Gene family sizes for homologous genes in groups of strains belonging to the same species, represented as in Figure 2. (a)Chlamydophila pneumoniae strains; (b)Streptococcus pyogenes strains; (c)Escherichia coli strains; (d)Staphylococcus aureus strains. Strain denomination and graph code displayed in the top right-hand corner. Only protein-coding genes are included. Zero on the y-axis indicates single-copy genes; 1 indicates a gene family formed of two members.