Which best describes a reduced pelvis in sticklebacks?

• A. A range of structures from a simplified girdle with no pelvic spines to an oval nub for a girdle
• B. A pelvis with two spines
• C. An enlarged girdle with spines
• D. No pelvis exists in reduced form

Answer: (D)

The concept here is evolutionary loss of a structure. In many stickleback populations, pelvic structures are reduced as an adaptation, and in some cases the pelvis is completely absent rather than just being smaller. So the best description of a reduced pelvis is that no pelvis exists in reduced form, reflecting a full loss rather than a partial reduction. The other options imply that some pelvic material remains or even that the girdle is enlarged, which doesn’t fit the common pattern of pelvic reduction in sticklebacks.