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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Exaptation refers to the evolution of a structure or trait for one function, and its subsequent use for another function. In this case, the swim bladder evolved from lungs that were originally used for breathing, but later became adapted for maintaining buoyancy. Changes in Hox gene expression are involved in differentiating body regions during development, but would not explain the evolution of the swim bladder. Paedomorphosis refers to the retention of juvenile traits into adulthood, and adaptive radiation refers to the diversification of species into different ecological niches, neither of which applies to the evolution of the swim bladder.

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