What happened as sticklebacks adapted to living exclusively in freshwater?

• A. They all changed identically
• B. They remained the same
• C. Over many generations, populations changed in many different ways, including in their skeletons
• D. They disappeared

Answer: (C)

Freshwater environments present different challenges and opportunities in each habitat, so natural selection pushes each population along its own path of adaptation. Over many generations, sticklebacks that move into freshwater acquire changes that better suit that environment, and those changes can vary a lot from one population to another. You’ll often see skeletal and other structural changes as traits that help with local food sources, predators, and life in still freshwater compared to the sea. Because these pressures differ across lakes, streams, and ponds, the result is diverse adaptations rather than a single uniform change.

That’s why the best choice describes a range of changes across populations, including in their skeletons. The other ideas don’t fit because the adaptations aren’t identical across all populations, they don’t stay the same once they encounter new environments, and they don’t disappear just by moving into freshwater.