Figure 1. Selection-mutation diagram for the case m = 2.

The lower-case letters a and b denote the unmutated copy of the two genes. The capital letters are mutations giving rise to a cooperating phenotype with respect to the given site. For example, "Ab" denotes a cooperator with respect to the first site, with an unmutated second site. Star indicates a cheater with respect to the given site. The mutations are represented by arrows and the corresponding mutation rates are marked. For simplicity, in this diagram we omitted back-mutations. The fitness of all the types in the presence and in the absence of cooperation is given at the bottom of the diagram, underneath the specific types. For illustration purposes, we used the smaller number of loci, m = 2. A larger number of loci would make a graphical representation intractable. Most simulations were run with m>2.