MULTIVARIATE-ANALYSIS OF GEOGRAPHIC COVARIANCE BETWEEN PHENOTYPES AND ENVIRONMENTS IN THE THREESPINE STICKLEBACK, GASTEROSTEUS-ACULEATUS, FROM THE COOK INLET AREA, ALASKA

Widely distributed freshwater populations of Gasterosteus aculeatus exhibit reduction in the bones of the pelvis and the numbers of lateral plates and dorsal spines. We investigated the relationship between skeletal reduction and the characteristics of the environments in which it occurs. A combination of environmental factors, including environmental ion composition lake productivity, geographic position, opportunity for gene flow, and the presence of other fishes, is associated with skeletal reduction. The principal correlates of reduction in all skeletal traits are low concentrations of calcium, magnesium, silicon, H+, and reactive phosphorus. We hypothesize that the ion composition of lake water is the main selective agent promoting skeletal reduction in sticklebacks in the Cook Inlet area, Alaska, and that other fishes and local gene flow may modify its extent. Our results show that a suite of skeletal traits responds in common to the interacting effects of at least three environmental factors (ion composition of lake water, presence of other fishes, local gene flow), and they emphasize that unitary explanations of the evolution of skeletal reduction are unlikely to be adequate.