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Why do sharks and dolphins have similar external body shapes?

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Leaf types — Morphology Common leaf morphologies.

Sharks and dolphins possess similar external body shapes due to a biological phenomenon known as convergent evolution.12

This process occurs when unrelated or distantly related species independently evolve similar traits to adapt to comparable environmental pressures or ecological niches. In the case of sharks (which are fish) and dolphins (which are mammals), both lineages evolved a streamlined, hydrodynamic body shape to solve the same physical challenge: the need for high-speed motion through a marine environment.12

Because there are only a limited number of efficient physical solutions for moving quickly through water, evolution in distinct lineages often trends toward nearly identical forms. This same streamlined shape has evolved independently in other marine animals as well, including tuna, seals, and various extinct Mesozoic reptiles.2