Before we can look at that, though, we have to understand how evolution works. Evolution, in its most basic form, is the process by which species change over time. Its driving mechanism is natural selection, or "survival of the fittest." In a given group of individuals, there are going to be differences. Some of these differences help an individual; others give it a disadvantage; most don't do anything at all. A great example of this is the peppered moth of England. Some individuals were a light color, matching the bark on the lichen trees where the moths live. Others were a sooty gray. Before the Industrial Revolution, the dark moths were pretty rare; birds could see and eat them pretty easily. During and afterwards, however, the soot on the trees gave the dark moths the advantage, and the light moths became rare. Now, as the pollution levels have fallen, the light moths are making a comeback. It's the classic example of natural selection in action.
Now, I'm sure by this point you're trying to figure out why I started with puppies when I was just going to talk about moths. I'm getting back to puppies now. A new study suggests that selection is well at work in domestic dogs, to an extent that doesn't occur in nature. This is artificial selection, rather than natural selection. See, domestic dogs don't face the same pressures as their cousins, wolves and coyotes. 
They don't have to chase down live prey and eat tough, raw meat. There is no shortage of easily accessible food, water, and shelter. Therefore, domestic dogs aren't playing the game of "survival of the fittest;" rather, they are playing "survival of the cutest." The study specifically looked at skull shapes of various carnivores, including several breeds of domestic dog. The results are impressive: it turns out a cat and a walrus have more in common, head-shape wise, than a collie does with a Pekingese. Because people are the main pressure selecting for characteristics, rather than practicality, a breed like a pug can survive as a pet. In the wild, a pug would be unable to breathe, eat, or avoid anything that wanted to eat it. Cute? Maybe. Practical? No.The pressures on dogs, and other domestic organisms, are an interesting study in evolution. Freed from the very limiting pressures of natural selection, these creatures can develop characteristics that would be totally impractical in the wild. From this diversity, humans pick the characteristics they like best, and breed those specifically, while other characteristics are deemed ugly, and are selected against. The process is fast, and easy to observe. This artificial selection is a proof-of-concept experiment for natural selection and the Darwinian model of evolution.
Source: Science Daily- "'Survival of the Cutest' Proves Darwin Right"
Charles Darwin's On the Origin of Species: A Graphic Adaptation, Michael Keller (Amazon)