Dogs have been selectively bred for thousands of years, sometimes by inbreeding dogs from the same ancestral lines, sometimes by mixing dogs from very different lines. The process continues today, resulting in a widening in appearance without speciation, "from the Chihuahua to the Great Dane."
All dogs should get as much exercise as possible. A daily walk is sufficient for most breeds. But most small dog breeds, for example Toy dogs such as Pugs, Maltese or Yorkshire Terriers, will get the majority of their required exercise needs just be walking, running and playing around indoors.
Today there are over 400 recognized breeds of dogs in the world. Many of them have historical origins dating back hundreds, even thousands of years. Wherever humans have lived, dogs have been alongside them performing various tasks. One of the reasons dogs have been such a successful species is because they are so adaptable. They have made themselves useful in countless ways to humans so we kept feeding them, providing shelter for them, and, yes, breeding them. It’s no accident that we have dogs able to hunt, herd, guard, track, and do so many other things at an expert level. Humans figured out early on that if you bred dogs that were good at these things, you would get offspring that were also good at doing them. All of these jobs performed by dogs were necessary for our own species to survive. It’s no secret that we owe a lot to dogs, just as we do to other animals.
All puppies are cute, but some breeds have that extra “je ne sais quoi” that makes them irresistible. For this reason, some of these breeds are popular in greeting cards, commercials and other marketing materials.