There are 4 types of molly that I know of, sailfin, lyretail, balloon, and the common black molly. I'm not sure if my yellow (24-karat gold) babies are sailfins or common yellows. The black ones are a result of selective breeding and do not exist in nature, or so I'm told. I've also seen an albino type molly, they are almost white. I have some yellow babies with a few black spots as a result of mixed breeding, I think they're called tiger mollies. Their siblings, born at the same time, don't have spots and are just yellow. (Different dads?) I also have dalmation mollies. They are all black with white spots. I found my original yellow mating pair at a local PetSmart. Different color mollies have different behaviors, the black ones love my penny wort plants, and the yellows swim near the top alot, and their babies school together, while the black babies are alone often. Very cool to watch.
So, as a result of selective breeding, we have many types and colors of mollies. I try to take some of my extra babies to a pet store and purchase an adult from the store so that in-breeding is minimized. I have read that in-breeding makes them less desirable (I don't know if that means that the babies will be dumb or have genetic problems, or unnatural colors.). Also, selective breeding (to get the prettiest fish) adds to their health problems by squashing their guts even more than they already are in nature, so they need frozen peas once a week in their diets to help keep them regular so they don't get constipated (Yes, it's happened to mine) and die. I've also heard that peas help prevent swim bladder disease. I thaw a plate of peas, about 1 for each fish, and pop them out of their skins, and chop them into tiny pieces, and watch the fish go nuts. They have all been very healthy ever since I started the addition of peas in their diet.
I love my mollies.
There is alot of information available, here's two;