How do marine mammals and fish differ in the way they move through water?

Although fish and marine mammals both live in water, they move in different ways because their anatomy evolved independently. Most fish propel themselves with side-to-side tail beats and body undulations. Dolphins and whales, by contrast, use vertical tail flukes and powerful muscles that drive an up-and-down motion. Sea lions and seals often use flippers for steering and can combine swimming styles depending on the species. This is a clear example of convergent evolution producing similar solutions from different ancestors.