Evolution, specifically mutation.
Remember Darwin's birds on the Galapagos. Well, those birds are examples of new species coming from an old one.
See, once a group of birds left Ecuador and landed on one of the islands. In order to survive, they mutated to fit the island's lifestyle.
Later, birds from that island migrated to the others, and repeated the process. Soon, there will multiple species of finches, living together. All because of one finch.
So look for mutation or migration in your answer