How are finches in the Galapagos Islands a good examples for adaptations?

Each species of finch has a special beak adaptation.
These finches are probably the best example- in fact, they are what inspired Charles Darwin to publish his theories on evolution by Natural Selection. All of the finches are very similar in shapes and size, meaning that they must have shared a common ancestor at one point. However, they all can have very exclusive diets based on the type of beak that they have. For example, thick beaks can break seeds from trees, thin beaks can penetrate cactus thorns, and so forth. This tells us that, at one point, their common ancestors' offspring started taking on different roles based on the random mutations they had and shared. Over the generations, these mutations became genetic species traits, giving them an advantage in their environment that none of the other bird species shared.