Spatial variation of morphology in a freshwater population of the threespine stickleback, Gasterosteus aculeatus

Abstract

Differentiation with respect to two functional components of morphology, the defensive complex and overall body form, was studied in a population of threespine sticklebacks (Gasterosteus aculeatus) from the Brush Creek drainage, a small coastal system in northern California with high- and low-gradient habitats. The vast majority of body-shape differences among localities was accounted for by size-related allometric variation, the differences being closely related to overall growth trends. Relative to general body size, small individuals had long spines and fins whereas larger individuals tended to be deep bodied and have shorter fins and shorter spines. Significant size-independent differentiation with respect to the defensive complex and overall body form was also observed. The spatial pattern of differentiation in the components of the defensive complex changed dramatically over the period of study, possibly in response to natural selection. Stable stepped clines for overall body form and lateral-plate morph ratio were observed. Both clines were centered on the ecotone between high- and low-gradient habitats, and were apparently maintained by differential selection in alternative stream-gradient habitats despite gene flow across the ecotone. Sticklebacks from the high-gradient habitat had a continuous row of lateral plates, were elongate, and had long fins, whereas those from the low-gradient or standing-water habitat tended to have only abdominal plates, were more robust through the midbody, and had shorter fins. The results of this study and previous work suggest that various aspects of stickleback morphology may respond independently and rapidly to different evolutionary forces and be functionally related to hydrodynamics.