Comparison of the taxonomic, phylogenetic and trait dimensions of betadiversity may uncover the mechanisms that generate and maintain biodiversity, such as geographic isolation, environmental filtering, and convergent adaptation. We developed an approach to predict the relationship between environmental and geographic distance and the dimensions of betadiversity. We tested these predictions using hummingbird assemblages in the northern Andes. We expected taxonomic betadiversity to result from recent geographic barriers limiting dispersal, and found that cost-distance, which includes barriers, was a better predictor than Euclidean distance. We expected phylogenetic betadiversity to result from historical connectivity, and found that differences in elevation were the best predictors of phylogenetic betadiversity. We expected high trait betadiversity to result from local adaptation to differing environments, and found that differences in elevation were correlated with trait betadiversity. When combining betadiversity dimensions, we observe that high betadiversity in all dimensions results from adaption to different environments between isolated assemblages. Comparisons with high taxonomic, low phylogenetic and low trait betadiversity occurred among lowland assemblages separated by the Andes, suggesting that geographic barriers have recently isolated lineages in similar environments. We provide insight into mechanisms governing hummingbird biodiversity patterns, and provide a framework that is broadly applicable to other taxonomic groups.