Q: If dogs are descended from...

Q: If dogs are descended from the wolf, where did all the dog breeds come from? --R.S. Dogs are members of the canidae family, which also includes wolves, coyotes, jackals and dingoes. The dog broke away from the others as it became less aggressive and was attracted to humans and their life style. The ancestral dog propagated by natural selection, and such dogs are still seen worldwide in socially and economically deprived urban areas, even in the United States. These dogs are small sheep dogs of the collie-shepherd type, with upright ears; they are yellow, light-brown, white or a combination of those colors. The length of their hair varies, and they weigh between 40 and 70 pounds. The ancestral dogs appeared to have fewer skeletal or genetic problems than the breeds of today, no doubt because of their propagation by natural selection.

About 12,000 BC in China and 6,000 BC in Europe and northern Africa, humans started to breed the ancestral dog selectively, and breeds were developed that now vary in size from 2 1/2 pounds to more than 200 pounds. Three general types of dogs emerged: herding, sporting and companion dogs. Their wide range in size, shape and structure predisposes the dogs of today to a variety of skeletal and metabolic problems. For instance, in small dogs, the body surface area exceeds the body mass; this can result in excessive loss of body heat. That is why small dogs cannot tolerate cold. Giant breeds, on the other hand, have large body mass relative to body surface and cannot effectively eliminate heat, so they often suffer from heat prostration and collapse in high temperatures. To a great extent, body weight, limb length and muscle power determine the skeletal diseases that affect the various breeds. Large breeds often suffer from hip dysplasia and degenerative joint diseases; small breeds tend to get bowed limbs and intervertebral disk and knee problems.

Some dog breeds are so abnormal in structure and function that they could not survive without man. The Chihuahua is too small, the bulldog lacks movement skills, and the size of the Great Dane and Saint Bernard limits their survival ability. Bigger is not better; increased size and height can lead to shorter lives and may be related to incidences of cardiovascular failure.

When humans began to change the size and shape of the dog, the result was an abundance of inferior animals able to survive only with the assistance of people. Interestingly, when domesticated dogs go feral (wild), they revert to packs, and in time, propagation by natural selection returns to the species many of the characteristics of the ancestral dog.

Dr. Clarke welcomes pet-care questions for use in this column, but regrets she cannot answer mail personally. Send your questions to Pet Doctor, Home magazine, Los Angeles Times, Times Mirror Square, Los Angeles 90053.