  In physics of living systems, a search for relationships of a few macroscopic
variables that emerge from many microscopic elements is a central issue. We
evolved gene regulatory networks so that the expression of target genes
(partial system) is insensitive to environmental changes. Then, we found the
expression levels of the remaining genes autonomously increase as a plastic
response. Negative proportionality was observed between the average changes in
target and remnant genes, reflecting reciprocity between the macroscopic
robustness of homeostatic genes and plasticity of regulator genes. This
reciprocity follows the lever principle, which was satisfied throughout the
evolutionary course, imposing an evolutionary constraint.
