Net evolutionary trajectories of body shape evolution within a microgeographic radiation of threespine sticklebacks (Gasterosteus aculeatus)

Following deglaciation of the Cook Inlet region of Alaska approximately 16 000 years ago, anadromous threespine stickleback (Gasterosteus aculeatus) r apidly colonized emerging lakes and rivers forming resident, Freshwater pop ulations. Although the precise body shape of the ancestral marine populatio n is unknown, marine sticklebacks sampled from both Pacific and Atlantic si tes present remarkably little body shape variation among populations, which suggests that the morphology of any of the marine populations could be use d to represent the ancestral phenotype. To infer the net evolutionary traje ctories of body shape change in the Cook Inlet radiation, derived body shap es of lacustrine samples were compared to the presumptive, primitive body s hape, represented by the mean shape of two anadromous samples from Cook Inl et. In general, some derived body shape traits are shared by all freshwater populations but many traits evolved in opposite directions. The principal axes of shape variation among freshwater sample means were computed using P rincipal Components Analysis. The strong correlation between the direction of the principal component axes and lake habitat variables suggest that pop ulations evolved toward selection peaks that are biased along the component axes due to biotic and abiotic features of the lakes.