Computational studies of an evolutionary model on a rugged phenotype
landscape suggest the existence of a phase transition as the maximum
mutation size is varied. I will discuss more recent results that show phase
transition behavior in on a neutral phenotype landscape, in which all
organisms have equal fitness (i.e., equal numbers of offspring). This
behavior occurs for organisms that undergo assortative mating and also in a
model where organisms reproduce by bacterial "fission". In contrast, the
phase transition does not occur when the organisms mate randomly. The
transition takes the system from a state of survival to a state of
extinction, and is thus an absorbing, non-equilibrium transition. The
system can be characterized by critical exponents that coincide with those
of the directed percolation (DP) universality class. Finally, I will
present evidence that an ordinary percolation transition occurs in the
system as well, for slightly different values of the critical parameter
than those at which the DP transition is observed, and I will discuss the
implications of the model for the problem of multi-level selection.
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