Which statement best describes the relationship between predator pressure and pelvic spine presence in these stickleback populations?

Which statement best describes the relationship between predator pressure and pelvic spine presence in these stickleback populations?

• A. Higher predator pressure correlates with more complete pelvic spines.
• B. Higher predator pressure correlates with fewer pelvic spines.
• C. Predator pressure has no effect.
• D. Pelvis presence is determined by mineral content.

Answer: (A)

Predation pressure shapes defensive traits through natural selection. In sticklebacks, the pelvic spine acts as a defense against predators that target the body. When predator pressure is high, individuals with a more complete pelvic spine have a better chance of surviving and reproducing, so the trait for a full spine becomes more common in the population. Conversely, in environments with low predator pressure, the spine can be costly to maintain (energetic costs, drag, reduced swimming efficiency), so there’s less selective advantage to keeping it, and populations tend toward reduced or absent spines. This explains why higher predator pressure is associated with more complete pelvic spines. The other ideas don’t fit the pattern: saying predator pressure leads to fewer spines would imply the spine is a net disadvantage under risk, which isn’t supported by the protective role these spines provide; and attributing pelvis presence to mineral content ignores the adaptive response to predation.

Predation pressure shapes defensive traits through natural selection. In sticklebacks, the pelvic spine acts as a defense against predators that target the body. When predator pressure is high, individuals with a more complete pelvic spine have a better chance of surviving and reproducing, so the trait for a full spine becomes more common in the population. Conversely, in environments with low predator pressure, the spine can be costly to maintain (energetic costs, drag, reduced swimming efficiency), so there’s less selective advantage to keeping it, and populations tend toward reduced or absent spines. This explains why higher predator pressure is associated with more complete pelvic spines.

The other ideas don’t fit the pattern: saying predator pressure leads to fewer spines would imply the spine is a net disadvantage under risk, which isn’t supported by the protective role these spines provide; and attributing pelvis presence to mineral content ignores the adaptive response to predation.