This dissertation explores the causes and consequences of range expansion and human-mediated introduction in marine mollusks. In chapter 1, Phenotypic consequences of range expansion in two intertidal snails, I investigate the phenotypic variation across latitude and wave-exposure gradients in Nucella lamellosa and Nucella ostrina. I also compare present-day forms to ancestral forms (both fossil and midden) in order assess phenotypic differences across time and space in two intertidal gastropods with different expansion histories following the Last Glacial Maximum. In Chapter 2, Latitudinal and microgeographic clines under plastic control in two Nucella rocky shore gastropods, I assess the relative contribution of plasticity to the phenotypic differences I find across wave-exposure and latitudinal gradients in Nucella lamellosa and Nucella ostrina by raising northern and southern populations from different wave-exposure levels under identical conditions and measuring phenotypic variation in the second generation. Finally, in Chapter 3, Bivalve invasions are asymmetrical in geography and niche expansion, we explore the prevalence of niche expansion and the factors that promote invasion success in a set of 120 alien bivalves.