The allele frequencies of a population are more likely to change if?

A change in allele frequency can be caused by various reasons. Mutations, recombination, gene drift, gene flow, and hybridization are some of the examples. Basically anything that would cause microevolution to happen would change allele frequencies. The Hardy Weinberg equilibrium might be helpful, too. 

The basic repeating unit of nucleic acids are known as nucleotides. a nucleotide consists of three distinct chemical groups.