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Traditionally, the Galapagos sea lion and Japanese sea lion were classified as subspecies of the California sea lion. However, a genetic study in 2007 found that all three are in fact separate species. The lineages of the California and Japanese sea lion appear to have split off 2.2 million years ago during the Pliocene. The California sea lion differs from the Galapagos sea lion in its greater sexual dimorphism. The Steller sea lion is the closest extant relative of the "Zalophus" sea lions, being a sister taxon.