From the very start (e.g. Darwin 1845, Wallace 1859), the faunas of island groups have played a special role in the growth of our understanding of evolutionary changes and speciation - and they continue to do so (e.g. Losos et al. 1997, 2004). Islands in archipelagos constitute repeated, discrete and relatively simple entities and therefore function as 'natura/laboratories' that can be used to test general theories (Whittaker 1998). The (often prominent) differences in phenotype (morphology, behaviour, ecology, life history) among island populations or between island and mainland populations are almost invariably attributed to genetic divergence, but it is often unclear which evolutionary processes (founder effect, genetic drift, natural selection, introgression, ...) induce these differences (Barton 1989, Clarke & Grant 1996). The alternative explanation, that the differences arise from phenotypic plasticity, is often not considered (Losos et al. 2000). In this project, we intend to take advantage of an exceptional opportunity to unravel the causes of fenotypic divergence among (island) populations.