Monday, November 14, 2011

Doggie Diversity


It is startling to consider the wide variation in size, shape and color of the dog. Yet all of the breeds listed by the American Kennel Club and all of the mixes found at the local animal shelter all have a common genetic source. The variations may appear dramatic, but in most cases they are actually superficial. Certainly dogs are all genetically compatible because all breeds can be mated together to produce fertile offspring. There does not appear to be any biological barrier to dog compatibility. The only barriers to mating are the physical ones of size and stature. Interestingly, there is also no difficulty in crossing wolves and dogs. Their offspring are also healthy and fertile in every case. Here alone we have substantial evidence of evolutionary parentage.

No matter how a dog may appear, each is biologically the same species, inside of which lurks a wild wolf. Even very small dogs will growl menacingly and do everything in their power to guard against an invasion of personal territory. The same little dog who barks at a postal worker, will not hesitate to enthusiastically display every single tiny tooth and fiercely attempt to repulse the advance of a dog ten times its size if the intruder deserves the treatment. Luckily for the smaller breeds, the big dogs have strong inhibitions against attacking another dog they perceive to be a puppy, even if the small dog is fully mature. In fact, large dogs are often perplexed by small, aggressive dogs who act in a mature, fully aggressive manner, rather than acting like playful puppies. It just doesn’t make sense to them.

The wide variety of dogs is a result of their long-term association with humans. From the outset of human/dog relations, selective breeding has been an integral part of the relationship. Undesirable qualities including excessive aggression, stubbornness, nervousness, were selected against. In the wild, wolves reach full maturity when they begin to kill regularly, but dogs never reach this stage and mature a notch below their wild relatives. The wolves with variations that allowed them to mature sexually before reaching the killer stage were most likely better adapted to live with people and could survive longer and reproduce more offspring.  Through the process, dogs regressed from the final stages of maturity exhibited in wolves and became more placid and playful. Not only was this true of their personality, but also their appearance. Unlike cats, another domesticated carnivore, dogs’ appearance and proportions change as they develop from pup to adult. Humans not only selected for immature behaviors, but also for underdeveloped physical features. The differences in breeds reflects the stage of development that was selected for.

The emergence of different varieties of dogs provides an evolution in the way humans live. Dogs are selected for hunting, protection, and even companionship. Unlike their predecessor, dogs are able to learn how to behave from their owners. Originally, the traits selected for were ones that helped in survival – not only for the dogs, but also for humans. Dogs that could track and chase, but not devour, were selected for as companions in the hunt. Later, some dogs were bred to be larger and stronger in order to help work in the fields. Others were selected for their herding ability, or their sense of smell. More modern breeds have been developed based mostly on looks. Humans were able to do this by selecting to breed only the ones with the most desirable traits. The breeding techniques employed in the production of a wide variety of dogs were not particularly sophisticated at first, nor difficult to perform. If a small dog was desired, all the humans had to do was to breed the runts of several litters over a period of time and pretty soon, a small breed exists. Or, if a fast dog is desired, it was much the same process of breeding together those animals that exhibited a propensity for speed. All one needs are dogs and time. This selection process would increase the number of desired alleles in the population and reduce the corresponding undesirable alleles, until eventually they would breed true for the desired traits each generation. This process also has worked for selecting for certain temperaments in dogs. Some breeds have been selected for by behaviors. Today’s bird dogs have been bred to have “soft mouths” so they can collect the birds, but not damage them. These same dogs have been selected for more recently as family pets because they do not bite.

For an entertaining overview of the principles discussed above, check out this Radio Lab episode wherein Brian Hare tells us the story of Dmitri Belyaev, a geneticist and clandestine Darwinian who lived in Stalinist Russia and studied the domestication of the silver fox.

References:


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