Complex sentence: Centuries of selective breeding by humans has resulted in dogs being considerable more genetically diverse than most other mammals, and as such dogs are the only animal with such a wide variation in appearance without speciation, "from the Chihuahua to the Great Dane".
Split into simpler sentences: Centuries of selective breeding by humans has resulted in dogs being more genetically diverse than most other mammals by a considerable margin. As such, dogs are the only animal with such a wide variation in appearance without speciation, "from the Chihuahua to the Great Dane".