To try to answer Blue Dragon's question...
Mutations can occur naturally in the wild. They may be good ones (such as a new color that blends into the background better) or bad ones (such as flies having curly wings making it impossible to fly), easily observed (again, a new color) or somewhat hidden (such as a better sense of smell). Pure albinos, having NO pigmant at all, is seen in almost every species. Partial albinism is also seen. If a specimen is lucky enough to survive in the wild (a white or red snake would be more easily spotted by a preditor) and reproduce at least once, it's offspring would carry that heterozygous gene and also pass it on to it's offspring. Someone catches one of these individuals and starts breeding it in captivity. By chance, or on purpose, it is bred to another snake carrying the same hidden gene. Suddenly, the person gets this hatchling that is homozygous for this mutated gene and shows it's effects (an amel, an aneryth, a motley, etc.) Now the person tries to reproduce it by inbreeding, and then...you eventually get the wide variety of corn snakes we now have, and there are probably more to come.