A swim bladder is a gas-filled sac that helps fish maintain buoyancy. The evolution of the swim bladder from the air-breathing organ (a simple lung) of an ancestral fish is an example of
A) an evolutionary trend.
B) exaptation.
C) changes in Hox gene expression.
D) paedomorphosis.
E) adaptive radiation.

Question

Quizplus · Accepted Answer

The swim bladder evolved from the air-breathing organ of an ancestral fish through exaptation, which is the evolution of a structure for one function that is later co-opted for another function. In this case, the gas-filled sac that originally helped fish breathe air was later adapted for use as a buoyancy organ. The swim bladder is not an evolutionary trend, as it is not a shared characteristic that has evolved in a linear fashion across multiple species. Changes in Hox gene expression and paedomorphosis are not directly relevant to the evolution of the swim bladder. Adaptive radiation may be relevant if multiple species evolved different swim bladder structures in response to different ecological niches, but the question only refers to the evolution of the swim bladder in general.

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