Which is the best evidence of an evolutionary relationship between two organisms?

In natural selection, the finches on the Galapagos Islands. The bug population greatly decreases, and only the finches with the strong beaks were able to survive as they could access the nut meats encased in hard shells. However, when a drought swept through the area, the trees didn't bear any nuts, and so the finches with the larger beaks died off and the finches with the slimmer beaks survived as they could reach the bugs deep in the crevasses of the tree bark.