1. Stephan Hoyer Physics 120 seminar March 28, 2008
Evolvability

2. What is evolvability? Two definitions:
A biological system is evolvable if its properties show heritable genetic variation, and if natural selection can thus change these properties
A biological system is evolvable if its properties can acquire novel functions through genetic change, functions that help the organism survive and reproduce
First superset second

3. “Evolvability” at different scales
Massimo Pigliucci. Is evolvability evolvable? Nature Reviews Genetics 9, (January 2008)

4. Extra eyes on Drosophila
Modular
Robust?
Halder, G., P. Callerts, and W. J. Gehring. Induction of ectopic eyes by targeted expression of the eyeless gene in Drosophila. Science 267, (1995).

5. Robustness
A biological system is robust if it continues to function in the face of perturbations
For living things:
Genetic robustness
Environmental robustness
Different levels:
Changes in genotype not reflected in phenotype
Overall fitness… to survive, reproduce, etc

6. Does robustness help evolvability?
Robustness hurts:
Less phenotypic variation => Less selective pressure => less evolvability.
Robustness helps:
Versatility: for other non-robust traits
Neutrality: robustness means more silent mutations to set stage for important ones.
As Capacitors: when robustness breaks, hidden variation is released

7. Evolvability in fitness landscapes
Q: What’s wrong with this picture?
A: Far too few dimensions!

8. Neutral spaces
Visualizing fitness landscapes gives you way too few dimensions!
Peak shift problem: how do you move between peaks?
High dimensional ( D) landscapes are qualitatively different
Neutral spaces allow for lots of variation
Techniques from physics: percolation theory and simulation
Gavirlets, S. A dynamical theory of speciation on holey adaptive landscapes. Am. Nat. 154, 1-22 (1999).

9. Why robustness? Clearly life actually is very robust
Easier to get to by chance most stuff exists in the large neutral space
Natural selection can select for robustness in neutral spaces

10. Modularity Modularity helps evolvability? Are modular systems robust?
Altering one module is less likely to disrupt others
But that one module can still disrupt everything if it’s essential
Also requires redundancy, most not in genes, but in alternate methods

11. How does evolvability evolve?
Clearly there are big steps in evolvability, e.g.
Multi-cellularity
Animals
Appendages
etc
Is evolvability a side-result or the end-goal of evolution?
Brings up issue of evolution at different levels: for individuals or for species?