The population on the first island would be unchanged. The population on the second island would change to avoid predation. The population on the third island would stay the same or become even more like the unpalatable species.
Work Step by Step
With no predators or minimal predation, as on the first and third islands, there is no known driver of evolutionary change. On the second island, however, there is a specific motivation to become more difficult to hunt. Without the protection of a similar-looking but unpalatable species, the flies on this island are easily preyed upon.