You may have seen on TV lately that PBS re-aired one of their Nature programs from 2008. "Dogs That Changed the World" is a two-part special about people's ancient symbiotic relationship with dogs. The first episode, "The Rise of the Dog" focuses mainly on how dogs evolved from wolves 15,000 years ago.
It reminded me tangentially of Christine O'Donnell wondering why monkeys aren't still evolving into humans. Most people have responded that evolution doesn't happen quickly, but actually it does sometimes happen in quick leaps, as mentioned in this Nature episode. (It's just difficult to document in the fossil record if you can't find transitional forms.)
The real reason that monkeys (and apes) are still around is that there's room enough for them and us humans. It's like asking how come wolves aren't still evolving into dogs? Just because one species transformed into a new species doesn't mean the old one is necessarily wiped out. Wolves still live in the wild and maintain their place in the food chain as predators, while dogs adapted and became our helper species. Both can exist at the same time, and nobody questions that. Why do creationists insist on thinking that evolution is so simplistic? Granted, we have to do our best in conserving the planet and fighting global warming so that more species (including us humans) don't go extinct now.
Anyway, the second episode of "Dogs That Changed the World" talks then about the Victorian fad for the dog fancy, and how roughly 40 working/sporting breeds were turned into 400 breeds in just 150 years. It touches briefly on the same ground as the Pedigree Dogs Exposed documentary, using again the example of the drastic change in the English bulldog. It's really sad that dog breeders got so obsessed with mere appearance rather than the health and function of the dogs. For 15,000 years the dogs were shaped by their uses in hunting, guarding, herding, etc, but in 1/100th of that time, we've broken them. Now the purebreds are so unhealthy and treated like living toys. Dog shows are just as artificial as beauty pageants, and the poor dogs are subject to so many genetic diseases.
Life is better for mixed dogs and family pets, but often the owners can misunderstand the needs and instincts of their dogs too. Which results in high-strung and destructive dogs that get sent to shelters. Still, the Nature special tried to end on a hopeful and positive note with dogs that help detect seizures in patients, or warn of natural disasters.