Global climate change has intensified the need to assess if, and how, natural populations adapt to abrupt shifts in their environment. The tempo of adaptation in natural systems has been the subject of theoretical and empirical investigation for decades. Recent evidence from genome-wide sequencing approaches has indicated that evolution may proceed at a pace previously deemed theoretically impossible. Such studies, however, have largely observed these processes in the context of model systems, and the extent to which these patterns will hold in ecologically-relevant species subject to the dramatic environmental perturbations associated with global change is unclear. Accordingly, this thesis investigates the capacity for, and mechanisms by which, the Mediterranean mussel, Mytilus galloprovincialis, may rapidly adapt to expected declines in global seawater pH. Reductions in seawater pH constitute a global change stressor impacting marine species globally, with anticipated impacts altering the structure and services of numerous ecological communities. Due to its experimental tractability, as well as its ecological and economic importance, M. galloprovincialis has become a model-species for exploring the physiological and morphological impacts of low pH seawater. Yet, the extent to which evolution may offset observed phenotypic consequences is unknown. To address this knowledge gap the present thesis explores the following: (i) the processes shaping and maintaining variation in low pH tolerance across the species’ native range; (ii) the extent to which the standing variation within natural populations of M. galloprovincialis can facilitate the magnitude of evolution necessary for persistence under global change conditions; and (iii) the molecular basis of low pH adaptation in marine bivalves and beyond. My results elucidate how contemporary gradients in pH variability shape distinct patterns of low pH plasticity across natural populations. Furthermore, my findings demonstrate that the standing variation within natural populations is sufficient for rapid adaptation to even extreme reductions in seawater pH. Lastly, I provide mechanistic links between the molecular mechanisms influenced by shifts in the external seawater pH environment and fitness-related abnormalities observed in M. galloprovincialis, a finding that likely explains observed low pH sensitivity across a broad range of marine metazoans. This thesis thus lends to our conceptual understanding regarding the dynamics of rapid adaptation in natural populations, while explicitly informing the management of an ecologically and economically important marine species as global change progresses.