Evolution of complexity

From: Pim van Meurs <pimvanmeurs@yahoo.com>
Date: Sat Sep 24 2005 - 20:37:33 EDT

Despite claims to the contrary, evolutionary science does understand
quite a bit as to how complexity may evolve. I mentioned Peter Schuster
and his work on RNA.

His talk was titled: Evolution in Simple Systems and the Emergence of

First he established what are the necessary conditions for Darwinian
evolution which are not surprisingly

Three necessary conditions for Darwinian evolution are:
            1. Multiplication,
            2. Variation, and
            3. Selection.

Now it is important to realize that selection acts on the phenotype
while variation acts on the genotype. This genotype-phenotype mapping
will become important.

We then get to a central paradigm of Darwinian evolution, namely that
the mutations are "random"

"One important property of the Darwinian scenario is that variations in
the form of mutations or recombination events occur uncorrelated with
their effects on the selection process."

Schuster then descibes the work by Lenski who is tracking bacteria
across tens of thousands of generations.

after 33 000 generations: One out of 12 Escherichia colicolonies adapts
to the environment and starts spontaneously to utilize citrate in the

Schuster then addresses RNA and their secondary structures. He defines
sequence space and structure space and the mapping between the two
spaces. Mapping a single structure to sequence space defines the neutral
network of this structure.

One can define connected and non-connected structures. Now we get to
some interesting experiments using a flow reactor. Evolutionary
trajectory now show some familiar characteristics namely, periods of
stasis followed by periods of fast change. Using a somewhat simplified
concept of fitness one can understand how neutral evolution can cross
chasms of reduced fitness.

Schuster then shows using the concept of compatible sets, how structure
space can overlap and shows an example using a ribozyme (one sequence,
two ribozymes implications for the emergence of new ribozyme folds by
Shultes and Bartel).

Peter Schuster has an extensive collection of reprints of his work and
the work of his students which lay out in full detail the intricacies of
these neutral networks and the impact on evolvability and evolution.
Check out his website at http://www.tbi.univie.ac.at/~pks/

And working papers at http://www.tbi.univie.ac.at/papers/

In http://www.tbi.univie.ac.at/~pks/Presentation/zuerich-05.pdf Schuster
goes into more detail of the RNA networks

Properties of RNA sequence to secondary structure mapping
1. More sequences than structures
2. Few common versus many rare structures
3. Shape space covering of common structures
4. Neutral networks of common structures are connected
Received on Sat Sep 24 20:40:21 2005

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