Final answer:
The frequency of a slightly deleterious allele can be maintained by mutation-selection balance, while genetic drift can cause rapid changes in allele frequencies, particularly in small, isolated populations such as those on islands.
Step-by-step explanation:
The frequency of a slightly deleterious allele maintained at an equilibrium frequency by mutation-selection balance would be higher, indicating that it has reached a stable frequency within the population. Typically, slightly deleterious alleles are removed from a population through natural selection; however, if mutation continuously reintroduces the allele into the gene pool, it can be maintained in the population. This balance is a delicate equilibrium between the rate of mutation introducing the allele and the rate of selection removing it.
On the other hand, genetic drift can also play a role in changing allele frequencies, especially in small populations. For example, on an island where populations are often smaller and more isolated compared to the mainland, genetic drift can lead to more rapid changes in allele frequencies because each breeding event can have a larger impact on the overall genetic makeup of the population. Therefore, genetic drift would likely happen more quickly on an island.