Non-migratory lineages experience higher diversification rates: global evidence from salmonid fishes

Biologists have long sought to understand the forces driving biological speciation. Contrasts in dispersal ability (e.g., migration) are hypothesised to influence rates of lineage diversification. However, past inferences of migration-diversification relationships have been contradictory, suggesting the need for further investigation and more nuanced explanations. Here we test for a relationship between diadromy (marine-freshwater migration) and diversification in salmonids, a dominant group of fishes native across the temperate Northern Hemisphere. We reconstruct a molecular phylogeny comprising 140 species and apply State-dependent Speciation Extinction models to quantify diversification. Diversification rates are higher in non-diadromous relative to diadromous salmonid lineages, echoing the findings of a recent study of homologous southern temperate freshwater fishes, but conflicting with global studies incorporating tropical lineages. Climate appears to be a key factor explaining diadromy-diversification relationships, with the negative relationship observed in temperate lineages likely reflecting increased isolation and speciation of non-diadromous lineages following recolonisation of glacially perturbed habitats. In contrast, the positive diadromy-diversification relationships observed in studies spanning tropical fishes may reflect the role of diadromy in facilitating colonisation of stable, low-competition habitats.