The island syndrome can be defined as a suite of predictable and consistent similarities among island organisms. In plants, it can result from processes of biased colonization and/or in situ evolution. Despite recent progress in identifying its components, the relative contributions of these processes remain unclear. Here, I disentangle and quantify the relative roles of these processes in driving morphological change in Coprosma, a plant genus widespread across the Pacific. First, I amassed data for two morphological traits and tested for correlated evolution between morphology and insularity. I then used phylogenetic comparative methods and directed network models to determine whether possessing larger leaves and/or stature favoured island colonization, whether these similarities are exclusively the product of in situ evolution, or both. Results indicate that large-leaved ancestors were favoured during island establishment, while both leaf size and stature consistently increased on islands as a result of in situ evolution. Overall, this study provides a framework to distinguish between biased colonization and in situ evolutionary processes in island plants.