Consider a population of finches originally observed on one of the Galapagos Islands about fifty years ago. At the time, it was determined, based on anatomical traits, that all of the finches in this population were a single species. You revisit this population and observe that there is still only one species of bird on the island but that this species is significantly different (shorter beaks, different plumage coloring) than the one observed fifty years ago. You know that there have been no migrations onto or off the island. What kind of evolution is this an example of