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The role of predation in the evolution of sympatric stickleback species Vamosi, Steven Michael

Abstract

The contribution of predation to the origin and divergence of sympatric species has not been examined in much detail. To determine whether there is an association between fish predators and the presence of stickleback species pairs (Gasterosteus aculeatus), I recorded the physical characteristics and fish communities of the six lakes containing stickleback species pairs and 47 lakes in the Strait of Georgia region of British Columbia that contained only a single stickleback species. A number of lakes were identified that had similar physical characteristics to lakes containing stickleback species pairs. These lakes, however, tended to have more fish species than lakes with two stickleback species, which had only one other fish species, the predatory cutthroat trout (Oncorhynchus clarki). The absence of certain fish species, thus, appeared to be important in allowing speciation of sticklebacks. Predation may have played a significant role in the morphological differentiation of sympatric stickleback species pairs. I measured the defensive armor of sticklebacks from lakes with one and two stickleback species. I provide evidence of character shifts in defense armor as a consequence of sympatry. Living in different habitats in sympatry may have been accompanied by adaptation to different predators in the two habitats. In lab predation experiments, I found that the species that lives in the pelagic zone was selectively preyed upon by predators commonly found in the littoral zone. The species from the littoral zone, in contrast, was selectively preyed upon by a piscivorous bird, the double-crested cormorant (Phalacrocorax auritus), which usually forages in the pelagic zone. Finally, I investigated whether the two species and their F1 hybrids are differentially vulnerable to predators. The species associated with the pelagic zone had significantly reduced survival in the presence of cutthroat trout, whereas the other species was largely unaffected by their presence. First generation hybrids had low survival regardless of treatment. Cutthroat trout predation may be a significant source of mortality for the species associated with the pelagic zone but does not contribute to reproductive isolation between the two species.

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