Answer:
The correct answer is - natural selection.
Explanation:
The change of an allele's frequency or microevolution in a particular population due to its fitness benefit it provides to the individuals of the population can be caused by the natural selection process. Relative reproductive success is a measure of Fitness.
The change of an allele's frequency in deleterious traits mostly decreases in a population and this process is known as adaptive evolution. There is no or very small number of relative reproductive success present.
