Morphological divergence of North‐European nine‐spined sticklebacks (Pungitius pungitius): signatures of parallel evolution

Parallel evolution is characterised by repeated, independent occurrences of similar phenotypes in a given habitat type, in different parts of the species distribution area. We studied body shape and body armour divergence between five marine, four lake, and ten pond populations of nine‐spined sticklebacks Pungitius pungitius (Linnaeus, 1758)] in Fennoscandia. We hypothesized that marine and lake populations (large water bodies, diverse fish fauna) would be similar, whereas sticklebacks in isolated ponds (small water bodies, simple fish fauna) would be divergent. We found that pond fish had deeper bodies, shorter caudal peduncles, and less body armour (viz. shorter/absent pelvic spines, reduced/absent pelvic girdle, and reduced number of lateral plates) than marine fish. Lake fish were intermediate, but more similar to marine than to pond fish. Results of our common garden experiment concurred with these patterns, suggesting a genetic basis for the observed divergence. We also found large variation among populations within habitat types, indicating that environmental variables other than those related to gross habitat characteristics might also influence nine‐spined stickleback morphology. Apart from suggesting parallel evolution of morphological characteristics of nine‐spined sticklebacks in different habitats, the results also show a number of similarities to the evolution of three‐spined stickleback (Gasterosteus aculeatus Linnaeus, 1758) morphology. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 403–416.