The genetic variant probably comes from ancient wolves.
Ancient dogs, domesticated from wolves in the past 30,000 years, differed in size to some extent. But the current extreme size differences — the largest breeds are up to 40 times bigger than the smallest — emerged in the past 200 years, as humans established modern breeds.
The researchers identified two versions, or alleles, of the variant. Across all breeds, dogs with two copies of one allele tended to weigh less than 15 kilograms, whereas two copies of the other version were more common in dogs weighing more than 25 kilograms. Dogs with one copy of each allele tended to be intermediate in size, says Ostrander.
The prevailing view among scientists used to be that small body size was probably linked to relatively new genetic changes, potentially unique to domestic dogs, says Robert Wayne, an evolutionary biologist at the University of California, Los Angeles. “This turns the whole story on its head. That’s what’s marvellous about the whole thing.”
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