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MENDEL'S DEMON reviewed by Gert Korthof
24 Aug 2001 (update: 3 Feb 2002)
DNA copying accuracy limits the increase of complexity of life on Earth. High error rates limit the number of genes that can be accurately copied. Furthermore and unexpectedly the Mendelian type of inheritance and sexual reproduction is a requirement for complex life. On the basis of this Mark Ridley predicts that complex life on other planets also must have the Mendelian type of sex.
IntroductionA year ago an alarming article on error rates in hard disks appeared in the Scientific American . It stated that if disks grow in size the total number of errors grows too. Assuming a constant relative error rate, we don't notice the errors on small disks, but there is a real risk of encountering errors on disks with Giga and Terabyte capacity. This means that without increasing the error-correcting rate big hard disks are just unreliable. Ridley did not use this example, but it would be a perfect introduction to the main problem of his book: keeping the error rate down by all kinds of error correcting mechanisms based upon redundant information in DNA. (See for more metaphors the box Technological metaphors).
Ridley uses the 'Chinese whispers' (or 'telephones' in North America) game as a metaphor. This metaphor does a marvellous job. It is the game in which a message is repeated from one person to the next, down a line. Ridley uses the message: WHEN THE TIGER COMES, FREEZE. (This one is easy to remember, but make it longer or try a string of 100 random letters). By the end the original message is laughably corrupted (see illustration). Ridley uses the following mutations of the original message:
WHEN THE TIGER COMES, SNEEZE
"Copying accuracy per letter is the same in all life except viruses" (p. 93). So for the rest of life the absolute number of errors simply depends on the length of the DNA message (compare with the errors on hard disks). Worm and fruitfly are just below 1 harmful error per offspring. Humans have a DNA length 2000 times larger than bacteria (about 33,000 genes) and produce 200 copying mistakes per offspring of which 2-20 are harmful. Stunning and alarming information. It's clear that it makes no sense to have 1 million genes. It would imply 6000 mutations of which 60-600 per offspring are harmful. No surviving offspring means extinction of the species. So there is an upper limit of complexity (number of genes). For other reasons Ridley states that "a life form cannot exist if it makes more than ONE mistake per offspring" (p. 78). So here is a paradox. We should all be dead. The human species should have been extinct for long. How we escape the paradox is the story of the book.
Unexpected predictions and explanationsRidley's analysis leads him to predict that if complex life exists on other planets, then it must use Mendelian inheritance. To me this was a really unexpected prediction. Ridley's prediction is based on the insight that the details of Mendelian inheritance are crucial for complex life. For example the following puzzling details of the design of the process that produces haploid gametes from diploid cells (meiosis):
Ridley's analysis leads him to predict that the simple one step meiosis should not exist. This is an eminently refutable prediction. I never realised the details of meiosis were unexplained. Ridley has an evolutionary explanation for it. In my view this proves the power and profoundness of his evolutionary analysis. No other alternative theory ever gave such detailed explanations. No other alternative theory pointed out the problem in the first place. If the success of a scientific explanation is measured by its ability to elucidate brute facts, then surely we have here a successful explanation at hand.
There are many more rewarding insights in Ridley's book such as the relation between the existence of sex and the existence of mutations (sex is pointless without mutations); Kondrashov's prediction about the effect of mutations in sexual reproducing organisms; why female chromosomes are expelled in meiosis at fertilisation; the reason for gestational diabetes in humans; why genes inherited from the father stimulate fetal growth and inherited from the mother limit fetal growth; why color blindness has increased in rich countries and why death rates of single versus married people differ. Ridley even proposes a new cure for AIDS based upon the principle of mutational meltdown. In short: Mark Ridley shows that evolutionary theory makes sense of puzzling details of cloning, sex, meiosis, Mendelian inheritance, gender, the Margulian merger, pregnancy and mutations rates (cloning of humans is in the news. He tells a few useful things about the strategy of cloning from a genetic-evolutionary point of view). I found it extra rewarding that Ridley applies all his knowledge to our own species (chapter 9 and 10). In fact the book is more about the humans species, than complex life in general. Many of the theories Ridley explained in his book turn out to be of great help to understand new findings in biology, which otherwise would be incomprehensible.
Is error elimination the core of neo-Darwinism?The Chinese whispers metaphor dominates the book and his thinking. "The Chinese whispers is actually a powerful way to think about mutation." (p. 63). "The analogy points to a property not just of children's games, but of life too." (p. 63). "There is only one way of being right (the original message) and so many ways of being wrong (anything other than the original message)." (p57). I have two problems with the Chinese whispers metaphor and the underlying idea:
(1) Typically the Chinese whispers metaphor starts with an error-free, uncorrupted, meaningful message: THE ORIGINAL. However evolution is not a game starting with a correct and meaningful message. (Do 'correct' messages exist anyway?). The CW metaphor obscures the question of the origin of correct and meaningful messages, because it simply assumes the existence of a correct and meaningful message.
(2) If the original is by definition error-free, then all DNA mutations must be corruptions of the original. There is no place for improvement. The notion of a 'creative mutation' seems absurd and a 'creative error' becomes self-contradictory. Ridley strengthens this idea by using a speech from Hamlet as the original message (3). No wonder Ridley is inclined to think that zero errors is the ideal (4). It looks as if life is just a Xerox machine. The goal of a Xerox machine to make 100% identical copies and nothing more. That's exactly what copying machines are for. We are not interested in a copying machine that makes all kinds of creative variations.
Please note that the Chinese whisper metaphor is compatible with the created and fixed species model of the Bible. Species contain the original God-given DNA message and that message must be safeguarded. There is no need to evolve new genes, because they are there from the Beginning. Surely Ridley does not support such a view!
Of course Ridley has the right to choose his subject: "our topic here is not evolutionary change. Our topic is mutational decay and how life preserves itself against it." (p. 62). In that context the metaphor is helpful indeed. But clearly Ridley wants more than explaining mutational decay alone. He wants to understand "the Evolution of Complex Beings". In that wider context the metaphor does more damage than good, because improvement and conservatism are antagonistic. For innovation the more mutations the better; for conservation : the less mutations the better. I think that finding a balance between too much and too few mutations, is what makes evolution an exciting subject. It's just too easy to focus on one aspect and forget the other. Proofreading enzymes cannot selectively correct harmful mutations. So if proofreading enzymes lower the total number of mutations, the number of useful mutations is lowered too.
Let's have a look at 8 examples of how the Chinese Whispers metaphor controls Ridley's evolutionary thinking:
Error reduction and innovations are intimately connected. Error reduction itself depends on the evolution of 50 new proofreading genes. Where did they come from? The secret of writing good books is not good spellcheckers, proofreaders, or editors, but a good author. Although Ridley discusses increase in complexity in chapter 2, that (complex!) story lacks a powerful metaphor and is contradicted by the Chinese whispers metaphor.
Generalising beyond Ridley's book: is the Evolutionary Synthesis (7) really completed as long as we don't have a theory that explains the origin of evolutionary innovations: new genes, new organs, new organisms? Wouldn't it be time to sort out if enough resources (time, reproductive capacity, number of beneficial mutations) are available to create complex adaptations? I hope Ridley's next book (Evolutionary Innovations (15) or Creative Evolution) contains a chapter discussing how life produced those 50 proofreading and repair enzymes. And a chapter discussing how all this can be done with a 'negligible fraction of mutations'. In short: we need a theory of evolution that does not take the core question for granted, but systematically explains how life got from 0 to 35,000 genes. And how animal body plans got from 1 to 35. I think it can be done, but somebody has to write it down and do the necessary research. Yes, I have somebody in mind! (14), (15).
Is there an alternative for the Chinese whisper metaphor? I think the 'trial and error' does a better job in the context of evolutionary innovations necessary for complex life. Mutations are trials. Trials can be successful or unsuccessful, but they always are trials. We can eliminate error completely by eliminating trial at the cost of never finding anything new, and be in a frozen state forever. This is incompatible with evolution. Stuart Kauffman uses the trial and error metaphor extensively, almost exclusively (8). In this metaphor evolution is a search in sequence space, protein space or shape space. A search is not assuming a correct message. Indeed we need to conserve our successes, but we have to discover them in the first place. A complete theory of evolution needs to integrate the conflicting metaphors of the Chinese whispers and the trial and error metaphor.
ConclusionIs the book difficult? Mendel's Demon is about concepts, not about animal and plants. Knowledge about fundamental biological concepts (mitosis, meiosis, Mendelian inheritance, mitochondria, Margulis symbiosis theory, haploid, diploid) is helpful, but these things are explained and illustrated in the book (glossary). Apart from that, one does not need to study many other books to understand Ridley (13). The success of Ridley's book lies in his creative use of well known metaphors: Chinese whispers, encyclopaedia, broken cars, meltdown, the holy writ, business merger, etc.
As an exposition of the limiting factors in evolution of complex life, Ridley's book is splendid. The explanation of many odd details of meiosis, Mendelian inheritance, sexual reproduction and gender demonstrates the fruitfulness of current neo-Darwinist research. Far from being side issues, these issues are fundamental to understand multicellular life. There is simply no competing theory in the whole field of biology that equals the explanatory power of neo-Darwinism. At the same time Ridley's book shows the limitation of current neo-Darwinism: the avoidance of the issue of innovation. I really hope that Ridley's next book is about evolutionary novelty. If anyone can do it, he is the man.
[ in 2004 the third edition was published ]
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|Copyright ©G.Korthof 2001||First published: Aug 24 2001||Update 3 Feb 2002. Links/Notes: 11 Oct 11|