What is likely to happen if a sab-species evolved in isolation on one island later spreads to an island occupied by another race of the same species? If the two population have not been isolated for long and differ in only minor ways, they may interbreed freely and so merge with each other but evidence from the study of insects suggests that if two population have been isolated for a long time, so many hereditary differences will have accumulated that their genes will not combine well. Any hybrid offspring will Got survive as well as the parent types. Hence natural selection will tend to intensify the gap between the two forms, and they will continue to evolve into two distinct species.