At the Westminster Kennel Club Dog Show this week, thousands of canine contestants will try to measure up to a set of exacting breed standards.
The perfect pug?
Look for a deeply wrinkled face with a distinct black mask and a tightly curled tail.
The ideal Afghan hound has the silky hair of a pageant queen set off by “long and punishing” jaws.
An exemplary Dalmatian should have spots between a dime and a half dollar in size.
A top-ranked basset hound is heavy-boned, with velvety, hang-low ears.
A blue-ribbon Bichon frisé should have a soft, springy coat reminiscent of clouds or cotton candy.
But how much does breed shape a dog’s health and behavior?
By Emily Anthes
Humans have a long history of reshaping their canine companions, which evolved from ancient wolves. At least 10,000 years ago, Arctic peoples were using selective breeding to create sled dogs , cold-tolerant animals with the stamina to pull heavy loads over long distances. Over time, people bred dogs for a diverse and increasingly specialized array of roles.
“Hunting things underground, hunting things overground, hunting things flying, hunting things in the water, running around in little wheels to turn your spit,” said Kathryn Lord, an evolutionary biologist at the UMass Chan Medical School and the Broad Institute. (The now-extinct turnspit dog was once a mainstay of European kitchens, where it was used to power fireplace meat-roasting spits.)
Still, historically, the priority was performance — breeding dogs that were good at their jobs.
That changed in the 19th century with the rise of Victorian dog fanciers, who began to codify existing breeds and to invent new ones. Rather than creating skilled working dogs, they aimed to engineer “the perfect physical specimen,” said James A. Serpell, an emeritus professor of ethics and animal welfare at the University of Pennsylvania School of Veterinary Medicine.
Breed clubs created prescriptive standards that specified exactly how the perfect pointer (or bulldog or foxhound) should look. They identified dogs that fit the bill and registered them as official members of those breeds. Then, they used those dogs — and only those dogs — to breed more just like them. As a result, each breed became a distinct, reproductively isolated population.
Alexey Kuznetsov/Alamy
A few genetic tweaks can yield very different dogs.
In humans, physical appearance is the product of many different genes, each of which typically has a small impact. For example, hundreds of genetic variants are known to affect height; many of these variants make less than a millimeter of difference.
That’s not uncommon in natural populations, said Adam Boyko, an expert in canine genetics at Cornell University. In the wild, a single genetic mutation that suddenly produced a miniature wolf would probably pose enough of a survival disadvantage to be weeded out by natural selection, he said.
But in dogs, “humans interfered with that whole process,” Dr. Boyko said. A mutation that caused a drastic physical change was an opportunity, allowing humans to, for instance, turn a giant schnauzer into a miniature one. So, rather than being removed from the population, some of these mutations became the underpinnings of breed-defining traits.
Today, many of the stark physical differences between breeds stem from a relatively small number of genes. The gene for a growth hormone known as insulinlike growth factor-1 is a key determinant of a dog’s body size. Small breeds and large breeds generally have different versions of the gene. And just three genes give rise to much of the variation in coats, determining whether a dog’s hair is long or short, smooth or wiry or straight or curly.
Breed is tightly linked to health.
Today, most breeds are genetically homogenous and highly inbred. Dogs of the same breed tend to be susceptible to the same diseases, and ailments that are widespread in some breeds are rare in others.
“When you go to a doctor, they ask you for a family history,” said Dr. Danika Bannasch, a veterinary geneticist at the University of California, Davis. “We don’t have to do that. As the dog walks in, and it’s a Labrador retriever, we already know what diseases its relatives are likely to have had.”
In some cases, these health problems arose as byproducts of inbreeding. Because breeds are genetically closed populations, a disease-causing mutation that just happens to pop up in one dog can quickly become common in future generations. “Especially if the animal with that mutation is otherwise a prize specimen,” Dr. Serpell said. “Because everyone will want to breed from that individual.”
The result is that many modern breeds suffer from ailments that dog lovers never would have deliberately selected for. Labrador retrievers are prone to a degenerative eye disease known as progressive retinal atrophy, while Cavalier King Charles spaniels often develop a heart condition called mitral valve disease.
In other cases, however, the physical traits that breeders are seeking can themselves be the problem, especially as these characteristics have become increasingly exaggerated. The breed standard for bulldogs explicitly calls for “very short” muzzles, but, over time, the dogs’ snouts have all but disappeared, Dr. Serpell said, resulting in serious respiratory problems.
Mutts and mixed-breed dogs can be healthier than purebred ones, especially if they have small amounts of DNA from many different types of breeds, Dr. Bannasch said.
But some mixed-breed dogs may still be highly inbred, she said, and some health problems are common in multiple breeds. Many large breeds are prone to bone cancer, for instance; mixing a few of them together may not have much benefit. “You can’t lump all ‘mixed breeds’ together,” she said.
The link between breed and behavior is murkier.
In creating modern breeds, humans generally put a much stronger emphasis on appearance than behavior, which is also shaped by a dog’s training and early environment.
“Behavioral traits definitely vary from breed to breed, but not nearly as strongly as the morphological traits do,” Dr. Boyko said. “You’re never going to get a collie that looks like a Great Dane,” he added. “But I see lots of dogs exhibit pointing behavior that aren’t pointers.”
Scientists have uncovered some general patterns. Terriers, which were bred to hunt and kill pests, are more likely to exhibit “predatory chasing” than herding dogs, for example. And, on average, Siberian huskies are more likely to howl than Labrador retrievers are.
But, overall, breed is a poor predictor of behavior, and there is more variation within breeds than between them, scientists have found. Someone who wants a friendly, fetch-loving dog that doesn’t bark can’t bank on getting exactly that just by bringing home a Labrador retriever. “You may get exactly the opposite of that,” Dr. Lord said.
Purebred dogs are not the norm.
Of course, plenty of pet owners opt for mutts or, in recent years, designer crossbreeds like the labradoodle. But most dogs are not pets at all. More than 80 percent of the world’s dogs are free-living animals that fend for themselves and are not under human control, Dr. Lord said.
A tiny fraction of these dogs might be abandoned purebred dogs, but the vast majority have no breed ancestry at all. Instead, they are descended from generations of free-ranging dogs, and they still have much of the genetic diversity that has been lost in purebred dogs.
“There’s no breed,” Dr. Lord said. “They’re just dogs.”
Fonte: www.nytimes.com
