Exaptation

Charles Darwin seated crop

Exaptation

Exaptation is a term used in evolutionary biology to describe a trait that has been co-opted for a use other than the one for which natural selection has built it. This concept is distinct from adaptation, which refers to a trait that has evolved through natural selection for its current function. Exaptation highlights the versatility and opportunistic nature of evolutionary processes.

History

The term "exaptation" was introduced by Stephen Jay Gould and Elisabeth Vrba in 1982. They proposed it to address the limitations of the term "adaptation" in explaining certain evolutionary phenomena. The concept has since become a fundamental part of evolutionary theory, providing insights into the complex pathways through which traits evolve.

Mechanisms

Exaptations can arise through various mechanisms, including:

• **Co-option**: A trait originally evolved for one function is repurposed for another. For example, feathers in birds initially evolved for temperature regulation and later became crucial for flight.
• **Functional shift**: A trait that serves one function begins to serve an additional function. For instance, the swim bladder in fish, which originally evolved for buoyancy, later became involved in respiration in some species.

Examples

Several well-known examples of exaptation include:

• **Feathers**: Initially evolved for insulation in theropod dinosaurs, feathers were later exapted for flight in birds.
• **Mammalian middle ear bones**: These bones originated from the jawbones of early synapsids and were exapted for hearing.
• **Human brain**: Certain cognitive abilities, such as language, may have arisen as exaptations of pre-existing neural structures.

Significance in Evolutionary Biology

Exaptation challenges the traditional view of evolution as a process of gradual, linear improvement. It underscores the role of contingency and the re-purposing of existing structures in the evolution of complex traits. This concept has implications for understanding the evolutionary origins of novel features and the adaptive landscape of organisms.

Related Concepts

• Preadaptation
• Co-option (biology)
• Functional shift
• Evolutionary developmental biology

See Also

• Adaptation
• Natural selection
• Evolutionary biology
• Stephen Jay Gould
• Elisabeth Vrba

Further Reading

• Gould, S. J., & Vrba, E. S. (1982). Exaptation—a missing term in the science of form. Paleobiology, 8(1), 4-15.

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