Thinking outside the barrier: neutral and adaptive divergence in Indo-Pacific coral reef faunas

Population divergence in marine species frequently occurs where there is no conspicuous geographic feature to mark a population boundary. In the absence of a discernible barrier to gene flow, strong conclusions about the processes that drive population divergence in the ocean are often elusive. Because our knowledge of patterns far outpaces our understanding of processes, there is a need to expand hypotheses regarding population divergence in marine species. Here, I posit that certain population-level processes (range expansion, spatial diffusion constraints and metapopulation dynamics) are primary causes of spatial structuring in marine populations rather than auxiliary factors that aid divergence across an incomplete physical barrier to gene flow. In particular, I use examples from Indo-Pacific coral reef organisms to illustrate this point. In connection, spatial patterns of adaptive divergence are unlikely to be correlated with patterns of selectively neutral divergence in many cases, as the processes affecting adaptive and neutral genetic markers are different.