What would be a sign that natural selection is at work in a population?

• A. All individuals always survive
• B. The environment remains constant
• C. The frequency of certain heritable traits changes over generations
• D. No new mutations occur

Answer: (C)

The key idea is that natural selection changes the genetic makeup of a population over time. It happens when there is heritable variation and individuals with certain traits leave more offspring than others. Because those successful traits are passed on, their frequency in the population increases across generations. So observing that the frequency of specific heritable traits shifts over generations is direct evidence that natural selection is at work.

For example, if a population has variations in a trait and the environment favors one form, that favored form becomes more common over time as it contributes more offspring. On the other hand, if everyone always survives, there’s no differential reproductive success to drive a change in trait frequencies. If the environment stays the same, selection can still occur if there’s heritable variation and differential success, but the hallmark to look for is the actual change in trait frequencies over generations. The absence of new mutations doesn’t stop natural selection from acting on existing variation.