Friday, October 22, 2010

Ravello meeting on Chance and Necessity, part II

I am at the Ravello meeting on Chance and Necessity in biology, on the 40th anniversary of Jacques Monod's seminal book, and will be posting a few entries while the meeting is going on this week.
The gathering is organized by Giorgio Bernardi, sponsored by International Union of Biological Sciences and Istituto Italiano di Studi Filosofici.
What follows are the raw and somewhat selective notes only, in order of presentation of the various speakers. Hopefully this will provide a feeling for what the meeting is about and generate some discussion. Throughout, parenthetical comments are my own, unless otherwise noted.
Werner Arber, Contingency of spontaneous genetic variation.
Talk started with a (somewhat peculiar) historical overview leading from Darwinism and the Modern Synthesis, through Watson and Crick and genomics, to a broader "synthesis" concerning molecular evolution. Different definitions of mutation if issue considered from phenotypic or molecular perspective, of course (just like the definition of gene itself). (Long-winded) introduction covering basics of molecular genetics. (Not clear at all what the point of this was, other than giving us a quick molecular genetics 101.)
Masatoshi Nei, Hugo de Vries and species formation: new perspectives from recent genomic data.
de Vries was famous for his experiments on mutations in Oenothera plants (and for contributing to the rediscovery of Mendel's work). These mutations were soon shown to be the result of chromosomal rearrangements and abnormalities, as opposed to the sort of point mutations discovered at the time by Morgan in Drosophila. Stebbins referred to de Vries' mutationist theory as a figment of imagination, even though polyploidy is very common in plants and other groups (this can't be right, Stebbins was well aware of polyploidy and it's role in speciation). Modern molecular genetics suggests that following genomic duplication there is a reduction in gene numbers that leads to incompatibility and speciation. (Lots of refs to Nei's own work on hybrid sterility back from the '70s and '80s.) Nei doesn't like Coyne and Orr's critique, in 2004, of his neutral model of hybrid speciation, proposed in 1983, suggesting that neutral models are under appreciated. (On this one I think Coyne and Orr were correct, actually.) (Overall, Nei seemed to want to significantly scale down the evolutionary importance of selection in favor of mutation, though I don't think his arguments were very coherent.)
Eviatar Nevo, Stress and evolution at micro- and macro- scales.
Importance of a variety of environmental stresses as major drivers of adaptive phenotypic evolution. (This has been a theme of Nevo for decades now.) Documented differences between, for instance, underground and above ground mammals, range across morphology, behavior, and even fine aspects of physiology. No question that life style drives phenotypic evolution. Evidence for a positive relationship between genetic diversity and levels of environmental stress. Similarly, indices of sexual activity, as opposed to asexual reproduction, increase with stress. (This morning we've steered pretty clear from Monod, chance and necessity. Hopefully better this afternoon, judging from the titles.)
Eugene Koonin, The role of extremely rare events in the evolution of life.
Major transitions in evolution are examples of extremely rare events and how important they can be, e.g., origin of life, nucleotides, cells, eukaryotes, or multicellularity. How do we explain the origin of replication and translation processes? Neither natural selection nor exaptation are adequate since both processes require replication and translation to get started. One popular answer is the RNA world type scenarios. However, known RNA replicases are ligases, not polymerases. (Somehow) the answer is related to inflation in cosmology... Which leads to a multiverse with island universes, of which ours is one, and in which the big bang becomes a local event... (Apparently) this is relevant because the number of times a given macroscopic history is repeated in an island universe is infinite. (Voila, by epistemological sleight of hand we solved the problem!) So anthropic (so called) selection would have preceded Darwinian selection.
Tomoko Otha, Near-neutrality, robustness and epigenetics.
Starts with brief history of neutral and near-neutral theories of molecular evolution. Neutral theory predicts that rate of evolution is same as rate of neutral mutation; near-neutral theory predicts rate of evolution to be inverse to population size. Much recent comparative genomic data compatible with near-neutral expectations. Robustness of gene networks made possible by near neutrality (this agrees with work by both A. Wagner and S. Gavrilets.) While robustness implies that many genotypes can result in the same phenotype, epigenetics results in the opposite: many phenotypes can be produced by the same genotype. (Not entirely clear what the role of epigenetics was here, but I take Otha to imply that it increases the range of near-neutrality as a theory of molecular evolution.)
Giorgio Bernardi, The neo-selectionist theory of evolution.
The two major determinants of gene expression are cis factors and chromatin structure. Lots of stats followed about the differential abundance of the various classes of DNA trinucleotides in the human genome. Selection favors certain types of chromatin structure in vertebrates, namely those that stabilize the thermodynamic properties of the chromatin itself. Indeed, patterns concerning the distribution of GC-rich chromatin is conserved across a hundred million years of mammalian evolution. (Not clear why this is “neo-selectionist,” however.)

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