Based on Experiment 2, what can be inferred about predation and pelvic spine frequency in the Nevada lake population?

• A. With no large predatory fish, the frequency of pelvic spines increased.
• B. Predation pressure had no effect on spine frequency.
• C. Spines increase only when there are many hiding places.
• D. The lake was initially populated by stickleback with complete pelvises, and because the lake did not contain large predatory fish, the frequency of stickleback with pelvic spines decreased over time.

Answer: (D)

Predation pressure shapes trait frequencies because defenses like pelvic spines help survive when big predators are present, but come with costs when they aren’t needed.

In Experiment 2, the Nevada lake population started with sticklebacks that had complete pelvic spines, yet the lake did not contain large predatory fish. Without that predator threat, spines no longer confer a survival advantage and can be costly to maintain (energy, mobility, or other trade-offs). Over time, natural selection favors fish with reduced or missing spines, so the frequency of pelvic-spine individuals declines. This shows how the presence or absence of predators directly influences which traits are favored.

The other options don’t fit this pattern: removing large predators would not lead to more spines being favored, the predation pressure clearly affects trait frequencies, and the idea that spines increase only with more hiding places isn’t supported by the study’s focus on predator presence.