Any dog lover knows that Labrador retrievers are friendly, Dalmatians are hyper, and Australian shepherds are smart (Scott & Fuller, 1974). Some dog lovers also know that Labradors are susceptible to hip dysplasia, while deafness and kidney stones run in Dalmatians. But why is this the case?
Breeding dogs for particular characteristics, or phenotypes, has been going on for centuries. Dogs are companions and workers, in service to humans, and they have thus been bred to accentuate desired traits. For instance, Dalmatians have long been coach dogs, in part because of their striking looks and their comfort around horses. Bred for endurance, they can run alongside horse-drawn carriages all day. When kept as a housebound family pet, however, a Dalmatian’s excess of energy can make the dog seem wired and can lead to less desirable behaviors, such as gnawing on furniture.
Dogs’ closest living relatives are wolves. Analysis of the two species‘ genomes has revealed differences that some scientists believe are a result of dogs being subject to artificial selection imposed by humans. It appears that with domestication, beginning as long as 14,000 years ago, came a relaxation of selective forces typical of nature (forces that continued in earnest on wolves), as well as an increase in variability in the dog genome compared with the genome of their ancestral stock (Björnerfeldt et al., 2006).
One question that tugged at Swedish researcher Carles Vilà is how dogs can have such a wide variety of phenotypes—imagine a tiny Chihuahua standing next to a Great Dane, or a Chinese shar-pei peering from under its skin folds at an Old English sheepdog who peers back through its long hair. In fact, the variation among breeds of dogs is far greater than the variation among other completely distinct species in the family Canidae.
If dogs evolved from wolves, which seems to be the case, then wolves must have had the capacity for this diversity somewhere in their genomes. Thus, Vilà and his colleagues decided to compare the mitochondrial DNA of dogs and wolves in an attempt to understand the genetic consequences of these species’ different lifestyles: domesticated versus wild. (Remember, both dogs and wolves evolved from a common ancestral wolf species, so wolves are an ideal control with which to study the consequences of dogs’ life with humans.) The mitochondrial genome was used because of earlier work by Vilà that showed the nuclear genomes of dogs and wolves to be too similar to study their molecular evolution. On the other hand, this research indicated that mitochondrial lineages are clearly distinguishable for the two species.
Vilà hypothesized that certain mutations—those that might be deleterious, but not strongly so—accumulated faster in populations in which natural selection had been relaxed, resulting in a decline in fitness. In other words, after dogs started to live with humans, less fit individuals were more likely to survive and reproduce than they were in the wild. In addition, it is highly likely that dogs were strongly selected for certain behavioral traits, such as tameness. “It is therefore possible that this process led to an increase in functional genetic diversity throughout the entire dog genome,” wrote Vilà, “including both genes and elements affecting gene expression.” Such a relaxation of selective pressures might have led to the wide phenotypic diversity in dogs, as well as the variety of diseases seen in dogs today (Figure 1).