Final answer:
In the absence of microevolutionary forces in a one-island model, an allele's frequency can either become fixed due to genetic drift or tend towards an average between the island and mainland allele frequencies due to migration. Fixation can occur randomly and is not dependent on the allele frequency surpassing a certain value like 0.5.
Step-by-step explanation:
When considering the one-island model in the context of population genetics and microevolutionary forces, we focus on what happens to an allele's frequency on the island in the absence of forces like natural selection, mutation, genetic drift, and migration. Based on the principles of the Hardy-Weinberg model, allele frequencies remain constant from one generation to the next unless evolutionary forces are at play. However, in an isolated island scenario where other evolutionary forces are not present, genetic drift can have a significant impact on allele frequencies, leading to the possibility of an allele becoming fixed in the population over time, especially if the population size is small.
Genetic drift tends to be more pronounced in smaller populations, such as on an island, as opposed to larger mainland populations. Over time, this can lead to one of two outcomes described in the choices provided: option 2, where the allele frequency will tend to average out between the island and the continent due to migration, or option 4, since genetic drift can result in any allele becoming fixed, even if its frequency is less than 0.5. However, the fixation here is due to random chance rather than it reaching a specific frequency threshold.