Incollection,
Collective properties of evolving populations of RNA molecules
Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, Genova, Italy, (9-13 July 2007)
Abstract
At early stages of biochemical evolution, the complexity of replicating
molecules was limited by high mutation rates. In the framework of the RNA
world hypothesis for the origin of life, small replicating RNA molecules are
at the same time carriers of the genetic information and responsible for
the biochemical function of the molecules. Therefore, RNA molecules, through
their dual appearance as sequence of nucleotides and spatially folded,
three-dimensional structure, represent a suitable model to study evolution of
biological function 1. Due to a relatively high mutation rate, a population
of RNA molecules consists of a large number of different sequences, commonly
denoted as quasispecies. The essential ingredient in this model is the
differentiation between genotype (molecular sequences which are affected by
mutation) and phenotype (molecular structure, affected by selection). This
framework allows a quantitative analysis of organizational properties of
quasispecies as they adapt to different environments, such as their
robustness, the effect of the degeneration of the sequence space, or the
adaptation under different mutation rates and the error threshold associated.
With help of large-scale numerical simulations, we investigate the structural
properties of molecular quasispecies adapting to different environments both
during the transient time before adaptation takes place and in the asymptotic
state, once optimization has occurred 2. We observe a minimum in the
adaptation time at values of the mutation rate relatively far from the error
threshold. The optimal value results from a trade-off between diversity
generation and fixation of advantageous mutants. We introduce and discuss
different quantities describing the collective state of the population in the
asymptotic regime, such as consensus sequence and structure of the consensus
sequence. Through the definition of a consensus structure, it is shown that
the quasispecies retains relevant structural information in a distributed
fashion even above the error threshold, hence providing structural robustness
to the population. Changing the system parameters (mutation rate, selective
pressure, population size, sequence length), we characterize the collective
properties of the population more in detail.
Our results indicate that certain functional motifs of RNA secondary structure
that withstand high mutation rates (as the ubiquitous hairpin motif) might
appear early in evolution of life on Earth and be actually frozen evolutionary
accidents. Experimental results available for natural RNA populations are in
qualitative agreement with our observations.
1) S.C. Manrubia, C. Briones, RNA 13 (2007), 97-107.\\
2) M. Stich, C. Briones, S.C. Manrubia, BMC Evol. Biol., submitted.
