Evolution of codfishes (Teleostei: Gadinae) in geographical and ecological space: evidence that physiological limits drove diversification of subarctic fishes

Aim To develop a holistic biogeographical history of codfishes in the subfamily Gadinae based on historical relationships, ecological niche, and evolution of physiological tolerances. Two alternative diversification scenarios were tested in two co‐distributed, Northern Hemisphere clades: (1) clade ancestors were temperate, and environmental niche has been conserved over evolutionary time, implying that speciation was driven by vicariance associated with ice sheet formation; and (2) clade ancestors were Arctic, and species convergently adapted to temperate environmental conditions, implying that speciation was driven by repeated adaption to temperate environments. Location Northern Hemisphere Arctic and subarctic oceans. Methods Fifty‐five new sequences of four genes from 23 tissue samples were combined with 10 GenBank sequences to generate a time‐calibrated phylogenetic hypothesis. Combining the phylogeny with information on species' ecological niche tolerances inferred from correlational models, I reconstructed ancestral environmental tolerances of each of the focal clades. These results were combined with Bayesian area‐based biogeographical analysis and regional palaeoclimatic history to develop a holistic biogeographical history of Gadinae. Results Of 18 environmental variables describing species' tolerances to salinity, temperature, sea ice concentration, and mixed layer depth, only mean, maximum and minimum sea bottom temperature, and mean and minimum sea surface temperature showed phylogenetic signal across Gadinae. Both ecological niche and geographical distributions of gadine fishes are largely conservative, but two clades contain both Pacific and Atlantic species. Focal clade divergence time estimates suggest a Pliocene origin for both, with further Pleistocene divergence. Main conclusions Reconstructed ancestral environmental tolerances of crown cods and tomcods support a temperate origin of both groups. The timing of diversification of these two clades and the intolerance of temperate species to sea ice suggest that cyclical Arctic ice formation drove divergence. Future sea ice reduction may have dramatic consequences for distributions and persistence of commercially important species when currently allopatric temperate species come into secondary contact.