Technical Abstract: Interactions between selection and gene flow can determine whether populations are able to adapt to local environmental conditions. To examine the roles of selection and gene flow on local adaptation, we studied populations of the clonal perennial, Iris hexagona. Along the Gulf Coast, I. hexagona occurs in freshwater and brackish conditions. We hypothesized that brackish marsh populations should be genetically distinct from freshwater populations, and would be better adapted to saline conditions. To test this, we conducted a common garden experiment using salinity as the agent of selection, quantified the genetic structure of natural populations, and estimated gene flow among them. We discovered that: 1) freshwater and brackish populations were significantly different based on microsatellite markers; 2) Salinity was a strong selection agent because it significantly reduced the number of flowers produced and delayed flowering phenology; 3) There was no evidence that populations were locally adapted to environmental salinity based on their similar performance in freshwater and brackish treatments. Although populations exhibited genetic structure, we propose that environmental salinity is too unstable to represent a directional selection pressure. Coastal habitats are under flux because of climate change-induced sea-level rise and hurricanes, which can rapidly and unpredictably alter local conditions.