Which of the following processes randomly effect (i. e. you cannot calculate the precise end result) the change in allele frequencies over time? (it is not all of them)
sexual selection
natural selection
genetic drift
gene flow
mutation

Mutations and Genetic Drift

Explanation:

A mutation refers to the random changes in the genome of the organisms. Being a random event, it is not possible to calculate the effect of mutations on the allele frequency of a population.  

Genetic drift refers to changes in the allele frequency by chance event alone. Since the occurrence of chance events can not be predicted, the resultant changes in the allele frequency of the population can not be calculated.