Divergence in the sea: comparative population genetics of north Atlantic marine invertebrates

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The origin of new species remains the central and most elusive question in evolutionary biology. In marine environments, the problem of genetic divergence and speciation is especially complex because the barriers to genetic exchange that are generally essential for allowing divergence to occur are far more subtle than in terrestrial systems. Nevertheless, species can and do form rapidly in the sea. I use population genetics to identify the fundamental processes that drive genetic diversification and speciation in aquatic organisms. By comparing patterns of genetic structure among several species, I ask whether the ability to disperse as larvae in the plankton provides the best explanation for the patterns observed. Does high dispersal potential lead to high population connectivity and reduce the likelihood that any one population will become genetically distinct through processes like random genetic drift or natural selection? Or, has the history of growth and geographic distribution left a lasting signature on the present-day patterns of genetic structure? If so, how important were these past events to the accumulation of genetic differences among populations or to the formation of new species? To isolate the evolutionary influences of biology and history I focus on the marine invertebrate fauna of the north Atlantic because many of these species have been profoundly influenced by recent patterns of glacial advance and retreat throughout the Pleistocene. During this time much of the marine intertidal community of the northwest Atlantic was completely replaced by colonizers that dispersed across the Arctic from the Pacific or the northeast Atlantic. These new arrivals brought with them new genetic material, and by applying coalescent theory I will use DNA sequences to look back in time and reconstruct the processes driving modern day patterns of genetic structure. This research will provide valuable insights into the mechanisms driving the accumulation of genetic divergence among populations and isolate the relative importance of both physical and biological factors to the process of speciation in the sea.