At the southern tip of the island of Santa Cruz in the Galápagos, a gorge known as Las Grietas is home to a species of parrot fish: a brilliantly colored creature about 18 inches in length. The pool where the fish live was created long ago, when large waves spilled over the island's raised edge and into a deep crevice. Today it is refreshed by rainwater that seeps through the porous volcanic rock that forms the steep gorge. Through it all, the small parrot fish population has been evolving in the pool, its water so clear one can see through more than 65 feet to the vertebrates lurking at the bottom.