How do you make a dog?
Simple: Just take the calmest most gentle fox you can find, and keep breeding and culling them for 40 years. And Yes, it's been done.
For an easy-to-read version of the story, click here. The original March 1, 1999 article from The American Scientist is here. The authors of this article note that:
"[After] 40 years and 45,000 [fox]... our experiment has achieved an array of concrete results. The most obvious of them is a unique population of 100 foxes (at latest count), each of them the product of between 30 and 35 generations of selection. They are unusual animals, docile, eager to please and unmistakably domesticated. When tested in groups in an enclosure, pups compete for attention, snarling fiercely at one another as they seek the favor of their human handler. Over the years several of our domesticated foxes have escaped from the fur farm for days. All of them eventually returned. Probably they would have been unable to survive in the wild."
As the foxes became tamer, some of their ears flopped (only elephant ears flop in the wild), their tails curved over their back like many domestic dogs (and like the wolf-dogs kepy by the American Indians), the legs got shorter, the heads got wider, their snouts got shorter, their bark became dog-like, and specific color phases occured, such as white added in the head region.
A Footnote: The Russian researchers which had spent 40 years doing this study became a victim of cost cuts after the fall of the Soviet Union, and they were forced to start selling tame foxes as house pets.
For a video version of the story, with Ray Coppinger doing the explaining and Dan Ackroyd doing the narration, see the clip below: