Erik B. Dopman1, Kerry L. Shaw2, Maria R. Servedio3, Roger K. Butlin4,5 and Carole M. Smadja6 1Department of Biology, Tufts University, Medford, Massachusetts 02155, USA 2Department of Neurobiology and Behavior, Cornell University, Ithaca, New York 14853, USA 3Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599, USA 4Ecology and Evolutionary Biology, School of Biosciences, The University of Sheffield, Western Bank, Sheffield S10 2TN, United Kingdom 5Department of Marine Sciences, University of Gothenburg, Gothenburg 40530, Sweden 6Institut des Sciences de l'Evolution de Montpellier ISEM, Universite de Montpellier, CNRS, IRD, Montpellier 34095, France Correspondence: erik.dopmantufts.edu

Coupling has emerged as a concept to describe the transition from differentiated populations to newly evolved species through the strengthening of reproductive isolation. However, the term has been used in multiple ways, and relevant processes have sometimes not been clearly distinguished. Here, we synthesize existing uses of the concept of coupling and find three main perspectives: (1) coupling as the build-up of linkage disequilibrium among loci underlying barriers to gene exchange, (2) coupling as the build-up of genome-wide linkage disequilibrium, and (3) coupling as the process generating a coincidence of distinct barrier effects. We compare and contrast these views, show the diverse processes involved and the complexity of the relationships among recombination, linkage disequilibrium, and reproductive isolation, and, finally, we emphasize how each perspective can guide new directions in speciation research. Although the importance of coupling for evolutionary divergence and speciation is well established, many theoretical and empirical questions remain unanswered.