Genetics And Breeding Strategies:
Essays For The Dog Breeder
By Dr. Susan Thorpe-Vargas
Reprinted with permission.
Have you ever wondered at the extraordinary diversity in the appearance of various dog breeds? How is it that a Yorkshire Terrier can be the same species as a Bullmastiff, or a Pug be related to a Saluki? What are the factors that have led to this incredible range and variety in appearance, not to mention behavior and temperament? It is not simply a question of phenotype vs. genotype, or dominant vs. recessive genes. Let's begin the journey by looking at how dogs evolved into the companion we know and love today.
Why so many dog breeds?
About 60 million years ago a small weasel-like animal lived in many parts of Asia. This ancestor of all modern day canids (dogs, jackals, wolves and foxes) was called Miacis. Cynodictis, the first true dog-like canids are thought to have descended from Miacis about 30 million years ago. This line eventually split into two branches, one in Africa and the other in Eurasia. The Eurasian branch was called Tomarctus and was, until recently, thought to be the progenitor of wolves, dogs, and foxes. However, new research has called this theory into question with a recent paper indicating now that the wolf is the domestic dog's only direct ancestor and that a recently shared ancestry with the fox and jackal is unlikely.2 This somewhat controversial paper also asserts that the first domestication of wolves may have taken place as long as 100,000 years ago. The actual time that such domestication occurred, of course, cannot be settled based solely on DNA analysis.
Research now suggests that the domestic dog line began to diverge from the wolf after the first wolf became domesticated. Over time, groups of wolves became adapted to a niche that made them ultimately better suited to domestication at some point as early as 100,000 years ago to as late as 14,000 years ago. The actual timing remains in dispute since the fossil records are not consistent enough to pinpoint an exact period of time. However it has been well established now that different domestication events did occur from multiple populations by researchers such as Robert Wayne3. This makes sense as both wolves and humans coexisted over a wide geographical area and it is likely that multiple domestication opportunities would have arisen. These multiple events in various parts of the world accentuated the diversity we see in dogs today.
As hunter/gatherers, humans would have found dogs very useful. Then, about 8,000 years ago, humans turned to a more settled way of life. This is when severe selection for specific behaviors and traits became important and 'modern' breeding practices started. And so it begins - dogs bred for many reasons, from companionship to guarding abilities and so on.
The concept of a "pure" breed is a relatively recent one; further back, local dog populations consisted of similar looking dogs bred for a specific purpose. Although there were some exceptions, the dog breeders of that time did not hesitate to breed a dog of one type to a newly arrived dog from another area. Thus, up until the 19th century the various dog breeds were more often than not strains of closely related and similar looking dogs that as a population had a great deal of genetic diversity. Only dog populations that lived in geographic isolation approach today's purebreds in terms of a restricted gene pool.
In searching for cultures without dogs since pre-historic times we come up empty handed. Thus we find early recognizable breeds coming from the Middle East, Africa and Asia. The Middle Eastern coursing hounds had become well established no later than 2,000 BC. The Basenji, a hunting breed of the African savannah, may predate the dogs of the Pharaohs. In the Far East, isolated areas such as Tibet and Mongolia produced a number of still extant breeds of ancient origin. Malta was occupied as early as 3,500 B.C and the dog brought to Malta may have had earlier Egyptian origins. The point here is that since relatively early times in recorded history, there has been a tremendous diversity in dogs. Contrast the Roman Mollosus - a mastiff-like creature (or what we think it looked like)--with the Maltese or the Tibetan terrier or the Lhaso Apso, and it immediately becomes obvious that there may be no "standard" dog. The tiny kingdom of Tibet, produced many different breeds, some now probably extinct, but which include the following breeds and their ancestors: Kuvasz (before Hungary); Lhaso Apso, Tibetan Terrier, Tibetan Spaniel, Tibetan Mastiff, just to mention a few familiar to Western dog fanciers. Even dogs we think of as "English" such as Mastiffs have had ancient origins. Recognizably mastiff-like dogs can be seen on Egyptian monuments circa 3,000 BC. They were in China circa 1100 BC and eventually went to England with invading Roman forces in the first century AD. We can safely say that certain dog strains have been breeding true for a very long time.
Neoteny - More fuel for diversity
There is a saying among dog breeders that "All puppies look alike." Newborn puppies of different breeds, except for size and of course color, look remarkably alike. How is it that they grow up to look so different from one another? The vast array of physical and behavioral differences in dogs is probably not due to selection for each individual trait, but more likely to selection for groups of traits that are all similarly affected by the same hereditary mechanisms. One such mechanism is the regulation and timing of developmental processes. Selection for one trait affected by developmental timing could inadvertently select for other traits also thus affected.
It is very likely that this process has played a vital role in the initial domestication and later diversification of dogs. As animals mature, they pass through different stages, each uniquely adapted to its particular circumstance. In wolves, neonates and juveniles are dependent upon parents to care for them, and they are extremely successful at eliciting that care. In comparison to adults, they are relatively tame and subservient. Wolves (or the wild ancestors of wolves and dogs) that tended to retain these immature qualities of tameness and subservience into adulthood would have been favored by early humans and would have formed the core of primitive domesticated dogs. This retention of immature characteristics in adults is known as neoteny. By choosing the individuals to reproduce that showed the favored immature behavioral qualities, concurrent selection for other juvenile traits -both wanted and unwanted--may very likely have occurred, laying the basis for the diversity seen in dogs today.
Now, let's take a look at some of the basic concepts of Genetics that every breeder needs to know. Then we'll discuss the mechanics of inheritance in Chapters 2 and 3.