  In this Letter, we study how cooperation is organized in complex topologies
by analyzing the evolutionary (replicator) dynamics of the Prisoner's Dilemma,
a two-players game with two available strategies, defection and cooperation,
whose payoff matrix favors defection. We show that, asymptotically, the
population is partitioned into three subsets: individuals that always cooperate
({\em pure cooperators}), always defect ({\em pure defectors}) and those that
intermittently change their strategy. In fact the size of the latter set is the
biggest for a wide range of the "stimulus to defect" parameter. While in
homogeneous random graphs pure cooperators are grouped into several clusters,
in heterogeneous scale-free (SF) networks they always form a single cluster
containing the most connected individuals (hubs). Our results give further
insights into why cooperation in SF networks is favored.
