What effect does natural selection have on the allele frequency of a population?

Natural selection is the process where individuals of a population are deemed unfit to survive or that more than one variation of individuals are deemed fit to survive and prosper. Depending on the situation, the allele frequency may be reduced or increased. Though, as mutations occur constantly, the allele frequencies of any population is ever changing.

I think it may cause micro evolution (aka a change in the allele frequencies), over time with fitness-increasing alleles becoming more common in the population over generations.