Evolutionary patterns of alternative life histories in fishes: assessing marine-freshwater and aquatic-terrestrial movement

Transitions between habitats driven by variation in physiological requirements at different stages of ontogeny have a wide range of effects on individuals, populations, and species as a whole. Where and when individuals can reproduce, how far they can disperse, and their physical ability to withstand particular environments affect gene flow and population size, which in turn may affect the potential for speciation and extinction. Explaining how individual-level behavior can alter diversification of species is a vast and challenging endeavor evolutionary biologists face. For my dissertation I address diadromous and amphibious behaviors in fishes. Diadromous fishes migrate between marine and freshwater systems and amphibious fishes move between aquatic and terrestrial habitats. Both behaviors restrict individuals' movements and are thought to be important in population and species level patterns. First, I use phylogenetic comparative methods in order to test the hypothesis that diadromy is an intermediate state between completely marine and freshwater life styles. I find that while in some cases diadromy seems to act as an evolutionary intermediate, it is also a state where lineages diversify before returning back to their ancestral environment. This suggests that diadromy is an important behavior in the diversification of some groups of fishes. Second, I use comparative population genetics to see if the high diversification rates found in diadromous fishes is associated with variation in population differentiation and genetic diversity. I find that there is no particular association with migratory life history and population structure in fishes. Migration life history does not predict levels of differentiation between populations nor genetic diversity within populations. Finally, I use population genetic tools to see if the terrestrial restrictions of amphibious behavior affect population structure. Assessing a mudskipper from East Asia, Periophthalmus modestus, I find that movement and connectivity of populations is driven by connectivity of land. Large, diverse continental island populations are likely the result of vicariance and small, less diverse oceanic island populations are likely the result of rare colonization events. These findings show that habitat restrictions are important in shaping patterns of biodiversity in fishes.