A framework for comparing processes of speciation in the presence of gene flow

How common is speciation-with-gene-flow? How much does gene flow impact on speciation? To answer questions like these requires understanding of the common obstacles to evolving reproductive isolation in the face of gene flow and the factors that favour this crucial step. We provide a common framework for the ways in which gene flow opposes speciation and the potential conditions that may ease divergence. This framework is centred on the challenge shared by most scenarios of speciation-with-gene-flow, i.e. the need for coupling among different components of reproductive isolation. Using this structure, we review and compare the factors favouring speciation with the intention of providing a more integrated picture of speciation-with-gene-flow.