I once read somewhere (sorry don't remember where) that if you allowed all purebred dogs to breed together, in about 10-15 generations they would reach a consistency of type. Much like modern pariah dogs today (dingos for example). A medium sized dog, 30-50lbs, with short/plush coat, probably earth tone in colouration, generally with prick ears and a wolfy muzzle.

Despite the wondrous varieties of breeds out there, Canis familiaris is only one species. There must have been a tremendous amount of genetic material available for manipulation to have developed such a diverse array of animals from the wolf. But if left on their own, mother nature would prevail and the dog would be back in its original all-purpose type, a square body with medium size.

Cats have much less variation in their genome. Just look at purebred cats. Structure changes very little. Primarily it is coat colour or length that is different among breeds. Wild cats are not much different from domestic ones.

Cheetahs once had a massive die off about 10,000 years ago. As a result all cheetahs worldwide are so genetically similar that they have many health problems and are poor breeders.

From an observer's standpoint, new bloodstock should be welcome when breeding any animal. Genetic health and long term gain should be more of a primary concern. How to do that when you've an established line of animals, I'm not sure.

The cheeta is an intresting example. Some defend line/inbreeding with that some animals in the wild also has small genepool and they seems okey. This cases are often freak of nature it´s not common that wild animals with small genepools could stay healty for a long time. They are also living a dangerous life,if one organism "crak" the code in the immunesystem in a small genepool, this organism could then crack all animals immunesystem and wipe out a whole species. Thats why small genepools are so dangerous, every animal have unique codes in their immunesystem, but with inbreeding this code gets similar for all animals in a certain group.

This is the danger with the linebreeding many does today in dogs, if many breeders use the same popular studs, after a while all the breedingstock becomes more or less inbreed. It´s not so hard to keep the health of a breed if people didn´t overuse certain animals. One example of this is the shorthaired dachshund here in sweden where I come from. This breed suffered from diseases caused by overuse of a few dogs, which leaded to small genetic variation, but with a new breedingpolicy, using more unrelated dogs in the breeding, this negative trend could be stopped.

Another intressting example is the clumber spaniel, this breed was almost extinct after WWII, but some people found a few dogs and started to breed them. This breed had a very limited genepool, because to few dogs left when they tried to save the breed. It´s also soon started to show illness caused by the weak immunesystem and other diseases due to small genetic variation. To better the health of this breed they have recently crossed in some similar dogs of other breeds, to get more genetic variation in the breed.

With all the dogs on this earth, health issues caused by small breedingstock shouldn´t be necessary. It´s when all different breeds was created for not long ago that problems started, small genepools and poor breedingmethods is the cause to the illness we see in many breeds today. It´s no excuse for breeders of dogs that today don´t show any serious problems to don´t care about the importance of responsible breedingpolicy. when problems are starting to show, it could only take about 20 years before a breed is in serious trouble.