Final answer:
Selection acts as an agent of evolutionary change when traits increase an organism's fitness, meaning its ability to survive and reproduce, thereby becoming more common in the population due to natural selection.
Step-by-step explanation:
For selection to be an agent of evolutionary change, the traits considered must increase fitness. In the context of evolution, fitness refers to an organism's ability to survive and reproduce, passing on its alleles to the next generation. Traits that increase an organism's fitness are likely to be favored by natural selection, because they help the organism to have more offspring, which inherit those beneficial traits.
Beneficial mutations increase the fitness of an organism, allowing it to have more offspring that carry the advantageous traits. Over time, these beneficial alleles can become more common in the population due to natural selection. In contrast, neutral mutations do not affect fitness and thus do not influence selection. Similarly, deleterious mutations lower fitness and tend to be removed from the population over time through selection. Consequently, traits must contribute to an increase in fitness—through improved survival and/or reproductive success—to serve as a mechanism by which selection drives adaptive evolution.