Promiscuity in Evolution

Posted 27 January 2005 by

No, this is not another post about the sexual habits of female apes. This is about enzymes, and their ability to catalyze different reactions with different substrates, even those that aren’t found in nature. It’s a property known as “promiscuity”, one that’s being increasingly recognized as important in enzymology and enzyme evolution.

The usefulness of enzymes derives in part from their specificity, in that they don’t just catalyze any old reaction with any old substrate. It would be hard for cells to maintain homeostasis if enzymes were highly nonspecific; helpful reactions would be coupled with harmful side reactions, regulation would be impossible, and things would get messy real quick. So it’s useful for enzymes to specialize in certain functions so that they can be applied for specific tasks at specific times. But because nature is a bit sloppy, enzymes are often able to catalyze many reactions weakly in addition to the “native” functions that they specialize in. These additional weak activities are referred to as promiscuous activities, and they’re potentially very important in enzyme evolution. Now a recent study (subscription required) published in Nature Genetics by Amir Aharoni and coworkers sheds some light on why enzymes are promiscuous, and what it means for their evolvability. (There is some good non-technical commentary on the paper here and here.) It also badly knocks down some bold claims made by leading ID proponents.

11 Comments

Great White Wonder · 27 January 2005

Nice stuff Steve. I might add that the "function" linchpin of Dembski's inane "theory" was, and remains, its most blatant and naive flaw after we allow ourselves to pretend with Dembski that mysterious alien beings with unprecedented powers actually exist. Of course, that's not where the problems end but looking much further begs the question.

But as we see here, a likely route towards novel gene evolution, which [Dembksi and Co.] did not account for, is one in which the new activity exists prior to gene duplication, with duplication simply allowing specialization.

Insofar as the concept of proteins having multiple functions -- including functions that have not yet been defined -- is obvious to any high school student who has taken a decent class in cell biology, the failure to deal with the issue responsibly has always been an unforgiveable oversight on Dembski's part. It's not reasonable to make such "mistakes" under the circumstances. Rather, such mistakes suggest selective ignorance of the facts. It's pathetic, actually.

frank schmidt · 27 January 2005

Very interesting, altho not all that surprising to a biochemist. And, as usual, it's a case study of how the IDC'ers take a textbook generalization

"Enzymes are highly specific catalysts."

and turn it into a rigid rule

"Enzymes only can catalyze a single reaction."

This allows them to reach the demonstrably false conclusion

"Within the next two years work on certain enzymes will demonstrate overwhelmingly that they are extremely isolated functionally, making it effectively impossible for Darwinian and other gradualistic pathways to evolve into or out of them."

I'm not concerned about Dembski; he's obviously incorrigible and has amply demonstrated the ability to ignore data that opposes his model (in this among other things, he clearly is not a scientist). But as a textbook author myself, I often have found myself having to make general statements to avoid a bewildering mass of disclaimers. I wonder about how we can teach the real lesson: that science draws verifiably true but non-rigid conclusions. It's a real problem, and it comes back to bite us sometimes.

Mark Perakh · 27 January 2005

Scientists usually shy away from making predictions. Recall that false prophets were to be stoned to death as Dembski and Co's favored book tells us, and the risk of becoming a false prophet calls for a caution in making predictions. Martin Gardner had defined a crank (in a book first published about 50 years ago). There are a number of features typical of a crank. A crank often has a penchant for introducing new terms (like Dembski's "unsimplifiability"), claiming to have discovered new important scientific laws (like Dembski's "law of conservation of information"), suggesting new, allegedly powerful methods of research (like Dembski's explanatory filter) etc. Behind this impressive facade there is in fact nothing of value. A crank may display enormous erudition, ingenuity in coming up with seemingly sophisticated arguments, and typically is absolutely convinced that his critics are stupid and either do not understand his great breakthroughs or willfully try to suppress his discoveries because of some non-kosher reasons. Gardner should have added one more feature typical of cranks - a penchant for dabbling in prophecies. In this contribution Steve Reuland gives a fine example of the abismal failure of one such bold prediction made by Dembski with his typical unbounded self-confidence. I doubt that the failure of Dembski's prediction will teach him a lesson. We can expect more such predictions from Isaac Newton of information theory. Perhaps there will be all kinds of arguments attempting to justify Dembski's uncautious wrong prediction, but no spin can deny the obvious.

Wedgie World · 27 January 2005

Another gap is closed for ID to hide in. While science uncovers more and more evidence and mechanisms, ID is doomed to remain ignorant.

While some people try to portray ID as scientific the reality is that there is NO scientific theory of ID. All there is, is a belief that science cannot explain all features and that this is evidence of 'design'. While admitting that ID was formulated to avoid addressing issues of 'who/what designed', the ID movement has made it clear that its designer is God.
Often ID can be observed claiming that it adds to science by providing means to detect design, something lacking in science while at the same time arguing that design detection has been applied succesfully in science. So in other words, the design ID is referring to cannot be about science since science already can detect design and thus this addition to science is, as expected, supernatural.

Not only is ID scientifically flawed and meaningless but it proposes that science can falsify God, who is forced to hide in gaps of our ignorance. A theologically risky and flawed position which may cause much damage to Christian faith.

Here we have a good example where ID is found to be wrong in its claims. Does this mean that ID has been falsified?

Jeff Low · 27 January 2005

To investigate what kind of evolutionary advantage promiscuity offers, the team created a speeded-up version of evolution in the lab. Mutations were introduced into the genes coding for various proteins in a completely random manner. Evolutionary pressure was then simulated by selecting those mutants with higher levels of activity in one of the promiscuous traits. After several rounds of mutation and selection, the scientists looked at their enzymes to see what had changed. As expected, they had managed to increase the activity they were selecting for by as much as a hundredfold and more.

So we generate mutations in a random manner, then intelligently select those mutations that increase the activity of the promiscuous traits, and then guess what? Surprise! Surprise! They increased the activity of what they were selecting for by as much as a hundredfold and more. This appears to be no different than simulating evolution with a computer program and simply shows what one can accomplish using intelligence.

Steve Reuland · 27 January 2005

I have a hard time believing anyone could buy such an illogical argument. The researchers did not "design" the proteins. They simply applied a selection criterion without any knowledge of the actual mutations involved. The results are proof of principle that mutation plus selection can create new proteins, the very thing that IDists have said couldn't happen. There is nothing stopping this from happening in the wild so long as there is selective pressure to encourage it to happen (which is precisely the case with PCP degradation, for example.)

By your reasoning, there is no such thing as a laboratory experiment that is not "intelligent design". Any variables that the researchers control means that they're using "intelligence", so therefore it's impossible to study any natural process whatsoever.

Great White Wonder · 27 January 2005

Speaking of promiscuous behavior, could someone explain to me what exactly is meant by the term "100% human brain" in the following context:

Mice With Human Brains Weissman has already created mice with brains that are about one percent human. Later this year he may conduct another experiment where the mice have 100 percent human brains. This would be done, he said, by injecting human neurons into the brains of embryonic mice. Before being born, the mice would be killed and dissected to see if the architecture of a human brain had formed. If it did, he'd look for traces of human cognitive behavior. Weissman said he's not a mad scientist trying to create a human in an animal body. He hopes the experiment leads to a better understanding of how the brain works, which would be useful in treating diseases like Alzheimer's or Parkinson's disease. The test has not yet begun. Weissman is waiting to read the National Academy's report, due out in March.

http://news.nationalgeographic.com/news/2005/01/0125_050125_chimeras.html

Steve Reuland · 27 January 2005

I can only assume they mean mice with brains that consist entirely of human neurons.

Great White Wonder · 27 January 2005

What "traces" of "human" cognitive behavior does he expect to observe in his little mouse dude?

I'll be frank, here: the remote possibility of benefits flowing from this research don't outweigh the sickening feeling.

From my perspective, that sort of experiment is no different from creating a human embryo and allowing the embryo to develop "traces" of "human" cognitive behavior so Dr. Weissman can experiment on it.

Steve Reuland · 27 January 2005

I don't really know, but my guess is that they'd like to see if the neurons form the same kinds of connections that human neurons do, or if the brain specializes the way human brains do, or if they act more "mouse-like". That would tell us if the factors responsible for human cognition (or at least human brain structure) are contained within the neurons themselves, or are influenced by other factors within the organism.

But this stuff isn't really my field. I prefer enzymes. (Hint.)

Great White Wonder · 27 January 2005

I put my follow-up thoughts on the "Little Mouse Dude" sub-thread up on the Bathroom Wall, in case anyone is remotely interested. ;)