It is proposed that equivalent phenotypic effects can be obtained by either structural changes in macromolecules involved in gene regulation or changes in activity of the structurally unaltered macromolecules. This equivalence between changes in activity (concentration) and changes in structure can come into play within physiologically plausible limits and seems to represent an important interface between environment and genome--namely, between environmentally determined and genetically determined gene expression. The equivalence principle helps explain the appearance of phenocopies. It also points to a general pathway favorable to the occurrence, during evolution, of frequent episodes corresponding to Waddington's genetic assimilation and is likely to represent one component of the system responsible for the high frequency of recurrence of parallel evolution.