Island colonisation leads to rapid behavioural and morphological divergence in Anolis lizards

Islands are hotspots of endemism and often function as engines of adaptive radiation. Nevertheless, we lack a deep understanding of the processes that generate phenotypic divergence when populations first colonise islands. Important questions include: (1) Do populations experience shifts in habitat use and behaviour with reduced competition and predation, and how fast do these changes occur? (2) Do shifts in niche occupancy result in morphological divergence from mainland populations? To investigate these questions, we transplanted 210 slender anole lizards ( Anolis apletophallus ) from mainland Panama to three islands in the Panama Canal that are likely species-poor compared to the mainland. We compared habitat use, flight initiation distance, and morphology among populations across two generations of divergence. We found that island lizards changed their behaviour immediately after colonisation, perching on lower and broader surfaces and allowing observers to approach more closely before fleeing. Although we found only weak evidence for an association between survival and morphological trait variation, trait means in the second generation often shifted in the direction expected if selection had acted on the founders. Our results indicate that colonising individuals can change their behaviour rapidly to exploit new structural niches, and that substantial shifts in morphology can occur after only a single generation. These changes, which are probably facilitated by ecological release, may represent the first steps in adaptive radiation of island lineages.