallacean models both describe aspects of the evolution of lepidopteran coloration, with Darwinian sexual selection largely acting on visible male traits in diurnal species, while Wallacean natural selection tends to act on female traits of nocturnal species: In butterflies, wing reflectance and color pattern traits exhibit the strongest dimorphism in dorsal and forewing areas, areas predicted to experience strong sexual selection, with male traits evolving more quickly and exhibiting greater between-species disparity than those of females (indicators of sexual selection), but only in wavelengths perceptible to butterflies. In contrast, in nocturnal moths, evolutionary rate and disparity are strongly female biased. Ancestrally nocturnal geometrid moths, where diurnal behavior has evolved repeatedly, confirm this trend of male-biased rates and disparity in diurnal species, especially on dorsal and forewing surfaces.