One central question in evolutionary biology is how phenotypic and genetic diversity build up and are maintained over both micro- and macroevolutionary scales. An emerging body of literature is attempting to bridge these scales. The main approach undertaken so far consists in comparing evolutionary rates in large comparative datasets. However, we are still lacking good model organisms and case studies to dissect the drivers of diversity maintenance at different scales and validate predictions from comparative studies. One promising complementary approach would consist in leveraging trans-species polymorphisms, i.e. polymorphic traits shared between closely related species, to investigating the factors that differ between polymorphic species and their monomorphic relatives. One such model is the female-limited colour polymorphism found in Ischnura damselflies. Females in many species come in two or three heritable morphs, one of which resembles males. Extensive studies of the ecology and population biology of these species revealed that these morphs are maintained by negative frequency-dependent selection caused by conflict between sexes over mating frequency, as common female morphs suffer from increased mating harassment compared to rare morphs. This project aims to decipher the macroevolutionary history of the morphs to elucidate its consequences on polymorphism maintenance and balancing selection. I will investigate the number of origins and losses of this polymorphism. For this, I will map the morph locus in species that appear to have gained the male-like morph independently to investigate if they are homologous. I will then search for signatures of ancient introgression to assess the contribution of adaptive introgression to the current distribution of this polymorphism. Finally, I will test the prediction that hybridization may be more frequent in polymorphic species.