Because island communities are derived from the mainland, they are often less diverse by comparison. However, reduced complexity of island communities can also present ecological opportunities. For example, amphibian diversity on the Indonesian island of Sulawesi is generally lower than in the Philippines, but this island supports a surprisingly diverse endemic assemblage of Sulawesi fanged frogs (genus Limnonectes). To explore evolutionary dynamics of this system, we examined molecular, morphological, and geographical variation of fanged frogs from these two regions. Using genealogical concordance – a conservative standard – we identified 12 species on Sulawesi, only four of which are described. One more species can be distinguished with morphology, and a Bayesian approach to species delimitation suggests our total species estimate on Sulawesi (n = 13) is still an underestimate. After accounting for evolutionary history, a model with multiple body size optima in sympatric Limnonectes species is significantly preferred over a “random walk” model where body size evolves by Brownian motion. Additionally, morphological variation is higher among sympatric than non-sympatric species on Sulawesi, but not in the Philippines. Taken together, these findings suggest that adaptive radiation of fanged frogs on Sulawesi was driven by natural selection to infiltrate ecological niches occupied by other lineages in the Philippines. Our study supports the concept of ecological opportunity in community assembly: diversification in mature communities, such as the Philippines, is limited by a dearth of unoccupied ecological niches. On Sulawesi however, evolutionary novelties originated in a predictable and replicated fashion in response to opportunities presented by a depauperate ancestral community.