Ok but breeds literally would not exist without inbreeding and line breeding. If you're not getting consistency, which only happens from narrowing the gene pool to the point that they "breed true" it simply isn't a breed. Wild animals do this too. Many species including feral chickens breed copiously within tight family groups until there's a loss of vigor or they run over the proverbial genetic cliff on some defect or another or outside blood takes advantage of the fact that the male (usually) is weak and can be usurped.All breeds were mutts at some point, and the notion of keeping the mutts we now call breeds pure to themselves for centuries with no outcrossing is a relatively new concept in animal breeding that hasn’t yet passed the test of time. In fact, every time we see animals both wild and domestic forced to remain pure to tight lines for more than a few decades, bad things happen.
The bulldog is an excellent example. They evolved a lot between the Renaissance and the end of the 1700s, starting out like a large mastiff and ending up small but healthy and tough by the early 1800s. We know from records they were outcrossed at various times to other breeds, constantly changing some traits but keeping them healthy.
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By the mid and late 1800s they were bred pure to themselves for many decades and for a long time they were athletic and healthy.
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Then they got too inbred and unhealthy, with breeders being unwilling to add new blood into the second century of keeping them “pure.”
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Good breeding of purebred stock just mimics this, except if you're good you make sure you never even come close to the genetic bottleneck from where there is no return.