Evolutionary Responses Of A Stream Fish To Water Impoundment
Although the evolutionary implications of natural differences in flow regime have been well-studied, the evolutionary implications of anthropogenic modification of flow regime are not well understood. To begin to characterize the evolutionary consequences of dam construction for small stream fishes, I conducted four related studies focused on the blacktail shiner (Cyprinella venusta). First, to characterize natural variation in body shape related to stream characteristics, I assessed the extent to which body shape corresponded to three environmental factors, phylogeny, and body size. Morphology was predominantly associated with variation in mean annual runoff; phylogeny was also a strong predictor of morphology. In contrast to previous studies that emphasize the importance of water velocity, these findings indicate that morphological variation is tightly linked to more complex aspects of hydrology and evolutionary history. Second, to initially characterize variation in body shape associated with dam construction, I compared the body shape of C. venusta from pairs of river and reservoir sites. River populations differ from reservoir populations in several aspects of morphology, demonstrating that reservoir characteristics drive changes in the morphology fish populations. Third, to characterize the pace and trajectory of phenotypic responses to impoundment, I examined a chronosequence of museum specimens originating from a reservoir and nearby stream. The rate of change in larger individuals was greatest in the first 15 years following impoundment, with less pronounced shifts in smaller individuals and no shift in the morphology of stream individuals. These results indicate that morphological responses to impoundment are decadal-scale and attenuate. Fourth, to determine if impoundment-related morphological shifts are genetically determined and are functionally significant, I conducted swimming performance trials with common garden C. venusta originating from reservoir and stream parents. Common garden juveniles exhibited differences in morphology similar to those between wild-caught individuals, indicating that morphological divergence following impoundment is an outcome of rapid evolutionary change. Offspring did not differ in swimming performance, suggesting that morphological evolution may be driven by reproductive or trophic shifts following impoundment. Together, these studies demonstrate that impoundment is a potent evolutionary force on fishes, but the mechanisms by which it does so remain relatively unknown.