Thursday, April 19, 2007

New Ways of Looking at Evolution

 
John Logsdon over at Sex, Genes, and Evolution recommends a new book, The Origins of Genome Architecture by Michael Lynch. John also points us to a review article by Lynch [The Origins of Eukaryotic Gene Structure]. I second both recommendations. Read the article. Buy the book.

Here's a quotation from the article by Lynch,
Despite the enormous progress in molecular genetics over the past 50 years, no general theory for the evolution of the basic architectural features of genes has been formulated. Many attempts have been made to explain the features of genes, genomes, and genetic networks in the context of putatively adaptive cellular and/or developmental features, but few of these efforts have been accompanied by a formal evolutionary analysis. Because evolution is a population-level process, any theory for the origins of the genetic machinery must ultimately be consistent with basic population-genetic mechanisms. However, because natural selection is just one of several forces contributing to the evolutionary process, an uncritical reliance on adaptive Darwinian mechanisms to explain all aspects of organismal diversity is not greatly different than invoking an intelligent designer.
Some of you will probably see why I like this guy! He warns against "uncritical reliance on adaptive Darwinian mechanisms."
This paper represents a first step toward the formal development of a general theory for the evolution of the gene that incorporates the universal properties of random genetic drift and mutation pressure. Although the ideas presented are unlikely to be correct in every detail, at a minimum they serve as a null model. For if verbal adaptive arguments are to provide confident explanations for any aspect of gene or genomic structure, something must be known about patterns expected in the absence of selection. This is a significant challenge because at this point it is difficult to reject the hypothesis that the basic embellishments of the eukaryotic gene originated largely as a consequence of nonadaptive processes operating contrary to the expected direction of natural selection. A significant area of future research will be to take these observations on gene and genome complexity to the next level, to evaluate whether natural selection is a necessary and/or sufficient force to explain the evolution of the cellular and developmental complexities of eukaryotes.
Everyone needs to start paying attention. Random genetic drift is just as important for evolution as natural selection. That's not speculation. As far as I'm concerned, it's hard incontrovertible fact.

One of the "new ways" of looking at evolution is to consider mutation pressure, loosely defined as differences in the frequency of mutation. I'm not a big fan of this but it does emphasize that modern evolutionary theorists are thinking outside the Darwinian box—not surprising since Darwin died 125 years ago (today). I prefer mutationism, which is a way of emphasizing the imprtant role of mutation in directing evolution. Mutationism and mutation pressure are not the same thing.

4 comments :

  1. Lynch's model isn't selection free. Rather, he choses to emphasize relaxed selective constraint due to small population size in taxa with complex gene structure. It's Ohta's nearly neutral theory extended to the evolution of genomes.

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  2. One of the "new ways" of looking at evolution is to consider mutation pressure, loosely defined as differences in the frequency of mutation. I'm not a big fan of this but it does emphasize that modern evolutionary theorists are thinking outside the Darwinian box—not surprising since Darwin died 125 years ago (today). I prefer mutationism, which is a way of emphasizing the imprtant role of mutation in directing evolution. Mutationism and mutation pressure are not the same thing.

    I'd sure like to see more on this from you, as and when you have time. Or maybe some references?

    RBH

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  3. Many thanks. Let me respond to just one snippet (though much else is worth comment, of course). You wrote

    Fair enough, but what other possible pathways could have been followed? If the actual pathway is only one of several million possibilities then why was that one particular design selected? Is there no possibility that it could have been accidental and selected? Isn't it possible that the actual end-product is as much due to the random mutations that occurred as it is due to natural selection?

    Dawkins seems to ignore this possibility when he draws attention to the non-randomness of evolution. I think that's a mistake. We can easily agree that natural selection is non-random but it doesn't necessarily follow that evolution now becomes predictable.


    In a train wreck of a thread on Infidels I was inspired recently to re-examine some of the data from Lenski, et al.'s 2003 Evolutionary Origin of Complex Features. While it's not part of that re-analysis, there's one striking result of that study that both illustrates your point above and modifies it. In a static selective environment identical across runs, with an identical Ancestor over runs, and varying only the initial random seed that determined the timing, kind and location of mutations through the runs, in 50 runs Lenski's Avida simulation evolved artificial agents that could perform a fairly complicated logic function (EQUALS) in 23 of 50 runs. What is of interest in this context is that the 23 lineages that evolved to perform EQUALS did so in 23 different ways -- the 23 evolutionary pathways were different from one another and the underlying opcode programs that were the artificial agents' 'genes' were different from one another in all 23 cases. They performed the same function, but they evolved that capability by very different routes, and in the end did it in very different ways.

    So in one sense the system is predictable: under conditions where there are complicated functions that are reproductively advantageous, 'Darwinian' mechanisms can often assemble complicated critters that perform those functions. On the other hand, it's an unpredictable system in that the particular evolutionary route (history) and the particular structure (genome) that enables performance of the functions are wholly dependent on the initial random seed: one can't predict the pathway, one has to run the model to see what it does given the particular randomization.

    That provides a nice illustration of your argument that

    The main conclusion of this essay is that a large part of ongoing evolution is determined by stochastic events that might as well be called "chance" or "random." Furthermore, a good deal of the past history of life on Earth was the product of chance events, or accidents, that could not have been predicted.

    I will not invoke Conway Morris' catalog of convergent structures here, but I'm put in mind of it. :)

    RBH

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