The late Cenozoic radiation of the Pacific salmon and trout (Oncorhynchus spp.) resulted in five species that are widely distributed in western North America and occupy distinct parts of river networks. The dramatic radiation of the Pacific salmon contrasts with the history of the Atlantic salmon, which have evolved little since the divergence of Oncorhynchus. Conventional hypotheses for the radiation of the Pacific salmon stress geographic isolation due to Pleistocene glaciations, but paleontological evidence dates their speciation to the middle Miocene to early Pliocene. Tertiary marine cooling may have spurred the development of anadromy in salmon and other fish, but does not readily explain the different evolutionary trajectories for the Pacific and Atlantic salmon. The timing of Pacific salmon speciation corresponds to significant physiographic changes around the Pacific Rim, suggesting that increased topographic diversity due to an active tectonic regime contributed to the evolution of the Pacific salmon.

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