PHENOTYPIC, GENETIC, AND ENVIRONMENTAL INTEGRATION OF MORPHOLOGY IN ASTREAM POPULATION OF THE THREESPINE STICKLEBACK, GASTEROSTEUS-ACULEATUS

Phenotypic, genetic, and environmental variances and covariances for 3 3 morphometric traits were estimated for a population of threespine st ickleback, Gasterosteus aculeatus, from the Brush Creek drainage, Cali fornia, by sib analysis of laboratory-bred families. Heritabilities of the morphometric traits ranged from -0.28 to 0.78, and were moderatel y low (mean h(2) = 0.26); the mean and range of heritabilities for fiv e phenotypic eigenvectors were similar. The average coefficient of gen etic determination of the traits and eigenvectors was high (0.57 and 0 .63, respectively), indicating a substantial genotypic contribution to variation in body morphology. The defensive complex, a functional set of bony armor structures, was genetically and environmentally integra ted: genetic factors (e.g., pleiotropy) are reinforced by environmenta l factors to produce a functional phenotype. Other components of morph ology, including body form, were environmentally, but not genetically, integrated. Given the importance of genetic factors to evolutionary c hange under natural selection, these results implicate natural selecti on in the evolution of the defensive complex; the role of natural sele ction in the evolution of other components of morphology is equivocal. Genetic integration of functionally (phenotypically) independent trai ts suggests that stochastic processes or pleiotropic mutation also hav e played a role in the evolution of morphology in this population of s ticklebacks.