Which of the following genetic phenomena leads to increased frequency of rare alleles?

The phenomenon that leads to increased frequency of rare alleles is random genetic drift. This process occurs in small populations where chance events can significantly affect allele frequencies from one generation to the next. In smaller populations, the random sampling of alleles during reproduction can lead to the fixation or loss of alleles, meaning that rare alleles can become more common simply due to chance rather than adaptive advantages.

While mutation introduces new alleles into a population, it does not necessarily increase the frequency of rare alleles on its own. Mutations can create new variations, but if those mutations are rare, they might not significantly impact the overall allele frequency unless they confer a selective advantage or accumulate over time.

Gene flow involves the transfer of alleles or genes from one population to another, which can affect allele frequencies but usually serves to homogenize populations rather than increase the frequency of rare alleles in a specific population.

Natural selection works on the premise of allele advantages; it tends to favor alleles that confer some sort of benefit to the organism's fitness in a given environment, which often means that more common alleles can become more prevalent, rather than rare ones.

In summary, random genetic drift is the key mechanism by which rare alleles can increase in frequency,