Many separate-sexed organisms have sex chromosomes controlling sex determination. Sex chromosomes often have reduced recombination, specialized (frequently sex-specific) gene content, dosage compensation and heteromorphic size. Research on sex determination and sex chromosome evolution has increased over the past decade and is today a very active field. However, some areas within the field have not received as much attention as others. We therefore believe that a historic overview of key findings and empirical discoveries will put current thinking into context and help us better understand where to go next. Here, we present a timeline of important conceptual and analytical models, as well as empirical studies that have advanced the field and changed our understanding of the evolution of sex chromosomes. Finally, we highlight gaps in our knowledge so far and propose some specific areas within the field that we recommend a greater focus on in the future, including the role of ecology in sex chromosome evolution and new multilocus models of sex chromosome divergence.

Significance: Recently, it was discovered that small insects like flies and wasps, with seemingly transparent wings, display vivid coloration against dark backgrounds because of so-called “wing interference patterns” (WIPs). It was proposed that such wing coloration could function in sexual selection and species recognition, but direct evidence for this hypothesis has been lacking. Here, to our knowledge we present the first experimental evidence that WIPs in male Drosophila melanogaster are targets of mate choice from females. Comparison of attractiveness of different male WIPs between black and white backgrounds revealed that two different color traits, saturation and hue, experience directional and stabilizing sexual selection, respectively. Our results suggest that vivid coloration in WIPs is a target of mate choice and might have evolved by sexual selection.