Island colonization followed by in-situ adaptive radiation has long been a fascination in biology, as it is thought to have generated the patterns of endemism we see on islands today. Nowhere is this more prominent than the Indo-Australian Archipelago (IAA), where four isolated biogeographic regions, separated by deep ocean channels, contain high levels of endemism. The rats and mice of the IAA (Rodentia: Muridae) are astonishingly diverse, both in species richness and niche breadth. Classical views predict that, following the arrival of an ecological generalist colonizer, subsequent speciation generates more specialist forms, increasing the overall ecological and morphological disparity within the resulting clade. We used a time-calibrated phylogeny for over 350 species of murids, along with measurements of body size and locomotor mode, to reconstruct the ancestral states at each instance of island colonization across the IAA. From these results, we test the prediction that clade age is positively correlated with ecological diversity, as indicated by locomotor mode and body size. Overall, we find increased locomotor diversity and body size in older clades, highlighting the role of in-situ island speciation in the generation of biodiversity.