| A puzzle that has intrigued biologists since Darwin’s time is why the fossil record shows new species seem to emerge suddenly and rapidly – in contrast to the gradual process that evolution is supposed to be. Researchers at University of Texas Southwestern Medical Center, Dallas, Texas, found an explanation in dog genes.
The explanation would be that many mutations occur in regions of the genes where a single, simple instruction is repeated many times. The mutations involve removing or adding one repeat. This differs substantially from what most scientists have traditionally considered the predominant mechanism of mutations, which alters only one “letter” of code changes at a time – a much smaller change.
The difference is somewhat like an instruction manual’s editor mistakenly changing the number of times an instruction is repeated, as opposed to changing one letter in the book. The former action is more likely to meaningfully alter the final result.
The chemical units that make up an organism’s DNA, or genetic code, are abbreviated with the letters A, C, T and G. Strings of these letters spell out the genetic instructions needed to carry out life’s functions. Many scientists believe evolution occurs through “single-point” mutations – a change from one letter to another among the billions of letters in the code.
The researchers found that bigger changes occur in genetic regions, called tandem repeat sequences, consisting of the same series of letters repeated many times over, for example, ACTACTACT. These mutations happen in these regions when such units – the ACT in the above example – are mistakenly added or subtracted as a group.
The researchers found that such changes help explain the length of dogs’ muzzles. The scientists combined genetic data from different dog breeds and data on the shapes of dog skulls, and found that that the length of a dog’s muzzle depends on the number of times specific tandem repeat units determining muzzle length are repeated.
Mutations in tandem repeat sequences occur up to 100,000 times more often than single point mutations, and are much more likely significantly change physical appearance, said John “Trey” Fondon, an evolutionary biologist and co-author of the study. | 
