Adaptive divergence in D arwin's small ground finch ( G eospiza fuliginosa ): divergent selection along a cline

We examine here, in a single year (2005), phenotypic divergence along a 560‐m elevation gradient in D arwin's small ground finch ( G eospiza fuliginosa ) in the G alápagos I slands. In this sample, four composite measures of phenotypic traits showed significant differences along the 18‐km geographical cline extending from lowlands to highlands. Compared with lowland birds, highland birds had larger and more pointed beaks, and thicker tarsi, but smaller feet and claws. Finches in an intervening agricultural zone had predominantly intermediate trait values. In a second, mark–recapture study we analyse selection on morphological traits among birds recaptured across years (2000–2005) in lowland and highland habitats. Birds were more likely to survive in the highlands and during the wet season, as well as if they had large beaks and bodies. In addition, highland birds exhibited higher survival rates if they had small feet and pointed beaks – attributes common to highland birds as a whole. Lowland birds were more likely to survive if they possessed the opposite traits. Selection therefore reinforced existing morphological divergence, which appears to reflect local adaptation to differing resources during the predominantly drought‐ridden conditions that characterized the 5‐year study. Alternative explanations – including genetic drift, matching habitat choice, deformation by parasites, and the effects of wear – received little or no support. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society , 2013, 110 , 45–59.