Overview map showing the position of the individual sampling locations in Lake Constance and locations of outgroup populations (insert) used in the genetic analysis. Population information and geographic coordinates for all samples are provided in Suppl. material 2. Pelagic zone samples were sampled by trawling in transects across Upper Lake Constance and the location in the map is not highlighting the exact location (see Methods). Outgroup samples from the United Kingdom are freshwater and marine samples from the River Tyne, whereas outgroup samples from Norway and Germany are only freshwater samples (lake and stream).