Evol Ecol Res 15: 241-269 (2013) Full PDF if your library subscribes.
Natural selection and the adaptive radiation of Haida Gwaii stickleback
Thomas E. Reimchen, Carolyn Bergstrom* and Patrik Nosil#
Department of Biology, University of Victoria, Victoria, British Columbia, Canada
Correspondence: T.E. Reimchen, Department of Biology, University of Victoria, PO Box 3020, Victoria, BC V8W 3N5, Canada. E-mail: email@example.com
Questions: What are the selective landscapes structuring inter-population variability on the Haida Gwaii archipelago? How much morphological variability is functional given the common potential for stochastic founder effects when new habitats are colonized?
Organism and sites: Threespine stickleback (Gasterosteus aculeatus) from 56 isolated watersheds with separate marine ancestry encompassing 102 freshwater localities ranging from large oligotrophic mountain lakes to darkly stained bog lakes and ponds on the Haida Gwaii archipelago, western Canada.
Field data: Biophysical attributes of the habitat (lake volume, depth, percent light transmission at 400 nm, calcium availability, pH, distance to marine waters, predation regime including puncturing, compression, and grappling species), morphological traits of the fish (body size, body depth, lateral plate number and position, dorsal spine length, pelvic girdle size, ascending process height, cross-sectional diameter, basal and lateral plate structural overlap), and geographical distance between localities. Based on previous surveys in this archipelago, we predicted the greatest expression of post-capture defences in lakes with high water clarity and the greatest reduction in small stained ponds, as this characterizes regular shifts in the predation landscape.
Analyses: Principal components, univariate linear models, and Akaike Information Criterion (AIC) analyses.
Results: Populations range from 30 to 90 mm in adult male standard length, from full-plated to naked, and from full-spined to un-spined. Again and again, populations converge towards functional multivariate or univariate phenotypes predicted by the biophysical attributes of each habitat, primarily lake volume, aquatic spectra, and predation regime. Across a broad diversity of lake volumes, body size is larger in stained lakes and we hypothesize that this is a defence adaptation for increased burst velocity and rapid access to the aphotic zones near the water surface. Defence armour is well developed in clear lakes, independent of lake volume, and in large stained lakes. Completely plated populations are only found in clear habitats. Reduction in posterior plates is associated with accentuated armour development in the anterior trunk in large lakes where predatory vertebrates are common. Anterior armour, including lateral plates and spines, is progressively reduced towards an unarmoured condition in shallow, stained ponds.
Conclusion: Threespine stickleback from pristine lakes and ponds on the Haida Gwaii archipelago demonstrate remarkable morphological differentiation among populations associated with habitat differences that is largely consistent with changes in predation regime, potentially mediated through a shift from post-capture to pursuit adaptations. Thus, even if founder effects occur when new populations are colonized, natural selection is a major determinant of morphological evolution. Our study emphasizes the efficacy of selective landscapes in this adaptive radiation and provides an opportunity for informed integration of emerging genome-wide data of these populations.
Keywords: body size, Canadian Galapagos, dystrophic lakes, morphological variability, predator–prey defences, Queen Charlotte Islands, stickleback.
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