Environmental confirmation of multiple ice age refugia for Pacific cod, Gadus macrocephalus
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- Bigg, G.R. Evol Ecol (2014) 28: 177. doi:10.1007/s10682-013-9662-y
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The concept of species surviving through quaternary climatic extremes by retreating to glacial refugia, and then evolving genetically during re-population movements of the following interglacial, has been in the literature for over 40 years. Recently, advances in genetic analysis have enabled this concept to be validated and theories regarding population expansions and contractions to be built. For the major Northern Hemisphere species of cod, Gadus morhua (Atlantic cod) and Gadus macrocephalus (Pacific cod), genetic analysis has suggested retreat to separate refugia on both sides of their respective ocean basins during the last glacial period. Ecosystem niche modelling has previously confirmed that environmental conditions during the last glacial were compatible with the existence of these separate refugia for Atlantic cod. Here it is shown that such modelling also confirms a reduced core glacial distribution for G. macrocephalus, but probable refugia on either side of the Pacific. Existing mitochondrial DNA analyses suggest two separate glacial populations in the northwest Pacific, which modelling confirms, with predicted separate marine refugia in the land-locked Sea of Japan basin and the Sea of Okhotsk. Existing mitochondrial DNA for the northeast Pacific populations is less conclusive regarding whether there were one or two separate refugia off this coast, and their location. Using environmental niche models this study shows the glacial NE Pacific environment could support two marine refugia, one centred in the Aleutians/Gulf of Alaska and the other off British Columbia. The intervening Cordilleran Ice Sheet, and the glacially sub-aerial and ice-free Queen Charlotte Islands shelf, is hypothesised to have constrained exchange between glacial stocks either side of the Islands. The postulated southern marine refugium is off-shore from an established terrestrial refugium, suggesting greater dependence of species and ecosystems during environmental change. An earth system approach to evolutionary change could enhance understanding of past and future ecosystems.