Final answer:
The banded and unbanded island water snakes represent an example of interplay between natural selection and gene flow, though migration is not explicitly influencing allele frequencies if there is no movement between the islands. Instead, mutation and genetic drift are contributing to the variation within populations. option 2 is the correct answer.
Step-by-step explanation:
The banded and unbanded island water snakes are an example of natural selection and gene flow. When considering the details provided, the situation describes allele A being favored in population B, likely due to natural selection, and allele A has become established in population B due to gene flow. Additionally, when mentioning no emigration or immigration, the scenario suggests that the evolution occurring is due to internal factors, not migration.
However, since the question compares two different island populations of water snakes (banded and unbanded), if there is no mention of gene migration between the two islands and natural selection is acting on both populations independently, then it's not a case of migration impacting the gene pool. It's essential to note that allele B's introduction to population A due to a mutation and its increase due to genetic drift points us back to the effect of small-scale random changes in allele frequencies, not migration.
Therefore, considering all information, the correct option would be migration in opposition to natural selection if there's migration between the islands. However, if there's no migration, the population dynamics are influenced by mutation, genetic drift, and natural selection occurring within each population separately.