EVOLUTION OF PELVIC REDUCTION IN THREESPINE STICKLEBACK FISH - A TESTOF COMPETING HYPOTHESES

Reimchen hypothesized that pelvic reduction in threespine stickleback is favored by an absence of piscivorous fishes and the resulting incre ase in predation by insects, but Giles hypothesized that the predation regime is unimportant and that a low dissolved calcium concentration favors evolution of pelvic reduction. Substantial pelvic reduction in threespine stickleback sampled from 179 lakes around Cook Inlet, Alask a is strongly associated both with an absence of predatory fishes and a low calcium concentration. However, the association of pelvic reduct ion with low calcium concentration appears to be contingent on the abs ence of predatory fishes. These results emphasize the importance of in teractions between seemingly unrelated environmental variables for sel ection of a single trait. However, these results also conflict with so me observations elsewhere and do not rule out the possibility that oth er environmental factors are important for selection for pelvic reduct ion in threespine stickleback.