19Colour polymorphisms are popular study systems among biologists interested in evolutionary 20 dynamics, genomics, sexual selection and sexual conflict. In many damselflies, such as in the 21 globally distributed genus Ischnura (forktails), female colour polymorphisms occur in some 22 species. Female-polymorphic species contain two or three female morphs, one of which is male-23 coloured (androchrome or male mimic) and co-exists with sexually dimorphic (heterochrome) 24 females. These female colour polymorphisms are considered to be maintained by frequency-25 dependent sexual conflict, but their macroevolutionary histories are unknown, due to the lack 26 of a robust molecular phylogeny. Here, we present the first time-calibrated phylogeny of 27 Ischnura, using a multispecies coalescent approach (StarBEAST2), incorporating both 28 molecular data and fossil information of 41 extant species (55% of the genus). We estimate the 29 age of Ischnura to be between 13.8 and 23.4 millions of years, i.e. Miocene. We infer the 30 ancestral state of this genus as female monomorphism with heterochrome females, with 31 multiple gains and losses of polymorphisms, evidence of trans-species polymorphisms and a 32 significant positive relationship between polymorphism incidence and current geographic range 33 size. Our study provides a robust phylogenetic framework for future research on the dynamic 34 macroevolutionary history of this clade with its extraordinary diversity of sex-limited female 35 polymorphisms. 36 or sexual selection, such as negative frequency-dependent selection or overdominance (Blows 62 et al.