Kibble for Thought: Dog diversity prompts new evolution theory

Science News ^ | 18 December 2004 | Christen Brownlee

[PatrickHenry]

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.

These results suggest that dogs have experienced significantly higher rates of tandem repeat mutations than the related species have, says Fondon. Because tandem-repeat sequences litter the genes that control the developmental plan in many species, Fondon suggests that mutations in these regions could have a strong bearing on evolution.

"As a new finding about the biology and genetics of dogs, I'm all for it. But in terms of applying this to [evolution in general], I think there's a question mark," says Sean Carroll, an evolutionary geneticist at the University of Wisconsin–Madison.

Carroll notes that because dog owners have coddled their companions over the centuries, mutations that would have killed wild animals may have persisted in the gene pool of domestic dogs. Because domestication diverges from a standard model of evolution, he says, further experiments are necessary to add weight to Fondon and Garner's theory.

[fr_freak]

I guess I'm a little confused as to how you think life, insects, plants, animals etc. came to being on this planet.

Really? If I have been defending Creationism this whole time, what do you think one of my possible answers might be?

I'm more confused about how YOU think all these creatures got here. They had to start from somewhere. If they all evolved, then there had to be a first organism. Where did that come from? The funny part is that you find it ridiculous that I might answer that an intelligent designer created them out of thin air, but yet the logical conclusion of your reasoning is that they were created out of thin air by RANDOM FORCES.

[orionblamblam]

> By this I mean there has to be hard-wired into the organism a trigger that causes it to put into motion the reproductive mechanisms that it has.

Considering that the precursor molecules to the very simplest things maybe called life are themselves self-replicating, and not due to any desire on their part, but just part of the mechanism... it would not be surprisign if early life was equally as prolific and yet as disinterested.

> Viruses, by the way, cannot replicate themselves.

They seem to be quite capable of it.

[fr_freak]

> Viruses, by the way, cannot replicate themselves.

They seem to be quite capable of it.

I'm not a Biology major, but if I recall correctly viruses require a host cell to infect in order to reproduce. I think it's safe to assume that if a virus was the first life on Earth, it did not have a host cell to infect.

The Difference Between Bacteria and Viruses

[Clorinox]

"but yet the logical conclusion of your reasoning is that they were created out of thin air by RANDOM FORCES."

Not thin air at all, but a planet called earth with various chemicals, air, liquid water, dirt, heat, an atmosphere capable of blocking solar radiation etc. I do believe life spontaneously evolved from these basic components. So do you think all forms of life were created at once, or did the creator parse out this life over long periods of time? Is the creator creating new species out of thin air as we speak, or are these species deriving mostly from sexual reproduction?

[SunkenCiv]

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Note: this topic is dated December 2004. Blast from the Past. Thanks PatrickHenry. Just adding to the catalog, not sending a general distribution. To all -- please ping me to other topics which are appropriate for the GGG list.