Contemporarily evolving systems provide a unique opportunity to characterize the direction, pattern, and rates of phenotypic change among multiple species. The damming of streams to create reservoirs results in a change from lotic to lentic environments and induces the evolution of phenotypes that optimize unsteady swimming performance in fishes. In my first chapter, I examined the spatial and temporal patterns of evolution of body shape in a widespread North American stream fish (Pimephales vigilax) in response to stream impoundment. I observed significant changes in body depth, head shape, and fin placement following dam construction in each of seven different Oklahoma rivers. The magnitude of change was greatest in the first 15 generations post-impoundment, followed by continued but more gradual change thereafter. In my second chapter, I compare the direction and rate of evolution of body size, body shape, and caudal fin shape following stream impoundment in eight cyprinids. My data show that these traits do not diverge in the same direction in all species. Rates of evolution during the period of directional selection were consistent among traits and species, perhaps due to similar amounts of underlying quantitative genetic variation as a result of their recent common ancestry. All of the above results indicate that changes in environmental conditions can result in rapid evolution of functionally important traits. In my third chapter, I tested the hypothesis that transitions between benthic and pelagic habitats resulted in convergent evolution of body shape in North American cyprinids. I built a phylogenetic hypothesis of 201 species using four genes, reconstructed the evolution of habitat type on the recovered topology, and evaluated evolution of body shape in benthic and pelagic species. I detected approximately 25 transitions between benthic and pelagic species with pelagic-to-benthic transitions being most common. Body shape was significantly different between benthic and pelagic species with benthic species developing deeper bodies with larger heads than pelagic species. However, benthic and pelagic species did not inhabit mutually exclusive regions of morphospace suggesting that convergent evolution of body shape is incomplete in this group of fishes. My results show that habitat transitions can drive convergent evolution of similar phenotypes among distinct evolutionary lineages. However, the magnitude of convergence among different lineages is likely constrained by their unique evolutionary history.