In this single group we can see wide variation in appearance and in life-style. Darwin provided what I believe to be an essentially correct interpretation of how the finches came to be the way they are. A few individuals were probably blown to the islands from the South American mainland, and today's finches are descendants of those pioneers. However, while Darwin saw the finches as an example of evolution, we can now recognize them merely as the result of recombination within a single created kind. The pioneer finches brought with them enough genetic variability to be sorted out into the varieties we see today.
Mutations are mistakes in the genetic copying process.
Each living cell has an intricate molecular machinery designed for the copying of DNA, the genetic molecule. But as in other copying processes mistakes do occur, although not very often. Once in every 10,000-100,000 copies a gene will contain a mistake. The cell also has machinery for correcting these mistakes, but some mutations still slip through. Some have no effect at all.