It's all because of us says this very interesting article from the PBS program Nova, one of my favorite programs on TV.
There are some "natural" breeds that developed by adaptating to different environmental niches; dogs in northern regions, for example, tend to be bigger because of the cold; however, the multitude of varieties came about from breeding for a particular purpose and then appearance:
For the next few thousand years, right up until the late 19th century, people bred dogs for certain skills: running down prey (greyhounds), hunting rodents in holes (terriers), flushing and fetching game (pointers and retrievers). It wasn't until the late 1800s, when kennel clubs first formed, that breeding for appearance rather than behavior began in earnest. Pure-breeding started then as well: to be registered as a purebred giant schnauzer, both your parents had to be registered as giant schnauzers. Breeders bent on designing the perfect breed are indubitably the reason why today we have dogs as divergent as the long-faced borzoi and squashed-faced bulldog, the Mexican hairless and lavishly hirsute Pekingese, the 150-pound St. Bernard and two-pound Chihuahua.
Another interesting tidbit I got from this article is that dogs are born with heads that grow to adult size but cats are born with adult sized heads. How strange is that?!
