In this thesis I investigated how mating related strategies may differ among populations and how these strategies evolve in response to their local ecological environments. I have used two different study systems of aquatic invertebrates: diving beetles (Dytiscidae) and the fresh water isopod Asellus aquaticus. These systems have in common that they present distinct genetic variation within the species: female polymorphism in the diving beetles and the occurrence of two ecotypes in the isopod. They also have in common that they are suggested to experience a conflict between the sexes over mating. In particular, I have addressed (i) how genetic polymorphism may be maintained in populations experiencing sexual conflict, (ii) the selection pressures acting on female polymorphism in diving beetles, (iii) the physical performance of the sexually antagonistic traits in diving beetles, (iv) parallel divergence and life-history consequences of precopulatory mate guarding in the two isopod ecotypes and (v) phenotypic plasticity in mating behaviour in response to demographic factors in the isopod ecotypes. I have also addressed the problem of how sexual conflict theory is conceptualised with regard to sex roles (vi). In my studies I have seen that behaviour and mating traits are to a large extent affected by ecological and demographic features. Within a species, the mating dynamics could differ dramatically between different populations and the evolutionary change in mating characteristics may be rapid. Mating characteristics can also change in parallel due to ecological similarities between populations. In the last study of this thesis it is shown that both the terminology and the features assigned to the sexes in theoretical models are highly sex-specific, and may reflect a stereotypic view of the sexes based on human sex roles
