Why are finches in the Galapagos islands a good example of speciation

All the finches in the different islands share a common ancestor, which is a group of finches that initially migrated into the islands and conquered the different islands. Due to the different ecologies in the different islands, adaptive radiation of the finches occurred and hence allopatric speciation. This is why finches in different islands have different types of beaks that are adapted to the food resources in the respective islands.