yes somethings do weed themselves out. no matter how tough & high their pain tolerance, a dog that has to run 100+ miles a week chasing fast critters like coyote, that dog won't last long w/ HD. likewise before the invention of quickclot such dogs w/ vonwilbrands would have died in the field on bear or cat or even just hitting a barbwire fence, which is why there is none known in fox & coonhounds, because young dogs like young men are reckless. dogs w/ autoimmune diseases that couldn't fight off infection the same. so yeah, depending on the job, some things do weed themselves out.