Morphological Divergence and the Organization of Trophic Characters Among Lacustrine Populations of the Threespine Stickleback (Gasterosteus aculeatus)

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Abstract
A multivariate solution for dealing with patterns of character covariance, when characters genotypic covariances are known, has been applied to the organization of trophic morphology among lacustrine populations of the threespine stickleback (Gasterosteus aculeatus). The quantitative genetics governing eight trophic characters were investigated using representative populations from each of three known trophic ecotypes (open-water, littoral, and intermediate) which occur in the same drainage. Character heritabilities (0.19-0.84) were all significant. Estimates of character genetic correlations (-0.65 < rG < 0.85) were obtained and their standard errors were calculated by bootstrapping. Cluster analysis of the genetic correlation matrices defined two character suites, head shape and gill raker structure. Selection gradients between trophic ecotypes indicated that directional selection had operated most strongly on characters related to food size (upper jaw length and gill raker number). These results support the hypothesis that interpopulation variability in trophic morphology is organized as an adaptive response to local selection pressures.