Kibble for Thought: Dog diversity prompts new evolution theory
Christen Brownlee

The wide range of variety in domesticated dogs-from the petite Chihuahua to
the monstrous mastiff-has powered a new view of what drives evolution.

Scientists have long known that the evolutionary changes that alter a
species' appearance or create new species frequently occur in rapid bursts.
One widely accepted theory holds that any evolutionary change results from a
random switch of a single genetic unit within DNA.

These single-point mutations occur in about 1 out of every 100 million DNA
sites each generation. This frequency is too low to cause rapid evolutionary
change, assert John W. Fondon and Harold R. Garner, biochemists at the
University of Texas Southwestern Medical Center at Dallas.

While examining human-genome data, Fondon found that small segments of
repeated DNA sequences, called tandem repeat sequences, are frequently
present in genes that control how an animal develops into its final
appearance. Unlike single-point mutations, tandem repeat mutations occur
when a cell's machinery for copying DNA makes a mistake and inserts a
different number of sequence copies.

Such mistakes, which happen 100,000 times as often as single-point
mutations, could alter an organism's appearance or function for successive
generations.
"I was stunned by what I found," says Fondon. "It occurred
to me that this
might be a nifty way for [organisms] to evolve very rapidly."

To evaluate this hypothesis, Fondon and Garner looked for tandem repeat
sequences in 92 breeds of domesticated dogs. For example, they examined a
gene that determines nose length. They found that the number of times a
particular sequence is repeated correlates strongly with whether a breed has
a short or long muzzle.
Many researchers explain dog-breed diversity as the emergence of hidden
traits in the genome. However, says Fondon, a more likely scenario is that
genetic mutations occur in dogs at a high rate.

By comparing skulls of dogs over decades, Fondon and Garner found
significant and swift changes in some breeds' appearances. For example,
between the 1930s and today, purebred bull terriers developed longer, more
down-turned noses.

Moreover, the researchers found more variation in tandem-sequence repeat
lengths among dogs than they found in the DNA of wolves and coyotes.

Full Text at Science News
http://www.sciencenews.org/articles/20041218/fob1.asp

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