Originally Posted by jeffegg2
All modern Betta's are some kind of deformity and not natural selection. I'm sure anything harmful will make the fish less likely to survive and therefore not likely to reproduce.
Breeding for longer pectoral or caudal fins is not going to adversely affect the health of the fish unless it is taken to the extreme.
The basic shape of a betta has remained pretty much the same. I have wild bettas and comparing them side-by-side with my domesticated splendens, I can still tell they belong to the same species.
By shortening the body, you would undoubtedly be selecting for all the attributes that a responsible breeder normally culls for. It would take a lot of ruthless culling to produce a consistent line of shorter-bodied fish, and I assume a number of those fish culled would have severe enough deformities that they could not be passed onto pet homes.
The trouble with domesticity is that we have removed the need for natural selection. Therefore, these fish can continue to propagate, and continue to pass on their 'bad' genes to future generations.
It's like how the fins of a swallowtail male guppy prevent him from being able to spawn. In the wild this gene would probably have died out, or become much scarcer, but by crossing a normal finned male onto a swallowtail female, we are able to keep the swallowtail gene in existence.