Proof is in the product. Many if not most of the most successful producers of gamefowl invest considerably in understanding how breeding systems can be used to promote improvement and sometimes simply maintenance of a quality line. The do not simply breed the best to best. They also do a lot of testing, especially when making crosses involving matings from different lines as sometimes best of best with such an approach can result in crap. For what I do which is not limited to games by a long shot, I must look at many matings and compare offspring of those matings. It is not just about quality of an individual, it is also about quality of an individuals siblings and offspring. Individuals that are good and have quality kin get major consideration over others where only an individual is good. To make the required, comparisons you need to be able to track genealogies and sometimes use a little math, Parties so vested also have enough interest in the heritabiltiy of other traits as that can sometimes be employed as a tool confirming parentage. With some parties not engaging in single matings using good confinement, that would also be a good tool. I do not know of too many people interested in any activity that brag about or refuse to learn more about what they are doing. The genetics can also be used to confer genealogies of birds not from your hands.