A swim bladder is a gas-filled sac that helps fish maintain buoyancy. The evolution of the swim bladder from lungs of an ancestral fish is an example of

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The swim bladder is believed to have evolved from the ancestral fish's lungs, which were originally used for breathing. This transformation is an example of exaptation, in which an existing structure takes on a new function over time.

A Swim Bladder Is a Gas-Filled Sac That Helps Fish