• Cocos Island;
  • colonization route;
  • divergence estimates;
  • Galápagos;
  • gene flow;
  • geographic isolation;
  • microsatellites;
  • Yellow warbler

Abstract The faunas associated with oceanic islands provide exceptional examples with which to examine the dispersal abilities of different taxa and test the relative contribution of selective and neutral processes in evolution. We examine the patterns of recent differentiation and the relative roles of gene flow and selection in genetic and morphological variation in the yellow warbler (Dendroica petechia aureola) from the Galápagos and Cocos Islands. Our analyses suggest aureola diverged from Central American lineages colonizing the Galápagos and Cocos Islands recently, likely less than 300 000 years ago. Within the Galápagos, patterns of genetic variation in microsatellite and mitochondrial markers suggest early stages of diversification. No intra-island patterns of morphological variation were found, even across steep ecological gradients, suggesting that either (i) high levels of gene flow may be homogenizing the effects of selection, (ii) populations may not have had enough time to accumulate the differences in morphological traits, or (iii) yellow warblers show lower levels of ‘evolvability’ than some other Galápagos species. By examining genetic data and morphological variation, our results provide new insight into the microevolutionary processes driving the patterns of variation.