Complexity: Darwin and neutral evolution
Is neutral evolution non-Darwinian?
„…Variations neither useful not injurious would not be affected by natural selection, and would be left either a fluctuating element, as perhaps we see in certain polymorphic species, or would ultimately become fixed, owing to the nature of the organism and the nature of the conditions….”
Charles Darwin, Origin of species (1859)
See THE MODERN MOLECULAR CLOCK for a good overview of neutrality, and the molecular clock.
The importance of neutrality
Various authors have addressed the importance of neutrality to evolution. Peter Schuster, Peter Stadler, Walter Fontana, Mark Toussaint and Martijn Huynenhave explored the importance of neutrality on evolution.
Concepts such as innovation, modularity, evolvability, robustness seem to be all intricately linked to neutrality. Since there are 3 codons of 4 possible bases which encode for amino-acids, there are \(4^3=64\) possible codes but there are only 20 amino-acids. This means that many changes in codons will neutral effects on the phenotype.
The effect of neutrality on evolution can be quite important. Without neutrality, evolution is often constrained to local fitness maxima, unable to progress to higher fitness values. Neutrality allows the genotype to ‘drift’ during these periods. This also explains why evolution often appears to go in spurts after periods of apparant stasis. And while the phenotype may be in stasis, because of the neutral evolution, the genotype isn’t.
Adapted from: Peter Schuster
In other words, not only is neutral evolution NOT non-Darwinian or anti-Darwinian but it seems to be essential for evolvability.
In future contributions I hope to explore more of the ‘scale free’ nature of DNA, protein, RNA networks, the importance of neutrality on evolvability, robustness and modularity as well as information theoretical approaches to show how simple processes could have created the world as we know it.
Various references
Martijn Huynen, Exploring Phenotype Space Through Neutral Evolution, J Mol Evol. 1996 Sep;43(3):165-9
RNA secondary-structure folding algorithms predict the existence of connected networks of RNA sequences with identical secondary structures. Fitness landscapes that are based on the mapping between RNA sequence and RNA secondary structure hence have many neutral paths. A neutral walk on these fitness landscapes gives access to a virtually unlimited number of secondary structures that are a single point mutation from the neutral path. This shows that neutral evolution explores phenotype space and can play a role in adaptation.
Erik van Nimwegen, James P. Crutchfield, and Martijn Huynen, Neutral evolution of mutational robustness, Proc Natl Acad Sci U S A. 1999 August 17; 96 (17): 9716-9720
M. Toussaint, C. Igel (2002): Neutrality: A Necessity for Self-Adaptation . Proceedings of the Congress on Evolutionary Computation (CEC 2002), 1354-1359
M. Toussaint (2003): The evolution of genetic representations and modular neural adaptation. Submitted (in April) version of my PhD thesis; a revised, official version published as a booklet will follow.