  Bipartite graphs have received some attention in the study of social networks
and of biological mutualistic systems. A generalization of a previous model is
presented, that evolves the topology of the graph in order to optimally account
for a given Contact Preference Rule between the two guilds of the network. As a
result, social and biological graphs are classified as belonging to two clearly
different classes. Projected graphs, linking the agents of only one guild, are
obtained from the original bipartite graph. The corresponding evolution of its
statistical properties is also studied. An example of a biological mutualistic
network is analyzed in great detail, and it is found that the model provides a
very good quantitative fitting of its properties. The model also provides a
proper qualitative description of the statistical features observed in social
webs, suggesting the possible reasons underlying the difference in the
organization of these two kinds of bipartite networks.
