Yes, as with any selective breeding program (cattle, dogs, flowers, etc.), the goal is to improve on the desired traits - health, look, reproduction, etc. You wouldn't purposely breed a skinny and ugly ray if you have a higher quality ray available. Also, through some natural selection the strongest and largest rays will likely breed first leaving the less healthy rays unbred. The best breeders have a strong understanding of genetics and and understanding of their livestock and they are able to produce outstanding results.
Some people are more lucky than good and that works too.