Explanation:
Natural selection as proposed by Darwin is a selection pressure which operates in a population and allow the best fitted genotype to survive in changing environmental conditions and eliminate the other genotype which are not fit
Causes of natural selection include: Genetic drift, mutation(changes allelic frequency) and sexual selection
In the given example, condition necessary for natural selection to occur on the mouse population is traits that affect the survival of the mice vary among individual mice, in this condition superior genotype will get the opportunity to reproduce and inferior genotype does not get opportunity to reproduce thus their genotype in population does not increase rapidly and hence they become eliminated leading to genetic drift