Three possible effects of a split on migration. (A) Graphical view of migration between A and B1,2 after the split of B. The ability of individuals from B1 or B2 to migrate to A (and vice versa) is exactly the same as before the split. (B) An alternative way for a split. Now, only migration between B1 and A is possible. (C) A second alternative showing uneven contact between A and B1,2.