Tens of thousands of years ago in prehistoric Eurasia, some daredevil made friends with a gray wolf. The millions of domesticated dogs we see today are all likely its descendants, and their enormous diversity masks a remarkable fact: Even after millennia of selective breeding, all of them, from the Chihuahua to the St. Bernard, remain members of the same species.
Canis familiaris displays by far the most variation of any mammal, and it’s still evolving at a steady clip. The mutation rate is so fast because breeders deliberately pair dogs with desirable traits or cross-breed distinct lineages to create novel “designer dogs.” In so doing, they speed up the leisurely process of natural selection, which is indifferent to human notions of canine perfection.
Today, our beloved pets come in 200 breeds (according to the American Kennel Club; the Fédération Cynologique Internationale, or World Canine Organization, uses vastly different standards and recognizes more than twice that number). In just the past century, many of them have undergone major transformations.
Entertaining as it is to compare our favorite dogs to their ancestors, their pedigrees often come with a cost. Because many breeds descend from just a few individuals, they lack genetic diversity, which predisposes them to a host of diseases that mixed breeds escape. In some cases, those diseases directly result from the anatomy dog fanciers have sculpted in their pursuit of each breed’s ideal (or “standard,” in kennel club lingo).
