The phrase that will best describe this change in gene frequency is the 'Genetic drift'.
Step-by-step explanation:
A random change in allele frequencies as a result of chance from one generation to next in a finite population is called Genetic drift. Genetic drift can occur as a result of certain circumstances like genetic bottleneck or founder's effect. Genetic drift is more intense in smaller finite population. It can bring about significant change in the allele frequencies and make an allele fixed in a population.
Thus the best phrase to define a change in allele frequency should be Genetic drift.