Hey, Blue nocturne, I answered your question ages ago with lots of information on the corruption of DNA, but you refused to read it because it was several links (about SEVEN different links stating the same thing actually)
When DNA replicates, it's not always 100% perfect, and error occurs during this point and it creates a mutation. This mutation can be small, like having an extra thumb, or it can be large, like having extra immunities or an extra arm or super-dense muscles.
Now, through this error, can you put 2+2 together and understand that a new trait can be created completely by accident, from the corruption of the DNA?