Actually, I think I can put some light into how mutations occur. The enzymes that do the DNA copying and editing are nearly perfect but not exactly perfect. There is a certain degree of miscopying (I don't remember the exact number but it is close to 1/10000000 DNA base pairs copied. Now, if you think that there are billions of base pairs in the DNA of any living organism (of course, there will be a variation in the amount of DNA from organism to organism, but a prototypical one) then you have thousands of spontaneous mutation per generation. Besides that, you have to add the mutations that are produced by environmental impact (UV light from the sun, chemicals, radioactivity, virus), and the amount increases. That happens in the somatic (in the body cells) as well as in germ line cells (in our case, ovules and sperm). Now, many mutations don't do anything or do little, many are harmful (if they are harmful enough that kill the new formed embryo right away and nobody knows that they even happened), and a few give an advantage to the new formed individual. That is my attempt to summarize evolution at the molecular level. I hope this is helpful, especially to diehard.