The evolution of freshwater diversity in the threespine stickleback (Gasterosteus aculeatus): site-specific differentiation of trophic morphology

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Abstract
To assess interpopulation levels of morphological variability populations of Gasterosteus aculeatus were sampled from lakes within the upper Cowichan River system (Vancouver Island, British Columbia [Canada]). Phenotypic divergence between populations is assumed to be a postglacial event. Nine characters were scored; eight were related to feeding and the ninth character was lateral plate number. All populations were the low plate morphs; however, populations of Gasterosteus in lakes lacking piscivorous fish had significantly fewer lateral plates than populations in lakes with predatory fish species. Two distinct trophic "morphotypes" were identified, each one associated with a specific lake environment. Populations inhabiting benthic-dominated enviroments were found to possess reduced gill raker number and reduced gill raker length but increased upper jaw length relative to populations from limnetic environments. We propose that the interpopulation variability in trophic morphology is a response to trophic resource differences between lakes.