Fig. 1. Stylized topology of the classical phylogeny of Pacific, Arctic, and Atlantic gadids on a world map centered on the Arctic.

The Arctic cod and Polar cod clade is an outgroup. Pacific cod and walleye pollock are thought to be separate invasions of an ancestral Atlantic cod into the Pacific. Greenland cod is a reinvasion of Pacific cod into the Arctic and Atlantic at Greenland. Walleye pollock is a sister taxon of Atlantic cod. Walleye pollock differs morphologically [forked tail and missing chin barbel, two traits that define genera within the Gadinae as shared derived characters in a cladistic analysis (16)] and ecologically (semipelagic and schooling) from its closest relatives. Its biology represents a niche shift on the invasion [cf. (44)] of the Pacific. Atlantic cod and the Pacific invaders, Pacific cod and walleye pollock, are ecological success stories. They are dominant players in the ecosystem, and their ecologies translate into the remarkable ability of these native species in their native habitat to support the world’s largest commercial fisheries.