Charles Darwin was fascinated by pigeons, believing that the secrets to his revolutionary theory of natural selection in 1859 was held in their beaks.It was believed the development of their beaks weren’t affected by natural selection, with 350-plus breeds of domestic pigeons having beaks of all shapes and sizes. It was thought that centuries of interbreeding showed beak length was most likely to be regulated by just a few heritable factors. And modern geneticists had failed to solve Darwin’s enigma by pinpointing the molecular machinery controlling beak size.

       But now, biologists from the University of Utah have found that a mutation in the ROR2 gene is in fact linked to beak size reduction in numerous breeds of domestic pigeons.

       Mutations in ROR2 also underlie a human disorder called Robinow syndrome.

       Elena Boer, study lead and clinical variant scientist at ARUP Laboratories, said: “Some of the most striking characteristics of Robinow syndrome are the facial features, which include a broad, prominent forehead and a short, wide nose and mouth.”

       The researchers bred two pigeons with short and medium beaks.

       The medium-beaked male was a Racing Homer, a bird that is bred for its speed with and has a beak length similar to the ancestral rock pigeon.

       The small-beaked bird, a female Old German Owl, is a breed known for its short, squat break.

       Michael Shapiro, the James E. Talmagem, senior author of the paper, said: “Breeders selected this beak purely for aesthetics to the point that it’s detrimental – it would never appear in nature.

       "So, domestic pigeons are a huge advantage for finding genes responsible for size differences.

       “One of Darwin’s big arguments was that natural selection and artificial selection are variations of the same process. Pigeon beak sizes were instrumental in figuring out how that works.”

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       The short – and medium-beaked parents produced an initial F1 brood of children with intermediate-length beaks.

       The biologists mated the F1 birds to one another, which caused the F2 grandchildren to have beaks of all sizes, big and small and everything in between.

       The researchers compared genomes and identified DNA sequence variants scattered in the genome.

       They then looked to see if those mutations appeared in the F2 grandkids’ chromosomes.

       Dr Shapiro added: “The grandkids with small beaks had the same piece of chromosome as their grandparent with the small beak, which told us that piece of chromosome has something to do with small beaks.

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       “And it was on the sex chromosome, which classical genetic experiments had suggested, so we got excited.”

       The team then compared the entire genome sequences of many different pigeon breeds.

       The results showed that all individuals with small beaks had the same DNA sequence in an area of the genome that contains the ROR2 gene.

       The study authors think that the short-beak mutation might cause the ROR2 protein to fold in a new way.

       The paper was published in the journal Current Biology.