Dog With His Head Out Of The Car Window

Dogs were domesticated from gray wolves at least twice. They must have been very valuable to early human settlements, for they quickly became ubiquitous across world cultures. For instance, it is believed the successful emigration across the Bering Strait might not have been possible without sled dogs. Dogs perform many roles for people, such as hunting, herding, protection, assisting police and military, companionship, and, more recently, aiding handicapped individuals. This impact on human society has given them the nickname "Man's best friend" in the western world. Currently, there are estimated to be 400 million dogs in the world.
Over the 15,000 year span the dog had been domesticated, it diverged into only a handful of landraces, groups of similar animals whose morphology and behavior have been shaped by environmental factors and functional roles. As the modern understanding of genetics developed, humans began to intentionally breed dogs for a wide range of specific traits. Through this process, the dog has developed into hundreds of varied breeds, and shows more behavioral and morphological variation than any other land mammal. For example, height measured to the withers ranges from a few inches in the Chihuahua to a few feet in the Irish Wolfhound; color varies from white through grays (usually called "blue'") to black, and browns from light (tan) to dark ("red" or "chocolate") in a wide variation of patterns; coats can be short or long, coarse-haired to wool-like, straight, curly, or smooth. It is common for most breeds to shed this coat.