Dog Size

Environmental & Science Education, STEM, Biological Evolution, Biodiversity, Nature, Wildlife, History of Science, Nature of Science.

Ed Hessler

Have you ever wondered why dogs vary so much in size and then chalked it up to selective breeding?

Ewen Calloway (Nature 27 January 2022) notes that "dogs differe more in size than any other mammal species on the planet. A mutation behind such variation has been traced to an unexpected source" ancient wolves.

He continues, "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."

Calloway reports that "two versions, or alleles, of the variant (gene) have been identified." The research on which he reports notes that when pooches have two of the small-bodied alleles, the dogs are smaller; when they have two copies of the large-bodied alleles are larger; and dogs with a copy of each are "intermediate" in size.

Did one pair come first. Right now the resarchers think the allele for small bodies is evolutionary the oldest. "Coyotes, jackals, foxes and most other canids they analysed, Calloway writes "had two copies of the ‘small’ version, suggesting that this version was present in a common ancestor of these animals."

Evolutionary biologist Robert Wayne (UCLA). "This turns the whole story on its head. That’s what’s marvellous about the whole thing.” What has been turned upside-down is that what was formerly thought to be due to "relatively new genetic changes, potentially unique" to our dogs. Geneticist Elinor Karlsson (U Mass Chan Medical School) adds that it could mean "dogs were domesticated from smaller-bodied wolves."

Caution is warranted here since we like to attribute things to genes, i.e., "the gene for."  This is complex since the gene in question 'accounts for about 15% of variation between breeds." 

A reminder that nature's mechanisms present very difficult research problems.