Selective agents in the adaptive radiation of Hebridean sticklebacks

Abstract

Question: What selective agents underpin the adaptive radiation of threespine sticklebacks (Gasterosteus aculeatus) on the Outer Hebridean island of North Uist? Hypothesis: The chief agents of selection for lateral plate number and pelvic score are predatory trout, the availability of dissolved ions, or an interaction of both. Methods: Adult threespine sticklebacks were collected from 26 lochs on North Uist. Fish were killed, stained, and scored for lateral plate number and pelvic score. We also measured the pH and concentration of dissolved calcium and phosphorous of each loch. We assessed the abundance of predatory trout in a subset of lochs by ‘fly fishing’ using a lure that mimicked the appearance of sticklebacks. Results: Dissolved calcium and phosphorous predicted stickleback lateral plate number and pelvic score, while trout abundance failed to predict either. Attack rates by trout on stickleback lures were higher in lochs with higher numbers of trout, with high water clarity, and at higher water temperatures. Our findings implicate a role for the bioavailability of dissolved ions in selection for reduced lateral plate number and pelvic score evolution, with indirect evidence for an effect of trout predation on the adaptive radiation of stickleback populations on North Uist