Adaptation to Climate Change: Contrasting Patterns of Thermal-Reaction-Norm Evolution in Pacific vs. Atlantic Silversides

How organisms may adapt to rising global temperatures is uncertain, but principles can be derived from studying physiological trait variations across existing spatial temperature gradients in nature. Many ectotherms, particularly fish, show increasing genetic growth capacities and efficiencies with latitude, but whether these constitute adaptations to gradients in mean temperature or seasonality is unclear. Using a new, large-scale comparative approach, we contrasted thermal growth reaction norms between Pacific (Atherinops affinis) and Atlantic silversides (Menidia menidia), two ecologically equivalent fishes occurring across weak (Pacific) vs. strong (Atlantic) latitudinal temperature/seasonality gradients. Novel evidence for countergradient growth variation (CnGV) in A. affinis challenges the paradigm that CnGV evolves mainly in response to strong seasonality gradients. More importantly, growth CnGV patterns directly mirrored the differences in environments encountered along the two coastlines. We thus posit that CnGV is the primary mode of reaction-norm evolution in ectotherms in response to even weak spatial – and by inference temporal - climate gradients, likely leading to a gradual poleward migration of growth-genotypes and a further broadening of species ranges, which is consistent with empirical evidence across taxa worldwide.