evidence: While more studies are needed to address the generality of the island effect on empirical grounds, a major stumbling block is the need of well-sampled phylogenies in both island and continental domains. The present study uses a completely sampled mainland-island system, the so-called arid clade of Hemidactylus geckos14 to test whether the patterns of phenotypic diversification in islands actually differ from those occurring in the continent. The arid clade represents a well-studied monophyletic radiation of 48 species distributed across the arid regions of northeast Africa, the Levant, Arabia, the adjoining areas of southwest Asia and, interestingly for our purposes, also in the Socotra Archipelago14,15,16,17,18 (Fig. 1). This archipelago is located in the northwestern Indian Ocean and originated as a continental fragment that detached from Arabia around 30 Ma19. It comprises two main islands, Socotra and Abd al Kuri (with areas of 3,625 km2 and 133 km2 respectively) and two small islets, Darsa and Samha (5.4 km2 and 40 km2, respectively), situated 380 km off the southeast coast of Arabia (Yemen), and approximately 100 km east of Somalia off the Horn of Africa (Fig. 1).