What is most likely to happen to a population of mice that becomes isolated on an island? Many different species will form as they evolve and fill different niches on the island.
The population of mice will adapt to life on the island but will still be able to breed with the mainland population.
Many different species that are capable of breeding with each other will form.
The population of mice will evolve into a new species of mouse that is adapted to all the different niches that are found on the island.