The white whales, dolphins (Delphinidae) and porpoises (Phocoenidae) together comprise the superfamily Delphinoidea, which are of likely monophyletic origin. Genetic evidence suggests the porpoises are more closely related to the white whales, and that these two families constitute a separate clade which diverged from the rest of Delphinoidea within the past 11 million years.[8] Fossil evidence shows that ancient white whales lived in tropical waters. They may have migrated to Arctic and sub-Arctic waters in response to changes in the marine food chain during the Pliocene.[9]