A wolf became a dog by selective breeding and domestication caused by humans bringing wolves to new environment once a bond was built, also known as artificial selection. Artificial selection means that the species changes due to man's intervention rather than nature's intervention, natural selection. This made the wolves adjust to new things creating new traits that would be passed down to their offspring. Each new breed was bred with another breed and so on. This created a wide variety of breeds with many different traits with only the "best" traits being passed down. This caused many genetic changes in phenotypes. If you look way back to when this started, you begin to realize that there is no such thing as a purebred.