This picture of the Galapagos finches explains geographic isolation. In this example the species of finches lived together on the island. Over time they spread to separate islands and started to adapt there. Changes in how the finches looked changed slowly. The beaks of the finches were the main thing that changed. Every island had slightly different environments, so the birds in those environments with the best beak shape for eating food and living survived and reproduced. Their children would get the beak shape that worked best and reproduce and the finches without a good beak would die. On every island the birds had different shaped beaks. All of this was caused because the same species of birds split onto different islands and were geographically isolated.