Dogs provide an interesting proof of evolution. Consider the astonishing variety of different dog breeds. There is the tiny Chihuahua, about six inches tall and weighing under six pounds. And other dogs are enormous, with the Irish wolfhound rising above a person when on his hind legs, and the Saint Bernard weighing over two hundred pounds. Some dogs are extremely intelligent, including the border collie, retriever, poodle and German shepherd. These dogs learn new commands with ease, and can perform complicated tasks. Other dogs, however, seems very dimwitted, often requiring hundreds of repetitions to learn, and even then usually failing to obey a command. There is such an incredible assortment of different dogs that it is easy to forget that they are all the same species, Canis lupus familiaris. This means that even a Chihuahua and a Saint Bernard (assuming that the obvious physical challenges could be overcome) could mate and produce live and fertile offspring.
So where did dogs come from? Darwin thought they might come from multiple sources, including the wolf, jackal and coyote, thereby in part explaining their diversity. The DNA evidence, however, shows that they are all derived from the wolf. DNA from all dogs is over 99% identical to that of a wolf, while the wolf and coyote DNAs, for example, are over 4% different from each other. This means, surprisingly, that all of the diversity of dog types in the world today came from a single source, the wolf.
How did the wolf get transformed into a woof? The precise order of events is a matter of conjecture, but it probably began when an abandoned litter was taken in and nursed by people. The DNA evidence, which shows a strong similarity for all dogs, suggests that there might have only been only a few such domestication events. These early wolf dogs would be subjected to what is called artificial selection. In the wild natural selection is at work with the strongest, fastest and smartest wolves surviving better to make more wolves. But once under the care of people survival depends on a new set of rules. For example, animals that liked to bite people probably did not fare well. But dogs are natural hunters and could help in the search for food. They also could provide an early warning system, barking when unwelcome visitors approach. So people friendly watchdogs, with their heightened senses of hearing and smell, would be very useful to early humans.
People have selected dogs for a variety of features, including hunting ability, companionship, intelligence, herding ability, and looks. Interestingly, there are over four hundred dog breeds today, and most of them were developed in just the last 150 years. This shows a remarkably rapid evolution of a great number of different dog breeds. Most of these breeds were made by first taking two very different existing dog breeds and crossing them. This maximizes genetic diversity in the offspring. Then there is a systematic selection, choosing the pick of the litter, those animals with the desired characteristics, and interbreeding them to make the next generation. The continued brother-sister matings coupled with continued selection rapidly results in a new breed of dog with a new set of characteristics. The new breed is genetically pure, because the repeated inbreeding removes genetic diversity. And the new dog can have a very distinctive set of features because of the artificial systematic selection for those very features. The Doberman pinscher, the Australian cattle dog, and the whippet were all developed in this manner.
It is remarkable to consider that the wolf had enough genetic diversity in its DNA to give rise to all of the dog breeds we see today. Wolves all look pretty much alike, and you’d think that if you keep breeding wolves you’d just get more wolves. Yet there are actually millions of base differences in the DNAs of different wolves, among the billions of bases total. This is clearly sufficient diversity to produce progeny with quite distinct traits when the power of artificial selection is applied over many generations.
The dog story is an interesting demonstration of evolution at work. In an extremely short period of time, in evolutionary terms, the wolf evolved into the dog, including all of the great variety of dog types we have today. This is one evolutionary event that was not only watched by man, but indeed was driven by man. It is but one example of the many domestic animals and plants that illustrate the incredible power of artificial selection.
Darwin proposed that given enough time the forces of natural selection could change the traits of species. The neck of the giraffe would get longer, to reach more vegetation, the gazelle could get faster, to better escape, and the cheetah could get faster, to better catch the gazelle.
The artificial selection that drove the evolution of dogs is simply natural selection on steroids. It proves the principle, and shows without a doubt that evolution is true.
About the author. Steven Potter, PhD, is a Professor of Pediatrics, in the Division of Developmental Biology, at Children’s Hospital Medical Center in Cincinnati. He has authored Designer Genes: A New Era in the Evolution of Man, published by Random House http://www.amazon.com/Designer-Genes-New-Era-Evolution/dp/140006905X/ref=sr_1_1?ie=UTF8&s=books&qid=1303812945&sr=1-1. He has also written over one hundred science papers and co-authored the third edition of the medical school textbook, Larsen’s Human Embryology.
