Moselio Schaechter


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December 01, 2011

A Gathering of Comments on a Talmudic Question

by Elio

First it may be appropriate to repeat what we mean by Talmudic Questions (and therefore Talmudic Answers). We use the term loosely to denote questions that may lack definitive answers but that open the door to ingenuity and dialectic. These questions embody the musings: “I have often wondered about…” We are fond of saying that answers to such questions are not likely to be found by a Google search.

Our Talmudic Question #81 (Given that most mutations are deleterious, why does the mutation rate not evolve to zero?) elicited especially lucid comments. Here we summarize a few only (and apologize to those whose comments are not mentioned).

Jan Spitzer points out that the human mutation rate is very slow indeed. But, he notes: something else is afoot: very fast growth and mutations of ‘memes.’

Daniel Smith posits that most mutations are in fact not deleterious. He says: I think the mutations we notice are certainly more likely to be deleterious. But what's science without a little sample bias?

John Wilkins cleverly points out that: Any mechanism that might keep the mutation rate low would be subjected to deleterious mutations...

Ramy Aziz tells us: Problem is: it cannot evolve to zero unless it is not zero. If mutation rate slows down to near zero, evolution will also slow down, and the probability of a ‘beneficial’ mutation of ‘no mutation’ will be astronomically low.

MB thinks that: A mutation rate of zero can be good for an individual organism, but deleterious to a population because of selective pressures. Hence good for population, bad for individual, and evolution/selection works on populations.

Liz says something similar: Zero mutation rate means no variation and thus the halt of evolution through the loss of environmental adaptability. This decreases the fitness of an organism that achieves this state versus one that allows some mutations, even if most of these would be deleterious.

Ami Bachar goes further: In earth or earth-like environments, where evolution is the base for all life, there cannot be such a thing as evolution to zero mutation rate.

Thinking about possible mechanisms, Mike Jones writes: Is there a level of noise that is unavoidable during replication?

And, similarly, Nick Matzke offers this: Because error-checking and correcting the DNA takes energy, organisms have a limited amount of energy, and thus at some point the cost of error-checking outweighs the benefits.

Mark Martin suggests that we pay attention to viruses: It also reminds me of the ‘no proofreading’ problem with RNA polymerases. RNA viruses using that enzyme for replication are extremely prone to mutation, and make many defective progeny (do we have a number for that?). Yet the ‘lottery winners’ have their shot, as well.

And lastly, Nathan Myers muses: So, here we are, bobbing about in a seething cauldron of mutating viruses with our exons and memes and vaccines and recombinant gene therapy. The Joneses, next cauldron over, have all that and mutations besides. Eventually the viruses cough up a mutation our civilized library of adaptability cannot best. Soon only Joneses bob about in our cauldron.

Comments

If the mutation rate goes to zero, the rate of lateral transfer must increase or the organism will become extinct.

Elio replies: A good point. Thanks for sharing it.

PS: Specific paper to read on mutation rate: http://www.indiana.edu/~lynchlab/PDF/Lynch183.pdf "Evolution of the mutation rate" (Lynch 2010 Tr Genet)

It like "crowdsourcing": many people sharing their ideas!

This was delightful. Every time i read something like this i thank my karma for making me a biology student:)
Indrani Roy

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