Here's an important thing to keep in mind. SELECTION is the main power behind evolution. The reason certain animals are well suited to their environments is because of natural selection. INDIVIDUALS who are not well suited to their environments, or any environment, are far less likely to procreate. But selection works best when there is ample DIVERSITY to select from. The constant random recombination of genes is one of Nature's most brilliant and elegant mechanisms. DIVERSITY and SELECTION work hand in hand to create the miraculous variety of living organisms on Earth.
Breeders play God, or Nature. We decide who procreates or not and with whom, according to criteria of our own making. Shouldn't we have enough respect for the intricacies of biology to follow, to the best of our abilities, the principles of Nature that are already established? For decades breeders have been eliminating whole families and pedigrees because there are siblings with disease somewhere. This has served only to eliminate both the bad and the good genes in those families. The equivalent in Nature would be to kill off an entire wolfpack or a litter if one wolf or cub was less than fit. And that would of course be ridiculous, but it's what many breeders have done for decades.
This might be useful if all siblings were the same, as COI and breeder culture suggests, and as inbreeding strives to produce, but obviously they are not. There is, however, only very limited inbreeding in Nature and there are multiple procreative mechanisms that favor the broadest diversity possible. Nature emphasizes diversity precisely so there are always some offspring that have a chance of survival in many different environments.
It's important to consider pedigrees when breeding, but if every purebred dog with a relative who was sick for one reason or another was removed, no gene pool would be viable. It's a backward way of thinking to look at pedigrees and automatically eliminate healthy dogs based on a few sick relatives. If a pedigree shows risk, that's what health testing is for, and why it's important to wait for healthy dogs who come from risky pedigrees to mature. Some diseases are late onset - or appear AFTER a dog reaches breeding age. In such a case, one strategy is to have one litter while young, and then wait till those offspring mature before having another.
It's also very important to be open about what's found in pedigrees. Often newer breeders and pet owners have no way of knowing what is in a pedigree and think everything will be fine, especially if there's health testing, usually for some unrelated condition. Breeders don't talk about problems often because perfectly healthy relatives will be tarred with the same brush by meanspirited gossip. And sometimes there aren't any problems, but meanspirited gossips will allude to one thing or another without proof or specifics.
In our breed, the Poodle Heath Registry is meant to provide both proof - since they only accept vet documents - and openness, since they are public. So unless I hear about sickness directly from a breeder or owner, or something is listed on PHR, I can only consider it gossip. Too many times I have tried to run down rumors only to find they were half truths or fiction. Such cases only served to ruin the reputation for reliability of the person who gossiped.
So while pedigree analysis is important, analysis of FACTS, not rumors, is most important when looking at pedigrees, and analyzing individuals should weigh at least as heavily as their pedigrees. All pedigrees have risk - and those risks should help determine when and to whom an otherwise sound dog should be bred. One of the great benefits of the new Genetic Diversity Test is that we have been able to see precisely how different siblings can be - which shows that just because one relative has a problem, it does NOT mean another will.
Hopefully an added benefit of the test will be that more people can be more open about health issues in individuals because it ensures that no intelligent person will assume family members will automatically have the same disease. There's no accounting for those who cannot understand or who are unwilling to learn. But this way we will be able to preserve the healthiest from each family, just as nature would do.
Ironically we have seen large movements in various breeds where a deadly disease is found and whole lines are eliminated by the most caring and stringent breeders. The remaining dogs tend to be those of breeders who refused to eliminate dogs, often amidst much clucking and consternation, and their lines survived precisely because of these breeders' unwillingness to respond to peer pressure. Imagine how much more diversity there would be in these breeds if individuals but not lines were removed.
Happily we have the tools now both to preserve diversity and to select from it. There will never be a guaranteed method to produce perfect dogs, but we can help them stay healthier by mirroring Nature.