In nature, survival of the fittest weeds out the weak. In nature if only 1 individual of a spawn survives to maturity, and spawns with another fit individual, that would be success.
In aquarium breeding, often times, many more survive than are beneficial to the long term health of the species.
And in line-bred fish such as EBJDs, selection is based almost solely on color, nothing else.
This at times, sacrifices and overlooks those genes that promote good growth and health, immunity, and robustness.