Researchers use variation across postglacial lakes to test how environmental conditions influence pelvic reduction.

• A. Environmental conditions have no effect on pelvic traits
• B. Pelvic reduction is identical in all lakes
• C. The variation across lakes supports predictions about environmental influences
• D. Only genetic drift drives pelvic reduction

Answer: (C)

Variation in environmental conditions across postglacial lakes creates different selective pressures on pelvic morphology in sticklebacks. Pelvic reduction often arises when the benefits of maintaining a pelvic girdle are outweighed by costs in a given lake environment, so populations exposed to different ecological factors—such as predation risk or resource availability—tend to diverge in their pelvic traits. When researchers compare multiple lakes with distinct ecological conditions, finding that lakes predicted to favor reduced pelvic structures actually show more reduction, while others show less, demonstrates that the environment is shaping this trait. That pattern matches the expectation that environmental influences drive morphological differences.

The other ideas don’t fit this pattern: if environmental conditions had no effect, we'd expect little variation across lakes; if pelvic reduction were identical everywhere, there’d be no ecological signal to follow; and if drift were the sole driver, the changes would be random rather than aligned with environmental differences.