Which statement about fossil stickleback data and Pitx1 is most plausible?

• A. The pelvic spine loss occurred about 10 million years ago.
• B. Pitx1 was not involved in pelvic spine loss.
• C. Left-side asymmetry cannot be linked to Pitx1.
• D. The pelvic spine loss occurred in a lake population about 10 million years ago.

Answer: (D)

The main idea is how ecology, fossils, and genetics come together to explain a trait. Pitx1 is a regulatory gene that, when its pelvic expression is reduced, leads to loss of the pelvic spine in sticklebacks. Fossil data show pelvic spine reduction in lineages that lived in freshwater lakes, not in marine populations, which tells us where the change happened. Putting those pieces together, the most plausible scenario is that pelvic spine loss arose in a lake-dwelling lineage and occurred around ten million years ago, a time frame that fits the divergence between marine and freshwater sticklebacks. This integrates the physical evidence from fossils with the genetic mechanism and the evolutionary timeline. The other options either omit the ecological context, declare Pitx1 uninvolved without support, or deny a link between Pitx1 and the observed asymmetry, which runs contrary to what the fossil-genetic pattern suggests.