Darwin observed that the finches on two different islands in the Galapagos belong to the same family but showed a lot of variations in their beak size and shape how can this variation be justified

The differences in the beak size and shape of two finches that belonged to the same family is due to the differences in the environments in the two islands where each of the finches lived. The differences in environment produce different results during evolution. One trait is probably favorable when living in one island and the different trait observed in the finches living in the other island is advantageous with respect to the environment of the second island.