Disentangling the relative roles of biased colonization and in situ evolution in driving the island syndrome in <i>Coprosma</i> (Rubiaceae)

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.