The adaptive significance of dorsal spine variation in the fourspine stickleback, Apeltes quadracus. IV. phenotypic covariation with closely related species

Summary The fourspine stickleback ( Apeltes quadracus ) and ninespine stickleback ( Pungitius pungitius ) have similar ecologies, they often coexist, and they share parallel polymorphisms for the number of dorsal spines. Spine number is positively correlated between them for 86 sites in eastern Canada. Dorsal spine length is positively correlated with spine number within each species, and spine length is positively correlated between them. Spine length for both species is also positively correlated with the presence of predatory fishes and negatively correlated with vegetation cover. Finally, spine number for both Apeltes and Pungitius is lower where they coexist with a new species of stickleback ( Gasterosteus sp.) which is found only in environments where predation risk is low. These patterns of covariation within and between species are evidence for natural selection, and they suggest that predators are a selective agent favouring the higher-spined morphs. We discuss these results with respect to the idea that geographic variation in Pungitius reflects historical processes of isolation and differentiation during the Pleistocene era.