EvolvabilityWhat is evolvability?“Evolvability” at different scalesExtra eyes on DrosophilaRobustnessDoes robustness help evolvability?Evolvability in fitness landscapesNeutral spacesWhy robustness?ModularityHow does evolvability evolve?Stephan HoyerPhysics 120 seminarMarch 28, 2008What is evolvability?Two definitions:A biological system is evolvable if its properties show heritable genetic variation, and if natural selection can thus change these propertiesA biological system is evolvable if its properties can acquire novel functions through genetic change, functions that help the organism survive and reproduce“Evolvability” at different scalesMassimo Pigliucci. Is evolvability evolvable? Nature Reviews Genetics 9, 75-82 (January 2008)Extra eyes on DrosophilaHalder, G., P. Callerts, and W. J. Gehring. Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila. Science 267, 1788-1792 (1995).ModularRobust?RobustnessA biological system is robust if it continues to function in the face of perturbationsFor living things:Genetic robustnessEnvironmental robustnessDifferent levels:Changes in genotype not reflected in phenotypeOverall fitness… to survive, reproduce, etcDoes robustness help evolvability?Robustness hurts:Less phenotypic variation => Less selective pressure => less evolvability.Robustness helps:Versatility: for other non-robust traitsNeutrality: robustness means more silent mutations to set stage for important ones.As Capacitors: when robustness breaks, hidden variation is releasedEvolvability in fitness landscapesQ: What’s wrong with this picture?A: Far too few dimensions!Neutral spacesVisualizing fitness landscapes gives you way too few dimensions!Peak shift problem: how do you move between peaks?High dimensional (100-1000D) landscapes are qualitatively differentNeutral spaces allow for lots of variationTechniques from physics: percolation theory and simulationGavirlets, S. A dynamical theory of speciation on holey adaptive landscapes. Am. Nat. 154, 1-22 (1999).Why robustness?Clearly life actually is very robustEasier to get to by chance most stuff exists in the large neutral spaceNatural selection can select for robustness in neutral spacesModularityModularity helps evolvability?Are modular systems robust?Altering one module is less likely to disrupt othersBut that one module can still disrupt everything if it’s essentialAlso requires redundancy, most not in genes, but in alternate methodsHow does evolvability evolve?Clearly there are big steps in evolvability, e.g.Multi-cellularityAnimalsAppendagesetcIs evolvability a side-result or the end-goal of evolution?Brings up issue of evolution at different levels: for individuals or for
View Full Document
Unlocking...