Evolutionary Learning in the 2D Artificial Life System "Avida"
  We present a new tierra-inspired artificial life system with local interactions and two-dimensional geometry, based on an update mechanism akin to that of 2D cellular automata. We find that the spatial geometry is conducive to the development of diversity and thus improves adaptive capabilities. We also demonstrate the adaptive strength of the system by breeding cells with simple computational abilities, and study the dependence of this adaptability on mutation rate and population size. 
Chris Adami and C. Titus Brown (Kellogg Radiation Laboratory, Caltech)