Domestication of the Dog, Part I, The

Phi Kappa Phi Forum, Summer 2004 by Tiffany-Castiglioni, Evelyn

Next week I shall take my dog to the veterinary clinic at Texas A&M University for her check-up. She is a black half-Labrador retriever with a little white on her muzzle and toes. Her mother was my neighbor's yellow Lab, and her father was a passing stranger. My dog has not had many health problems, aside from one recurring ailment diagnosed as "dietary indiscretion," though several years back she had an unusual dermal cyst. The latter condition is so breedspecific that it confirmed that her father was part Rhodesian Ridgeback. I had seen him in the neighborhood, a brown mutt with the hair along his spine growing forward instead of back, hence forming a "ridge." In the waiting room we shall see other dogs, possibly a small, trembling Chihuahua that nevertheless barks fiercely at us, possibly a large, muscular Rottweiler with hip problems.

All this variety raises questions. How can these dogs with such pronounced differences in size, weight, coat, body shape, and disposition be the same species? And, how did humans and dogs form the unique and varied inter-species living arrangements that we have now? For thousands of years we have manipulated dogs' genes, first to domesticate them from their wild ancestors, and then to produce specialized breeds. The ancestral wolf, the concept of species, and a modern reenactment of domestication are the focuses of this column.

IS THE WOLF THE SOLE ANCESTOR TO THE DOG?

With all of the variety in dog breeds today, did dogs have single or multiple ancestral populations? It may seem intuitive that any domestic species must be derived from selective breeding of a handful of founder animals, because multiple derivations would require that later in their natural history the separate lines would still be similar enough to be the same species. However, such is not the case with pigs (Sus scrofa), which were domesticated separately in Europe and Asia about nine thousand years ago, and horses (Equus caballus), which appear to have been domesticated from two or more separate wild horse populations about six thousand years ago. In dogs, the idea of multiple ancestors (including both wolves and jackals), or at least multiple independent domestication events from one species, is attractive because it seems compatible with the diversity of modern dog types. Nevertheless, powerful new genetic approaches provide a firm answer that dogs were domesticated from wolves in a single event.

Evolutionary biologists have long favored the idea that the ancestor of dogs was the wolf (Canus lupus], though until very recently, the idea could not be ruled out that other members of the dog genus, such as jackals, were part of dog ancestry. However, in 2002, Dr. Peter Savolainen at the Royal Institute of Technology in Sweden and his colleagues presented unusually strong evidence (Science, Vol. 298, p. 1610) that dogs were domesticated from the grey wolf in Eastern Asia, possibly China, about 15,000 years ago. The group counted mutations in mitochondrial DNA, which is found in the energy-generating structure in the cell called the mitochondrion and is inherited only maternally. They studied DNA samples from 38 Eurasian wolves and 654 dogs from Europe, Asia, Africa, and Arctic America. The founders of all modern dog species were probably three female wolves.

The date is significant because dogs were widespread around the globe by nine thousand years ago, as evidenced by prehistoric art and fossil remains, and therefore had dispersed rapidly after their domestication, presumably because of their usefulness to migrating human populations. No new domestication event of dogs took place in the New World, according to Dr. Jennifer A. Leonard and Dr. Robert K. Wayne at UCLA and Dr. Carles Vilas at Uppsala, Sweden (Science, Vol. 298, p. 1613; 2002), who studied DNA from ancient dog bones taken from archéologie sites in Mexico, Peru, and Bolivia. Therefore, dogs may have accompanied human colonizers of the Americas as early as 12-14,000 years ago. New Eurasian dogs were later brought to the Americas by colonists, but those dogs still were of Eurasian wolf origin. Subsequent breeds of dogs have arisen all over the world, mainly in the last few hundred years, by intensive selective breeding for desired traits, but all the breeds are still one species.

HOW CAN CHIHUAHUAS AND GREAT DANES BE ONE SPECIES?

Species is a human, not a canine, concept, and it is defined in the simplest terms as a group of animals that can interbreed to produce fertile offspring. Chihuahuas and Great Danes are the same species because they can readily interbreed to produce fertile offspring, whereas horses and donkeys are different species because they produce infertile mules if they interbreed. It is easy to explain why such crosses between dissimilar dog breeds are fertile: they carry the same genes, and their chromosomes look identical. It is likewise easy to explain why mules are infertile: their parents do not have matching chromosomes. Horses have sixty-four chromosomes, and donkeys have sixty-two that are structurally rearranged compared with the horse. As a result, mules cannot make eggs and sperm that are fertile.