The Domestication of the Dog, Part I
Tiffany-Castiglioni, Evelyn, Phi Kappa Phi Forum
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 breed-specific 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 archeologic 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.
More puzzling is that dogs and wolves have nearly identical genetic material and identical chromosomes, and they can interbreed to produce fertile offspring, yet we classify them as different species. Indeed, all members of the dog genus--dogs, wolves, coyotes, and jackals,--have seventy-eight chromosomes and can interbreed to produce fertile offspring. The definition of species among animal behaviorists additionally includes the likelihood that groups of animals would naturally interbreed, which may be limited by geographical isolation, differences in social behavior, and the incongruity of their reproductive cycles.
The reasons that dogs and wolves generally live as different "species" are perhaps their very distinct habitats and behavioral characteristics. Wolves live in the wilderness in packs, have a hierarchy--one alpha male and one alpha female in a pack--and breed seasonally. In contrast, dogs live with humans and can breed more often. These behavioral differences, together with physical variations, have led scientists to classify dogs and wolves into different species. Under discussion is the idea that dogs should be classified as a wolf subspecies, Canis lupus familiaris. The analysis of the dog genome is nearly complete. DNA-sequence data in the dog would perhaps be a good starting point to verify whether dogs and wolves are different species, or just different breeds, and indeed will help refine our concept of species.
IS THE DOG JUST A TAME WOLF WITH FLOPPY EARS?
If the wolf and dog are genetically similar, why not call a tame wolf a dog? There is a difference between taming an individual and domesticating a species. The Latin root of the word "domestic" is domus, or house. Domestication means cultivating through generations of selective breeding animals that live with humans and serve human purposes. On the part of the animal, it means adaptation to a captive environment by an accumulation of genetic changes spanning several generations. Domesticated species have the common characteristic that they are tolerant of humans and often dependent on them. Their "fight or flight" adrenal response in the presence of humans is muted.
A remarkable study of the Siberian silver fox (Vulpes vulpes) reenacts how domestication may have taken place. The study was initiated in the 1950s in Russia by Dr. Dmitri Belyaev and is being continued by Dr. Lyudmila N. Trut (American Scientist, Vol. 78, p. 160; 1999). The project started with thirty male and a hundred female foxes from commercial farm stock that had been bred for fur for more than fifty years. These animals were therefore tolerant of caging and isolation from other foxes, but they were very afraid of humans, difficult to handle, and therefore still wild foxes.
During a period of forty years, researchers selectively bred farm foxes for a single characteristic: friendliness to humans. Monthly, beginning at one month of age, the fox kit was tested for its reaction to an experimenter, who offered it food and attempted to pet or handle it. Animals were categorized at seven to eight months of age as Class III if they tried to bite or flee, Class II if they allowed themselves to be touched but were not friendly, and Class I if they were friendly with the experimenter. Only the tamest animals were bred, which even in 1999 was less than 5 percent of males and 20 percent of females. After six generations, a new class emerged, Class IE, the domestic elite, who sought out human attention, licked the experimenters' hands, and wagged their tails in a dog-like fashion. After twenty generations, 35 percent of the animals were Class IE, and after thirty-five generations this proportion rose to 70-80 percent. In all, the experiment has involved 45,000 foxes.
[FIGURE 1 OMITTED]
Though breeders were selected for tameness alone, an astonishing result of the farm-fox study was the appearance of phenotypic, or physical and biochemical, changes in some elite animals. Some of these foxes were spotted or patched black and white (piebald, see Figure 1) or had a white star on their foreheads. In addition, many of the new, unusual physical traits in the elite foxes appear to be the retention of kit-like characteristics in the adult, such as barking, floppy ears, tails that curve over the back, broader heads relative to length, shorter snouts, and smaller skulls. This phenomenon is called pedomorphism and is believed to be a feature common to many domestic mammals. These characteristics, rare in the fearful fox population, increased in numbers in each generation as the experiment progressed. Reproductive behavior also changed, with elite foxes reaching puberty earlier and having longer breeding seasons.
The physiological basis for these changes is the subject of ongoing study by Russian investigators, who have shown that the foxes' endocrine (hormone-secreting) systems and brain chemistry have changed. Elite foxes have lower stress-hormone levels than fearful foxes, and their brains have higher levels of the neurotransmitter serotonin, which is associated with calmness. In selecting and breeding animals that were friendly to humans, the genetic pool has become skewed towards a different response of the adrenal cortex towards humans. The wonder is that this process took only thirty to thirty-five generations of foxes and forty years of human time.
To become the commensal species that dogs and humans are, that is to say, species that live together to their mutual benefit, an event unique in natural history took place. Our human forebears and dogs' wolf forebears must have been resourceful beings to establish this enduring relationship. The relationship is still evolving, and with recent revelations from canine genomics, it is evolving in unforeseen ways. In my next column I shall discuss two topics: speculations on how wolves became proto-dogs and the creation of pure breeds. Purebred dogs have been created by generations of tightly regulated breeding to a conformational standard. The offshoot of this activity seems to be new models for human hereditary diseases. I'll be back in six months with more.
Authors Note: I thank Dr. Bhanu Chowdhary and Dr. Kimberly Greer at Texas A & M University for providing information about canine genomics and evolution.
Evelyn Tiffany-Castiglioni, PhD, is associate dean for Undergraduate Education and head of the Department of Veterinary Anatomy and Public Health, College of Veterinary Medicine, Texas A & M University. She conducts research on the neurotoxicity of environmental contaminants. She is on the editorial boards of Neurotoxicity and the International Journal of Developmental Neuroscience.…
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Publication information: Article title: The Domestication of the Dog, Part I. Contributors: Tiffany-Castiglioni, Evelyn - Author. Magazine title: Phi Kappa Phi Forum. Volume: 84. Issue: 3 Publication date: Summer 2004. Page number: 6+. © 2008 Honor Society of Phi Kappa Phi. COPYRIGHT 2004 Gale Group.
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