Reductions in prolonged swimming capacity following freshwater colonization in multiple threespine stickleback populations.

We compared ancestral anadromous-marine and nonmigratory, stream-resident threespine stickleback (Gasterosteus aculeatus) populations to examine the outcome of relaxed selection on prolonged swimming performance. We reared marine and stream-resident fish from two locations in a common environment and found that both stream-resident populations had lower critical swimming speeds (Ucrits) than marine populations. F1 hybrids from the two locations displayed significant differences in dominance, suggesting that the genetic basis for variation in Ucrit differs between locations. To determine which traits evolved in conjunction with, and may underlie, differences in performance capacity we measured a suite of traits known to affect prolonged swimming performance in fishes. While some candidate traits did not evolve (standard metabolic rate and two body shape traits), multiple morphological (pectoral fin size, shape and four body shape measures) and physiological (maximum metabolic rate; MMR) ...