Why are modern dog breeds so different from one another?

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John Lindo, PhD
by John Lindo, PhD

Short-haired. Long-haired. Floppy ears. Pointy ears. Guard dogs. Lap dogs. If all dogs are descended from the gray wolf, why do different breeds look—and sometimes act—so  different?

It’s a question that has long intrigued both scientists and dog owners.

Back in 1868, Charles Darwin was among the first to propose an answer. In his book The Variation of Plants and Animals under Domestication, Darwin introduced a theory now known as “Domestication Syndrome.” Essentially, his idea was that as humans first began domesticating wolves (or the progenitor of any other domesticated species), they selected for “docility” and inadvertently triggered a cascade of other traits that hitchhiked along for the ride.

Today, of course, we have a much more sophisticated understanding of how physical and behavioral traits are inherited. We know more about how artificial selection, breeding domesticated animals for specific characteristics, alters a species physically and behaviorally.

So let’s take a deeper look at why modern dog breeds are so different from one another. In addition, we’ll explore how some of these differences can impact your dog’s health.

Most modern dog breeds were developed in the 1800s

Current genetic and archeological evidence suggests that wolves began to be domesticated some 30,000 to 15,000 years ago. However, modern dogs appear very different from those first domesticated wolves. That is because, over the ages as domestication continued, humans kept selecting for observable characteristics, or “phenotypes,” that they found most useful and appealing. Although there are a few dog breeds that can be traced back to about 5,000 years ago, most of the breeds that we see today were developed during the Victorian Breed Explosion in the 1800s.

A timeline showing the domestication of wolves into modern dog breeds today. Starting with Wild Gray Wolves 30,000 years ago to Domesticated Wolves 15,000 years ago to the First Dog Breeds 5,000 years ago to the Victorian Breed Explosion in the 1800s to Modern Dog Breeds Today. Under the timeline is the phrase "Domestication of wolves" and "Artificial selection: Animals bred for specific characteristics"
The evolutionary process from the Grey Wolf to the Domesticated Canine

What phenotypes did dog breeders typically select for?

That depends on the breed. Generally speaking, humans needed domesticated animals to perform specific jobs. Dog breeds were developed to help with everything from hunting, herding, and hauling to protection, performing, and companionship.

Dogs were bred to retain puppy-like characteristics in adulthood 

Dog breeders also favored neoteny—the retention of juvenile characteristics in adulthood. This is why many modern adult dogs have the physical features of puppies, such as rounded heads, proportionally large eyes, short muzzles, and small body size. Modern adult dogs often exhibit juvenile behaviors, as well. These could include tail wagging or licking (a begging behavior used by puppies, and a sign of submission or appeasement in adult dogs), and a general demeanor that is more friendly and playful rather than aggressive. 

Of course, modern dog breeds vary dramatically in the extent to which adults retain the physical and behavioral characteristics of puppies. Some breeds, such as those intended for racing or for guarding and hunting roles, differ from their wolf ancestors in ways that don’t make them look or behave like puppies. Others, including many of today’s most popular breeds, are intended primarily for companionship, and these show some of the most extreme selection for juvenile characteristics.

On the left of the arrow pointing right is a depiction of an Adult Wolf skull. Beneath is an illustration of an Adult Wolf howling. An arrow points from the wolf to the right with the caption "Alternation of anatomy towards an exaggerated juvenile form". On the right is a smaller skull labeled "Skull: Smaller, Rounded, Short face, Large eyes". Below this is an illustration of an Adult French Bulldog.
This comparison of an adult wolf skull compared with that of a toy breed dog shows the extreme alteration of anatomy towards an exaggerated juvenile form.

Adults of companion breeds often have small, rounded skulls, short faces, and large eyes. Such features make these dogs less threatening and more appealing to humans. Even facial expressions typical of puppies, such as raising of the eyebrows, can make adult dogs appear cuter and increase their chances of being adopted from a shelter.

Neoteny has created canine faces humans find less threatening and more appealing.

Neoteny can impact your dog’s health

Unfortunately, while neoteny can make dogs more appealing to humans and more likely to be adopted or bred, it can also create serious health problems. Toy breeds with the most extreme physical alterations from the ancestral form often have difficulty with overheating or poor tolerance for exercise, due to the short muzzle and other features that impede breathing. The size and shape of their heads can also lead to brain disorders, dental disease, and difficulties during birth.  

Over thousands of years, domestication caused variation in both the appearance and the behavior of dogs. Then, intensive artificial selection for specific features and personalities altered our canine friends even more. While these changes have led to what many consider the most lovable creatures in the world, some have perhaps gone too far and created health issues for modern dogs. 
At Loyal, we are working to better understand the factors that shaped and continue to shape dog health. This will help us find ways to improve the overall wellbeing of humankind’s best—and longest—friends.


Waller BM, Peirce K, Caeiro CC, Scheider L, Burrows AM, McCune S, et al. (2013) Paedomorphic Facial Expressions Give Dogs a Selective Advantage. PLoS ONE 8(12): e82686.

Jumbo, miniature american shepherd whispers in human's ear
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