The ecology of adaptive radiation in Darwin's finches

The ecological theory of adaptive radiation postulates that divergent natural selection between populations occupying contrasting environments, or using different resources, can drive adaptive divergence and thereby cause speciation. My thesis investigates this hypothesis in Darwin's finches of the Galapagos Islands. I use field observations, genetic tools, and statistical analyses to study patterns of sympatric coexistence, niche use, adaptive divergence, and gene flow within and among four closely-related species of Ground finches (Geospiza spp.). I also quantify possible impacts of human disturbances on this adaptive radiation. My first major finding is that the coexistence of sympatric, closely related species is facilitated by partial differences in diets (imperfect generalism), promoted by high spatial and temporal environmental heterogeneity. My second major finding represents a demonstration that a single generalist species (Geospiza fortis ) might be composed of a number of individual specialists. This individual specialization might be important in promoting the initial stages of adaptive divergence. My third major finding is of strong reproductive isolation between coexisting large versus small beak size morphs with G. fortis. This result suggests the possibility that adaptive divergence drives genetic divergence in the face of initially high gene flow for sympatric populations on a single island. My final major finding is that human activities appear to be negatively impacting adaptive divergence between the small and large beak size morphs of G. fortis. I specifically show how the diet and performance (bite force) differences that characterize the two morphs at an undisturbed site where the morphs remain distinct have degraded at a human-disturbed site where the morphs are no longer distinct. These findings confirm some of the major predictions of the ecological theory of adaptive radiation by revealing how resource (diet) differences that cause adaptive (beak) divergence promote ongoing ecological speciation in Darwin's finches. Overall my thesis illustrates how the study of ecological interactions is central to understanding the factors that promote and maintain the process of adaptive radiation.