  Topological features of gene regulatory networks can be successfully
reproduced by a model population evolving under selection for short dynamical
attractors. The evolved population of networks exhibit motif statistics,
summarized by significance profiles, which closely match those of {\it E. coli,
S. cerevsiae} and {\it B. subtilis}, in such features as the excess of linear
motifs and feed-forward loops, and deficiency of feedback loops. The slow
relaxation to stasis is a hallmark of a rugged fitness landscape, with
independently evolving populations exploring distinct valleys strongly
differing in network properties.
