Two ecologically and morphologically divergent species of the Gasterosteus aculeatus (threespine stickleback) species complex occur in Enos Lake, Vancouver Island, British Columbia. The limnetic species swims in open water most of the time and feeds on plankton, and the benthic species lives near the shore in a structurally complex environment where it eats larger benthic prey. Based on design criteria inferred from hydrodynamic considerations, from other fishes, or from other populations of Gasterosteus, and on ecological differences between the two species, we expected that limnetic sticklebacks would have a more fusiform body, longer pectoral fins and snout, and larger eyes. Although some of these expected differences had been observed in a previous study of this species pair, others were verified for the first time in this study. Shape divergence between our samples results both from allometric trends coupled with differences in overall mean size and from size-free shape differences. Standard length is a poor proxy for general size (i.e., the first principal component of the morphometric variables) because several major features which differ between the species are dorsoventrally oriented. The marked morphometric divergence between these species has probably evolved within about the past 13 000 years, since deglaciation of Vancouver Island.
Body size differences do not arise from divergent mate preferences in a species pair of threespine stickleback