SEA-LEVEL CHANGE MEDIATES GENETIC DIVERSIFICATION OF COASTAL FISHES

Geologic and climatic processes shape the physical world in which species evolve. Genetic data can illuminate the evolutionary consequences of how species respond to those processes. Coastal estuaries are an excellent system in which to study the interplay of physical and biological processes, as coastlines migrated substantially during glacioeustatic sea-level change. We performed GIS-based paleohabitat modeling from San Francisco, USA to Nayarit, Mexico using coastal geomorphic parameters to infer regions permissive to tidal estuary formation from the Last Glacial Maximum to present at kyr-scale resolution. Using genetic data of 524 individuals from three codistributed fish species inhabiting those estuaries we assessed diversity and population structure. Finally, we used a Bayesian statistical framework to test the degree to which the paleohabitat-defined extirpation-recolonization scenarios explain the genetic patterns of these fish species relative to alternative hypotheses.

Results indicate there was a dramatic (83%) reduction in estuarine habitat at glacial lowstand relative to today, with only three lowstand refugia along the Pacific Coast from San Francisco, California to Point Cabo, Mexico (~2,000 km). Incorporating regional tectonic and sediment processes reveals that tectonically steepened (narrow) coastal shelves host fewer, smaller lowstand refugia compared to broader shelves, and that steep-coast refugia produce more genetically distinct populations than those along broad (wide) coasts. Habitats formed during postglacial sea-level rise host populations that are genetically mixed from proximal refugial sources.

In summary, we propose a framework in which tectonic and sediment processes control regional shelf topography, and that topography dictates where estuarine habitats form. As sea level changes against that heterogeneous topography, it eliminates and isolates populations to produce distinct genetic groups. This sea-level change mechanism operates globally, and could have contributed to diversification of coastal species on deeper timescales through changes in global ice volume or large-scale tectonic events.