Planet of the Apes: The unique diversity of man's best friend

Several weeks ago, I got a question via voice mail that reflected an issue Charles Darwin himself raised: "I was curious about the situation with dogs," this reader said. ". . . Some look entirely different from others. They say all dogs came from the wolf." While he understood that dogs were shaped by breeding, he wondered whether the diversity of dogs could be considered a form of evolution.

Dogs do hold the record as the world's most diverse land mammal, said Elaine Ostrander, a geneticist who studies dogs at the National Institutes of Health. The largest dogs are 40 times the size of the smallest ones. They come with different coats, head shapes, snouts, and behaviors. Ostrander's work uncovers the DNA differences that make this variability possible.

Darwin used dog diversity in the very first chapter of On the Origin of Species to help make a case for evolution and explain the mechanism behind it as a natural analogue to breeding. "Who can believe," Darwin wrote, "that animals closely resembling the Italian greyhound, the bloodhound, the bulldog or the Blenheim spaniel ... ever existed freely in a state of nature?"

Darwin recognized that these breeds were not tame versions of wild greyhounds and bulldogs. He didn't know that all dogs originated from one single species - the wolf - as we do today thanks to DNA.

But he wrote that if such a common dog origin were true, it would make a powerful case for transformation of species in nature: "Such facts would have great weight in making us doubt ... the immutability of the many very clearly allied and natural species."

Today, said Ostrander, there are about 350 dog breeds. "What Darwin was recognizing is what we learn from dogs will be true for all variable species, whether human or plant or animal," she said. Today, with the ability to compare DNA, we know that many of the same genes control growth and stature in humans and dogs, and we and dogs share many of the same genes that predispose us to cancer.

And DNA work may soon answer another question: Why are dogs so much more variable than cats, cows, or pigs, which are also shaped by artificial selection?

Ostrander said two possible genetic explanations exist for dog variability. One is that something latent in the DNA of wolves allowed them to be transformed into both Great Danes and dachshunds. Under that view, she said, pushed-in noses and floppy ears and spots were all embedded in the wolf genome.