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. 

B] nitrogen fixationmight be wrong, so correct me.want extra on your answers? just go on my account and go on questions and answer, or ask in the comments!