Final answer:
Selection is an agent of evolutionary change when traits are heritable, allowing advantageous traits to be passed on to subsequent generations. Natural selection favors traits that enhance survival and reproductive success. Over time, these traits increase in frequency within the population, leading to evolution.
Step-by-step explanation:
For selection to be an agent of evolutionary change, the traits considered must be heritable. This implies that the traits have a genetic basis, allowing them to be passed on from one generation to the next. These traits usually appear through gene mutations or gene flow, contributing to the genetic variability of a population.
Natural selection acts on this genetic variation when certain traits provide some individuals with an advantage over others, particularly in terms of survival and reproduction. As a result, organisms with advantageous traits are more likely to survive and have more offspring. Over time, these traits can reach a high frequency in the population, especially if they code for favorable adaptations.
For instance, consider an environmental pressure such as the presence of a predator. Individuals with traits that enhance their ability to avoid predation—be it through speed, camouflage, or mimicry—will have a higher probability of surviving and reproducing. Gradually, these traits become more common within the population, reflecting the process of evolution acting on the population level, rather than on individuals.