What is a key limitation of natural selection?

The correct answer is that natural selection acts on existing variation within a population. This means that for natural selection to occur, there must already be some diversity in traits among individuals. It's important to understand that natural selection does not create new traits; rather, it selects from the variations that already exist. For example, if a population of birds has individuals with different beak sizes, natural selection can favor individuals with beak sizes that are more suitable for accessing food in their environment.

This limitation is significant because it highlights the role that genetic variation plays in evolution. If a population lacks sufficient genetic diversity, natural selection has fewer options to work with, which can impede adaptability and survival as environmental conditions change. As a result, while natural selection is a powerful mechanism of evolution, it fundamentally relies on the pre-existing variations within a population.