Final answer:
Natural selection can lead to a population that is enriched for a particular trait through directional selection.
Step-by-step explanation:
Natural selection can lead to a population that is enriched for a particular trait through the process of directional selection. This occurs when the environment changes, selecting for phenotypes at one end of the existing variation. For example, in the case of butterflies with different alleles for a pigmentation gene, if the habitat changes due to predation, butterflies with alleles that result in pigmentation that blends well with the new habitat will have a higher survival rate, increasing the frequency of those alleles in the population.
Similarly, in the case of a hypothetical butterfly population, if its habitat changes, such as the flora within which they live becomes darker, the darker pigmented butterflies will likely become more common following natural selection forces.