A swim bladder is a gas-filled sac that helps fish maintain buoyancy. Evidence indicates that early fish gulped air into primitive lungs, helping them survive in stagnant waters. The evolution of the swim bladder from lungs of an ancestral fish is an example of
a. An evolutionary trend.
b. Paedomorphosis.
c. The gradual refinement of a structure with the same function.
d. Exaptation.

Verified step by step guidance

1. Understand the concept of exaptation: Exaptation refers to a trait that evolves for one function but is later co-opted for a different function. In this case, the lungs of ancestral fish evolved to help them survive in stagnant waters but were later adapted into swim bladders for buoyancy.
2. Analyze the evolutionary process described: The problem mentions that the swim bladder evolved from lungs, which indicates a change in function over time. This aligns with the concept of exaptation.
3. Compare the other options: a) An evolutionary trend refers to a general direction of change in traits over time, which is not specific to the functional shift described here. b) Paedomorphosis refers to the retention of juvenile traits in adult organisms, which is unrelated to the transformation of lungs into swim bladders. c) Gradual refinement of a structure with the same function does not apply because the function changed from respiration to buoyancy.
4. Focus on the correct answer: The transformation of lungs into swim bladders represents a shift in function, which is characteristic of exaptation.
5. Conclude that the correct answer is d. exaptation, based on the evidence provided and the definition of the term.

Key Concepts

Evolutionary Trend

An evolutionary trend refers to a general direction in which a species or group of organisms evolves over time, often leading to increased complexity or adaptation to specific environments. In the context of the swim bladder, it highlights how certain traits can become more pronounced or refined as species adapt to their ecological niches.

Exaptation

Exaptation is a process in evolution where a trait that originally evolved for one function is co-opted for a different purpose. The swim bladder's evolution from primitive lungs exemplifies exaptation, as the original function of gas exchange in lungs was repurposed to aid in buoyancy control in aquatic environments.

Gradual Refinement

Gradual refinement refers to the slow and continuous process of change in a biological structure or function over generations. This concept is crucial in understanding how the swim bladder evolved from lungs, as it illustrates how small, incremental changes can lead to significant adaptations that enhance survival in specific habitats.