Selective pressure on metabolic network structures as measured from the random blind-watchmaker network

A random null model termed the blind-watchmaker network (BW) has been shown to reproduce the degree distribution found in metabolic networks. This might suggest that natural selection has had little influence on this particular network property. We investigate here the extent to which other structural network properties have evolved under selective pressure from the corresponding ones of the random null model: the clustering coefficient and the assortativity measures are chosen and it is found that these measures for the metabolic network structure are close enough to the BW network so as to fit inside its reachable random phase space. It is, furthermore, shown that the use of this null model indicates an evolutionary pressure towards low assortativity and that this pressure is stronger for larger networks. It is also shown that selecting for BW networks with low assortativity causes the BW degree distribution to deviate slightly from its power-law shape in the same way as the metabolic networks. This implies that an equilibrium model with fluctuating degree distribution is more suitable as a null model, when identifying selective pressures, than a randomized counterpart with fixed degree sequence, since the overall degree sequence itself can change under selective pressure on other global network properties.