The bottleneck effect is used to describe an event where a population decreases. One example involves the northern elephant seal. They have a reduced genetic variation due to a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000, but they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted. Which phrase best explains this change in gene frequency

The phrase that will  best describe this change in gene frequency is the 'Genetic drift'.

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.

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