The unique diversity of man's best friend
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."
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.
The evidence against this, she said, is that we never see wolves born with pug noses or polka dots.
The other view is that the genes underlying these traits don't exist in the wolf, but that wolf DNA is very good at spinning out new variants - that it's particularly "plastic."
What makes an animal's DNA plastic? One answer might be found in the parts of the DNA that don't make up the genes, but nonetheless control how those genes work. Seven percent of the dog's DNA, for example, is made of strings of code called SINEs that appear to have copied themselves throughout the dog chromosomes.
The SINEs are like parasites in that they don't hold the code for anything the dog needs, but they copy themselves selfishly through the dog's DNA.
Between dog generations, SINEs can copy themselves in new spots on the chromosomes. And sometimes, the location of these SINEs does influence traits, Ostrander said. Australian shepherds, for example, have blue-gray coats thanks to the invasion of a SINE into the middle of a gene for coat color.
SINEs crop up in humans and other animals, she said, though dogs may be particularly rich in these and related bits of variable and movable DNA. These can act as engines of variability, often ramping up or down the activity of genes, rather than destroying them.
Stanford geneticist Greg Barsh, who studies coat colors in animals, said some color genes in dogs may have been present in wolves in a "cryptic" form, meaning they float around unseen because they need to be inherited in unlikely combinations to have an influence. Such genes can become more common through breeding.
A few mutations influencing dog coat color appear to go back thousands of years, including one that causes many dogs to be completely black, said Barsh. By contrast, some mutations leading to red or brown coats seem to have occurred much later.
Spots on most dogs trace back to a gene called MITF, he said, which is involved in pigmentation, eyesight, and hearing. In boxers and bull terriers, MITF mutations interfere with pigment-producing cells called melanocytes. These cells tend to die in clusters, leading to the white neck and underbelly areas of many breeds. MITF also contributes to dalmatian spots, he said, though to Barsh, dalmatians are black dogs with very large white spots.
In the coat-color department, cats are as variable as dogs, Barsh said, and that will be the subject of a forthcoming paper.
As Darwin noted, cats aren't easily bred, thanks to their "nocturnal ramblings."
Darwin wrote that he saw breeding as a form of evolution - a demonstration of the ability of animals and plants to change dramatically. That thought wasn't original with Darwin; his main contribution was to figure out a mechanism for such change in nature - the revolutionary concept of natural selection.
In Origin, he argued against other naturalists who proposed that all variation in dogs came about through the crossing of existing, God-created forms. Darwin argued that canine breeds result from variation and selection, albeit of an artificial kind.
He saw breeding as a process analogous to natural selection, in which nature selects variations useful to the reproductive ability of the organisms themselves, rather than those useful to breeders. There's lots of variation out there, he noted, but nature had a lot of time.