Riverscape Genetics: Insights Into the Drivers of Divergence in Coastal Brazilian Fishe

The movement of organisms in spatially structured landscapes is affected by constraints imposed by geographic and physical properties of the environment, and by the response of the organisms to this environment (i.e., ecological requirements). Freshwater environments, especially rivers, are known for imposing stronger movement constraints than terrestrial and marine environments. These constraints are associated with the isolation of different river drainages and the properties of a river itself, such as shape and water flow. Because of these unique characteristics of riverine landscapes (riverscapes), our understanding of neutral demographic processes in these environments is still lacking relative to that of other environments. This dissertation research aims to help fill this knowledge gap by advancing the understanding of the effects of riverine environments on neutral demographic processes. I combine simulated and empirical data to ask how riverine basins over spatial scales (i.e., local and regional) and temporal scales (i.e., present and past) interact with organisms to promote the observed patterns of genetic diversity in freshwater fishes. The Brazilian coastal drainages are an ideal area for this study as a series of isolated basins that were cyclically connected and disconnected because of Pleistocene sea level changes lead to a great diversity of endemic fishes. In my dissertation, I first demonstrate that paleodrainage structure during the Pleistocene is the main factor explaining population genetic differentiation in one species. Then, I give insights about how riverine landscapes and their physical properties (including during past time periods) structure genetic diversity within drainages. Finally, I used a comparative approach to elucidate whether sea level changes in coastal areas affected the freshwater community as a whole, or if responses were species-specific. The work presented here advances knowledge pertaining to the evolution of freshwater fishes, particularly those of the Neotropics. Overall, by exploring relevant hypotheses in order to identify processes that structure genetic variation within and between basins and species, my dissertation distinguishes the evolutionary mechanisms operating at different spatial and temporal scales, and provides insights into patterns of genetic diversity in freshwater fishes, especially along the coastal Brazilian basins.