Which statement best describes the trajectory of the stickleback population in the ancient lake now known as the Truckee Formation?

Which statement best describes the trajectory of the stickleback population in the ancient lake now known as the Truckee Formation?

• A. Initially the lake population had no pelvic spines, and spines increased.
• B. Initially the population had pelvic spines, but over 15,000 years the frequency shifted to fish without pelvic spines.
• C. Pelvic spines remained the same frequency over time.
• D. Spines fluctuated with no overall trend.

Answer: (D)

The key idea is that evolutionary change in a trait often tracks changes in the environment, not a straight line in one direction. Pelvic spines in sticklebacks are a classic example of a trait that can be favored or disfavored depending on ecological conditions like predator pressure, lake level, and resource costs. In the ancient lake captured by the Truckee Formation, those conditions likely shifted many times over thousands of years. At some times, having spines would help fend off predators and be advantageous; at other times, the costs of maintaining spines or a different predator lineup could make unspined fish more successful. When the environment swings back and forth, the frequency of spined versus unspined fish can rise and fall rather than move steadily in one direction. That leads to a trajectory characterized by fluctuations with no single overarching trend, which is why the best description is that spine frequency varied over time without a consistent directional change.

The key idea is that evolutionary change in a trait often tracks changes in the environment, not a straight line in one direction. Pelvic spines in sticklebacks are a classic example of a trait that can be favored or disfavored depending on ecological conditions like predator pressure, lake level, and resource costs. In the ancient lake captured by the Truckee Formation, those conditions likely shifted many times over thousands of years. At some times, having spines would help fend off predators and be advantageous; at other times, the costs of maintaining spines or a different predator lineup could make unspined fish more successful. When the environment swings back and forth, the frequency of spined versus unspined fish can rise and fall rather than move steadily in one direction. That leads to a trajectory characterized by fluctuations with no single overarching trend, which is why the best description is that spine frequency varied over time without a consistent directional change.