Adaptive divergence in contiguous populations of Darwin's Small Ground Finch (Geospiza fuliginosa)

Abstract

Hypothesis: The ecological theory of adaptive radiation predicts divergent morphological adaptations to different environments, and different evolutionary trait utilities in varying environments. Organism: Darwin’s Small Ground Finch (Geospiza fuliginosa). Times and places: Santa Cruz Island, Galapagos Archipelago, in two study areas of different habitat type: highland forest (c. 500 m elevation) and lowland arid zone (c. 50 m elevation). The two contiguous populations of Darwin’s finches were separated by a flight distance of 18 km. Data were collected from both study areas between November and March in the years 2000, 2001, 2002 and 2004. Analytical methods: This natural experiment examined two contiguous populations for evidence of the first phase of speciation, namely adaptive divergence in morphological parameters between environments. We used mist-netting to capture and then measure birds between habitat types, and observational methods to record foraging behaviour. Foraging behaviour was recorded as a single observation per bird that was observed along transects through the study plots. Morphology data for beak and foot dimensions were analysed separately, and as principal components factor scores, and were compared between habitats and years. Results: Birds in the lowlands of Santa Cruz Island differed significantly from birds in the highlands in foot dimensions (longer foot and claws) and beak size (shorter beak). Lowland birds spent more time foraging on the ground, and used the foraging techniques of picking and chipping at prey with their beak, and scratching the ground with their feet, whereas highland birds tended to forage in low vegetation, sliding their longer beaks through vegetation to remove seeds. This study is the first to document adaptive divergence in clinal populations of Darwin’s finches. These findings highlight the role that adaptive response to ecological gradients has over relatively short distances in contiguous populations. The findings also generate testable hypotheses about species richness in lineages that show morphological differences within clines.