| Summary: | Environmental variation is ubiquitous in natural systems. The genetic and physiological mechanisms governing population-level responses to this variation will impact the process of speciation and the capacity for populations to persist in a changing climate. Until recently, population-level responses to environmental selection remained largely unexplored in marine systems due to the historical assumption that the inherently dispersive nature of most marine taxa would preclude their ability to specialize to local environments. This dissertation represents the first investigation of population-level responses to environmental variation in a Caribbean reef-building coral. This research integrates ecological, physiological, genetic and genomic methods to (1) determine patterns of local adaptation in the Florida Keys, (2) identify stressors driving adaptive responses, (3) distinguish the physiological and genetic mechanisms underlying coral adaptation and (4) assess the potential for future adaptation in the common reef-building coral Porites astreoides. Results demonstrate that corals adapt and/or acclimatize to their local habitat and that this specialization incurs fitness costs. Temperature differences between reefs likely play a selective role in differentiating inshore and offshore coral populations. Genetic and gene expression differences indicate that coral hosts play a substantial role in driving these population-level differences. Inshore corals exhibit greater gene expression plasticity, which may be involved in stabilizing physiological responses to temperature fluctuations experienced at inshore reefs. In addition, naïve juvenile coral recruits from inshore reefs exhibit a growth rate advantage over offshore recruits under elevated temperature treatment, suggesting that thermotolerance differences observed in adult populations could continue to evolve in response to climate change. Taken together these results provide novel insight into the drivers of reef decline in the Florida Keys and the role of the host in coral adaptation capacity. === text
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