Pelvic reduction in sticklebacks is studied because:

• A. It provides a clear example of adaptation to different environments
• B. It is the only trait that varies
• C. It occurs in freshwater lakes but not oceans
• D. It has no genetic basis

Answer: (A)

Pelvic reduction in sticklebacks shows how natural selection can sculpt anatomy when populations encounter different environments. Across many freshwater lakes, descendants of marine ancestors repeatedly evolve smaller or absent pelvic structures, illustrating parallel evolution driven by the distinct ecological pressures of freshwater habitats. The reduced pelvis likely reflects an adaptive trade-off: in these environments, the energy and drag costs of maintaining pelvic spines can be higher than the benefits, so individuals with smaller or missing pelvic bones tend to leave more offspring. This convergence across independent populations makes it a classic example of adaptation to different environments.

It's not the case that this is the only trait that varies, nor that it only happens in freshwater and never in oceans. In addition, there is a genetic basis for the change—regulatory changes affecting a developmental gene (Pitx1) alter pelvis formation—so the trait is heritable and can respond to selection.