The population structure of organisms is the outcome of historic processes (climate and migration) and remains subjected to gene flow, mutation, selection and genetic drift. Our own research on the population structure of gobies of the genus Pomatoschistus has shown that historic processes, selection and migration play indeed a role in the North Sea. This research will be continued with highly variable markers and will be extended with common sole. Moreover, the parallel evolution of the goby complex Pomatoschistus and the parasite complex Gyrodactylus will be studied.