Colin says "man made mutation", I say "selective breeding". No different than thoroughbred race horses or pure breed dogs. Man has been doing it since the dawn of time!
At some point, through a twist of genetics a fish demonstrates a different color, body shape, fin shape, or some other interesting characteristic. Finding it interesting and/or attractive, breeders will line breed that fish to produce more of the same. A very many of the colorful and interesting fish we have in the hobby are the result of selective line breeding.
And so it goes...
Genetics is an amazing thing. I had some red and pineapple swordtails. I put three trios of reds in my 110g stock tank. Some pineapple fry were added from my 60g. They are colony breeding and I have reds, pineapples, sunsets, red wags, pineapple wags, and just maybe a few greens. I now have 100+ fish that periodically go to the local fish store a couple dozen at a time.