> This breed doesn't suffer health problems due to inbreeding, because it isn't a (UK) Kennel Club breed
The paper aligns with what you say, it remarks that Collies are highly inbred but not suffering that much from it. Can you explain the significance of not being a Kennel Club breed? Can that be the whole story, though?
> There were interesting exceptions to the correlation of inbreeding and health. The Border terrier, Basenji, Collie, and English setter breeds have high inbreeding but low morbidity. Likewise, the Malinois, Pomeranian and Russian Tsvetnaya Bolonka (Russian Toy) have lower inbreeding and high morbidity. These example breeds are neither brachycephalic nor particularly known for extreme morphologies.
> In the case of healthy breeds with high inbreeding, it may be possible that these breeds have been purged of deleterious alleles as has happened with inbred mouse strains
> In the opposite situation, the recorded morbidities could be high allele frequency Mendelian diseases or potentially conditions linked to phenotypes under selection in the breed. These discrepancies could also exist due to population differences between the insurance data and the inbreeding data.
Well, the Kennel Club rates breeds in terms of their conformance to "spec", which is a lot about appearance. Inevitably, breeders breed to the spec. Border Collies aren't bred to any spec. (Collies are a different breed, which I believe is rated by the Kennel Club)