A few birds of a particular species arrive on a small island from a neighboring larger island. There are new food plants available on the small island, and after several generations, the average beak size and shape in the population on the small island is slightly different from the population on the larger island. About once a year, a few more birds from the neighboring larger island arrive and join the population on the small island, and a few birds from the small island migrate back to the larger island. If you wanted to ensure that the population on the small island became a separate species from the population on the larger island, you should:
prevent any more birds from migrating between the two islands
transfer more birds from the larger island to the small island each year
transfer more birds from the small island to the larger island each year
provide many new foods to the bird populations on both islands to give them a chance to adapt to many different types of food
transfer some food plants from the larger island to the small island each year