Everybody Into the Gene Pool


Dogs and people have a very similar number of nucleotide bases that make up our DNA, that is, all of our genetic material. Their genome contains 2.4 billion bases, while ours contains 2.6 billion. And while two dogs of different breeds are as different as any two people, dogs within a breed are 99.9 percent similar. Why is that?

It has to do with something called linkage disequilibrium. When breeds were created some 200 years ago, there was a kind of bottlenecking of genes within each one. Each breed was created from very few dogs, with a relatively static number of genes getting through the bottleneck to cause their makeup and all the other dog genes being left out. That means that unlike in random mating where dogs turn out differently from each other, there’s a disequilibrium in the amount of genetic material linked together on each chromosome to create dogs who not only share physical traits but also the same potential for developing particular diseases.

The breed with the highest level of linkage disequilibrium — meaning each dog is most closely related to the next because of the long linkages of genetic material that appear exactly the same from dog to dog — is the Shiba Inu, a Japanese breed. The Shiba was nearly wiped out during World War II, and the relatively few who survived and went on to mate passed down even fewer genes than the larger Shiba population before them.

Breeds like greyhounds have the shortest linkage disequilibriums, meaning they have shorter chains of genetic material that are exactly the same from one dog to another. It’s the same with golden retrievers. In fact, the Dutch and American populations of golden retrievers are as genetically different as the human populations of Europe and East Asia. (Not that those two populations are as different as they look. Every single person is more than 99 percent genetically identical to the next.) Interestingly, dogs of two different breeds are more genetically different than any two people. A dachshund and a German shepherd have less in common gene-wise than someone from Kenya and someone from Norway.

But all humans share with all dogs something very significant: the ability to digest starch. It’s believed to be a crucial genetic step in what allowed dogs — probably the first animal to be domesticated — to live among people. Think about the fact that people started farming close to the time that some wolves became dogs. Thus, the human diet began to consist less of hunted animals and more of starchy foods like wheat.

Wolves, on the other hand, have always thrived on a carnivorous diet. For the wolf to become a dog who could eat what we do, changes in the ability to break down and absorb nutrients from starch were going to have to occur. That’s exactly what happened. Mutations at three genes occurred for dogs to evolve from wolves in such a way that would allow for efficient starch digestion, making them many times better at breaking down starch during at least one part of the digestion process. These changes may have been selected for as dogs started hanging around people. Perhaps scavenging in waste dumps near increasingly common human settlements during the dawn of the agricultural revolution may have been just the change in ecological niche that allowed the early ancestors of today’s dogs who were able to digest starch to keep mating and produce new generations, hypothesize researchers from Harvard and MIT’s Broad Institute and Sweden’s Uppsala University writing in the journal Nature. It’s a “striking case of parallel evolution,” they say. “An increasingly starch-rich diet during the agricultural revolution caused similar adaptive responses in dog and human.”

Genetics researcher Elinor Karlsson, PhD, says that what happened with wolves and dogs also happened with non-human primates and people. People’s genes allow them to digest starch in a way that chimpanzees’ genes don’t. Humans gained an ability to be nourished from a wide variety of food sources through the same genetic adaptations as dogs.

It’s worth noting that wolves didn’t evolve into dogs in a strictly linear fashion. Yes, dogs have lots of wolf genes, but wolves have dog genes, too. It’s the reason some wolves are black. They inherited a gene back from dogs after dogs became an animal of their own but before the two species separated for good.


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