Hybridization between distant lineages increases adaptive variation during a biological invasion: stickleback in Switzerland

The three-spined stickleback is a wide spread Holarctic species complex that radiated from the sea into freshwaters after the retreat of the Pleistocene ice sheets. In Switzerland, sticklebacks were absent with the exception of the far northwest, but different introduced populations have expanded to occupy a wide range of habitats since the late 19th century. A well studied adaptive phenotypic trait in sticklebacks is the number of lateral plates. With few exceptions, freshwater and marine populations in Europe are fixed for either the low or the fully plated phenotype respectively. Switzerland, in contrast, harbors in close proximity the full range of phenotypic variation known from across the continent. We addressed the phylogeographic origins of Swiss sticklebacks using mitochondrial partial cytochrome b and control region sequences. We found only 5 different haplotypes but these originated from 3 distinct European regions, fixed for different plate phenotypes. These lineages occur l...