Influence of network modularity on the retention of duplicates. (a) A hierarchical clustering was carried out to delimit modules in metabolic networks. Colors denote different modules in EcoKegg. (b) Metabolic pathways (branches in the trees) within and across modules were compared using a measure of evolutionary distance (ED). Modules comprising related branches are indicated by color as in (a). A value of (ED) closer to zero (the darker squares) implies a greater retention of duplicates between the two given pathways. (c) Observed (ED) values were compared against those expected by chance - after random shuffling of protein-domains. A Z-score < -3 (green) refers to significant (ED) values (P < 0.001).