This thesis examines the process by which continental fish faunas arise, using the Great American Biotic Interchange as a case study. The Interchange allowed South American fishes to colonize North America. I examine the history of this colonization in the Characiformes (Actinopterygii: Ostariophysi), finding repeated colonization events and a surprising non-monophyly of the group. I explore the morphological variation in the standing diversity of North American Characiformes, finding it to be a non-biased subset of the variation found in South America. I also examine intraspecific morphological variation along environmental gradients, finding that this variation can be equal to variation among species. Finally, I use ecological niche modeling to predict the future spread of characiform fishes in North America, finding that their projected future ranges may be regulated by climate, but mediated by diet.