A central goal in evolutionary biology is to understand the origins of biological diversity and the associated factors that promote speciation. The study of island species has provided many important insights, by showing how microevolutionary processes may drive the early stages of diversification (Clegg et al., 2002a,b; Emerson, 2002; Arbogast et al., 2006; Warren et al., 2006; Illera et al., 2007; Ricklefs & Bermingham, 2007; Grant & Grant, 2008; Phillimore et al., 2008; Miláet al., 2010). Some of the most studied insular taxa are those in the Galápagos archipelago. Extensive molecular studies carried out on its endemic birds have provided temporal resolution for divergence times and evolutionary trajectories. For example, the ancestor of Darwin’s finches is estimated to have arrived about two to three million years ago (Grant & Grant, 2008) – Galápagos mockingbirds two millions years ago (Arbogast et al., 2006), whereas the ancestors of Galápagos hawks, magnificent frigatebirds and yellow warblers, are estimated to have colonized the archipelago <300 000 years ago (Bollmer et al., 2006; Browne et al., 2008; Hailer et al., 2010). Yet, the evolutionary outcomes for these groups have been surprisingly variable: at one extreme is the radiation of finches with more than thirteen species and four species of mockingbirds and at the other limited intraspecific variation in morphology among the populations of Galápagos doves (Santiago-Alarcón et al., 2006). This degree of variation in evolutionary outcomes of colonizing taxa is not new to archipelagos. The Hawaiian honeycreepers and thrushes are examples of ancestors arriving to Hawaii at similar times, yet having undergone completely different evolutionary trajectories, one leading to yet another spectacular radiation of more than 50 species of honeycreepers, whereas thrushes have evolved into a mere four species (Lovette et al., 2002). These patterns raise important questions about island speciation including, How much time after colonization is required for genetic and morphological variation to arise in natural populations? What are the respective roles environments and geographic features play in morphologic diversification? How do different taxonomic groups respond to the same amount of time since isolation and novel environmental conditions? A good starting point to address these questions is to study young island species. Such studies provide insight into microevolutionary processes in diversification, and factors, such as how time since colonization, geographic isolation and environmental conditions might influence genetic and phenotypic divergence (Clegg et al., 2002a,b; Emerson, 2002; Warren et al., 2006; Arbogast et al., 2006; Ricklefs & Bermingham, 2007; Illera et al., 2007; Grant & Grant, 2008).
Here, we examine one such recent colonization to the Galápagos, the endemic yellow warbler. The yellow warbler arrived to the Galapagos between 10 000 and 300 000 year (Browne et al., 2008) allowing one to examine whether time since colonization would be sufficient to produce morphological divergence as might be expected based on other young island systems (Clegg et al., 2002a). Yellow warblers are commonly found on almost every island in the archipelago and across steep environmental gradients from dry forest in the lowlands to wet cloud forests in the highlands. Under these circumstances, phenotypic variation might potentially be expected to arise first, by drift alone among isolated islands if gene flow is reduced and second, within islands if selective forces operate, despite gene flow, along an environmental gradient (Endler, 1977; Grant et al., 1985; Smith et al., 1997; Schneider & Moritz, 1999; McCormack & Smith, 2008).
The objectives of this study are threefold: (i) to identify the mainland source populations of aureola yellow warblers and confirm arrival time estimates; (ii) to quantify genetic population differentiation and investigate the probable colonization route that yellow warblers may have used to reach the Galápagos and Cocos Islands; and (iii) to explore the patterns of morphologic variation across islands and habitats to examine evidence for differentiation.