Inversions and the origin of behavioral differences in cod

How does adaptation manage to occur in the face of overwhelming gene flow? One popular idea is that the suppression of recombination, for example the fixation of a chromosomal inversion, can maintain linkage disequilibrium between groups of locally adapted alleles that would otherwise be degraded by gene flow. This idea has captured the imagination of many geneticists and evolutionary biologists, but we still have only a basic understanding of its general importance. In this issue of Molecular Ecology, Kirubakaran et al. ( 2016 ) examine the role of recombination suppression in a particularly fascinating example of adaptation in the face of gene flow: the evolution of migratory differences between interbreeding populations of cod. Along the north coast of Norway, two types of cod breed in the near‐shore waters: a ‘stationary’ form that lives near the coast year round, and a ‘migratory’ form that lives far offshore and only returns to the coast to breed. Using a combination of approaches, Kirubakaran et al. ( 2016 ) deftly demonstrate that the migratory form has completely fixed two adjacent inversions containing a suite of genes closely connected to migratory behaviour and feeding differences. This work provides an excellent example of how recombination suppression can facilitate adaptive divergence, and helps us understand the geographic and temporal scales over which genomic structural variation evolves.