Of Dogs and Wolves
Posted April 1, 1998
What do a toy poodle, a Great Dane, a tiny Chihuahua, and a collie have in common? All of these breeds of domestic dogs are probably descended from their wild cousins,
the wolves. Biologists studying the mitochondrial DNA of canid species, including wolves, dogs, jackals, dingoes, and coyotes, have determined that the 400 breeds of domestic dogs all
evolved from ancestral wolves.
Biologists extracted DNA from blood samples or hairs from coyotes, jackals, 162 wolves, and 140 dogs, representing 67 breeds and 5 mixed breeds, from North America,
Asia, and Europe. They studied a particular region of mitochondrial DNA that has a high mutation rate, and compared the sequences of bases on these regions. The scientists identified
27 different sequences among the wolves and 26 different sequences among the dogs. Dog DNA sequences differed from coyotes and jackals by about 20 nucleotides. But wolf and dog sequences
differed by only 12 nucleotides, indicating that wolves are the ancestors of all dogs. What was most interesting about these DNA sequences was that those identified for dogs did not
sort by breed. In other words, individual dogs of the same breed carried different sequences, an indicator of diverse ancestry for the breed.
Once the different sequences were identified, scientists used phylogenetic techniques to place similar sequences into groups called clades. All the dog sequences
fell into four distinct clades. In two of the clades the sequences clearly showed that these dogs came from two unique common ancestors. Scientists agree that this means these two clades
were formed at two different times. Why is this important? It means that dogs have a diverse ancestry and may have been domesticated more than once, each time with the introduction of
new wolf genes. The other two dog clades shared some sequences with wolf sequences, indicating that at some later time, domesticated dogs mated with wild wolves again. Because nearly
three-quarters of the dog sequences form just one clade, biologists hypothesize that most modern dogs descended from a common domestication event involving a single female line.
Besides discovering that wolves are the ancestors of domestic dogs, scientists also wanted to know the timing of domestication. Fossil evidence indicates that
dogs were domesticated around 14,000 years ago. The DNA evidence indicates that dogs separated from wolves more than 100,000 years ago. Some scientists argue that the mitochondrial DNA
is not reliable for identifying time, but others say dogs are not found in the fossil record prior to 14,000 years ago because until that time they were undistinguishable from wolves.
Wolf bones have been found with human fossils dating back to 100,000 years ago.
References
Morell, Virginia. "The Origin of Dogs: Running with the Wolves." Science, 276, June 13, 1997, pp. 1647-1648.
Mlot, Christine. "Stalking the Ancient Dog," Science News, 151, June 28, 1997, pp. 400-401.
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