Module conservation is analogous to sequence conservation. For sequences (left) coding regions are usually much more conserved than the genome as a whole. Similarly, in the network setting, modules are more conserved than the entire network. In addition, coding regions can often tolerate synonymous mutations that change the DNA sequence itself but do not alter the protein product. Similarly, modules may be able to tolerate loss of specific interactions as long as the two interacting orthologs remain in the same module (often through redundant interactions or interactions with other module members).
Zinman et al. BMC Systems Biology 2011 5:134 doi:10.1186/1752-0509-5-134