Figure 1. Map showing the sampling locations within Europe.

Different marker shapes represent different habitat types (circle = sea, square = river, triangle = pond, inverted triangle = lake), whilst different colours represent the Eastern clade (black), and the Western clade (white). Markers with two colours show sites where haplotypes from multiple clades were found. In addition, there are two sites in Canada that are not shown – one is a pond site, and one is a lake site. Both sites form their own clade, however one sample from the pond site has haplotype E3, the most common Eastern haplotype. The distribution of the nine-spined stickleback is overlayed in grey (from Paepke [32]), and the major seas are labelled.