Tests of selective neutrality of polymorphism within each group. The joint distributions of (D, Z) tests statistics expected under neutrality in the Eastern group (group 1, panel 3a) and the Western group (group 2, panel 3b) are plotted using a blue shading for the probability density. Symbiotic genes are plotted as green dots and flowering genes as orange dots using each candidate gene abbreviation. Control loci are represented as black dots. The joint distribution for (D, Z) within each group was obtained through 105 coalescent simulations from models parametrized to fit patterns of polymorphisms in the set of controlled fragments (See Methods for further details). Note that in order to generate a unique graphical representation of the neutral joint distribution, simulations for each of the 53 loci were pooled and the resulting (D, Z) distribution was binned using 104 categories (rigorous p-values computed using null distributions tailored for each locus length and polymorphism are available in Additional file 7, Table S5).