Form or function? Evolution takes different paths

Apr 05, 2010

Biologists long have known that both the appearance of organisms and their inner workings are shaped by evolution. But do the same genetic mechanisms underlie changes in form and function? A new study by scientists at the University of Michigan and Taiwan's National Health Research Institutes suggests not.

The research is scheduled for online publication in the during the week of April 5.

In the study, U-M evolutionary biologist Jianzhi "George" Zhang and colleagues Ben-Yang Liao and Meng-Pin Weng set out to systematically test a hypothesis proposed by molecular biologist Sean Carroll in 2005. Carroll posited that changes in morphology (such things as shape, color and structure of external and internal parts) occur through different genetic mechanisms than changes in physiology (inner workings). Carroll backed up his assertion with examples, but the idea, which challenged previous dogma, was controversial, Zhang said.

To test the hypothesis, Zhang's team turned to a database of knockout mice---lab mice that have been engineered to lack particular .

"We found about 5,200 genes that have been knocked out in the mouse and the resulting effects studied," said Zhang, a professor of ecology and . "From those genes, we looked for genes that, when knocked out, affect only morphological traits, not physiological traits. We got about 900 of those genes, which we call morphogenes."

The researchers also found about 900 "physiogenes"---genes that affect only physiological traits, not morphology.

"Next, we compared the two groups of genes to see if there are differences in the molecular roles of their products," Zhang said. "We found very large differences." Morphogenes were more likely to carry instructions for transcription---the step that determines whether a gene should be turned on and how much gene product should be manufactured. Physiogenes were more likely to be blueprints for enzymes, receptors, transporters and ion channels (molecules that control the flow of ions across cell membranes).

The next step was to examine patterns of evolution in the two groups of genes.

In a classic paper published in 1975, evolutionary biologists Mary-Claire King and Allan Wilson argued that evolution of both morphology and "ways of life" (physiology and behavior) occurred through changes in the way genes are turned on and off, rather than through direct changes in gene products themselves. In the parlance of geneticists, these traits were shaped over time through changes in , not changes in protein sequence. King and Wilson supported their claim with the example of chimpanzees and humans, which are remarkably similar at the protein sequence level, but quite different in appearance and behavior. It was this influential paper that Carroll commemorated 30 years later, but he suggested instead that physiological changes are due to protein sequence changes, while morphological changes result from changes in gene expression.

With their new analysis, Zhang and colleagues found that, at the protein sequence level, physiogenes evolved much faster than morphogenes. "This is consistent with the idea that physiological changes tend to be caused by changes," Zhang said.

Next, the researchers examined gene expression data, looking to see how similarly or differently genes are turned on or off in identical tissues from different species, such as the livers of mice and humans. Greater differences indicate more rapid evolutionary change.

"We found more differences in morphogenes than in physiogenes," Zhang said. "In other words, morphogenes evolve faster, with respect to expression patterns, than do physiogenes---a finding that supports the idea that morphological changes result mainly from gene expression changes."

The finding that and physiology are shaped by different evolutionary genetic processes can not only aid in future evolutionary studies, but can also be helpful in the study of human disease, Zhang said. "Our analysis of the knockout mouse data suggests that morphological defects are more likely due to problems with gene expression. This knowledge could help identify the disease-causing mutations more quickly, because it narrows the set of candidate genes and mutations that one needs to search from."

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More information: Proceedings of the National Academy of Sciences: www.pnas.org/

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User comments : 3

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breadhead
1 / 5 (3) Apr 10, 2010
What definition of evolution is used here? Variation in a kind? Is the article's assumption that this gene tailoring will eventually produce a banana, or something other than a mouse? Instead of the first sentence in this article, I can say, "I have known that both the appearance of organisms and their inner workings were shaped by creation", But, I would be correct, and it would put the article in the trash, where it belongs.
kevinrtrs
1 / 5 (3) May 12, 2010
So the real research is more what diseases pop up when damage occurs to gene expression than it has to do with how one species changes from X to Y, where X and Y cannot interbreed.

Form and functional challenges will frustrate evolutionists for eons to come. The reason is simple - if one is to construct ANY new structure, one has to know in advance what the outcome is to be. And there-in lies the problem for evolution. It needs to generate the instructions to create components, assemble the components into a new form, then get the feedback mechanism to confirm that assembly is complete. Then once the structure is fully formed it needs to be primed for use, i.e. integrated with existing elements. After that instructions are necessary in some brain/nervous system to enable use of that new structure. Currently there exists no known evolutionary mechanism to generate such instructions/code. This is a major stumbling block for evolutionary research.

Skeptic_Heretic
1 / 5 (1) May 17, 2010
So neither of you two geniuses have ever worked on a raw problem.

When developing a new tool one doesn't engineer the tool directly, there is a perfection process of adapting another tool to fit the new task. That is evolution within design.

The difference here is evolution in the real world doesn't require the hand of god to craft it. The hands of the environment do so without the need for magical beings making wishes and waving wands.