Final answer:
In order for Natural Selection to work on a particular population, variation, fitness, and selective pressure must be present.
Step-by-step explanation:
Natural selection can only take place if there is variation, or differences, among individuals in a population. Importantly, these differences must have some genetic basis, otherwise natural selection would not lead to change in the next generation because there would be no way to transmit those traits from one generation to the next.
Additionally, natural selection occurs when there are differences in fitness among members of a population. Some individuals pass on more genes to the next generation, causing allele frequencies to change.
Furthermore, for natural selection to work, there must be selective pressure, such as a shortage of food, a predator, or a mating preference. The survivors of this selective pressure must be able to reproduce and pass on their DNA to their offspring.