Threespine sticklebacks, small fish with a circumglobal distribution in coastal marine and freshwater of the northern hemisphere, present a remarkable scope of variation in body and fin shape among populations. The repeated evolution of divergent body shapes in a radiation of stickleback from Cook Inlet, Alaska suggests that diversification has proceeded by extensive parallel selection. To explore this hypothesis, hydromechanical equations of fish propulsion and descriptions of stickleback foraging and anti-predator behaviours were used to develop a series of hypotheses that predicted the evolutionary effects of native predatory fishes (NPF) and relative littoral area (RLA) on body shape. Body shape was measured using Cartesian coordinates of anatomical landmarks transformed by the generalized resistant fit super-imposition. In general, the results were consistent with the hypotheses that (1) RLA has a direct effect on selection for foraging behaviour and morphology, (2) RLA has an indirect effect on selection for morphology employed in predator evasion, (3) presence of NPF has a direct effect on selection for evasive morphology, and (4) presence of NPF has an indirect effect on selection for foraging behaviour and morphology. The magnitude of the divergence of body shapes present in the Cook Inlet system suggests that extreme phenotypes have evolved by opportunistic expansion into new habitat relatively free of interspecific competition.