A particular order of birds, called Struthioniformes, is a group of ground dwelling birds with large legs and no keel bone. Both the ostrich and emu are members of this order. The ostrich, living on the African plains is a large bird with only two toes. The emu living in New Guinea (an island) is a smaller bird with three toes. Genetic evidence suggests these two birds come from a common ancestor. Which factor listed below most likely caused the ostrich to have two toes and the emu to have three toes?