A method for modulating the selection stringency during viral-assisted evolution of a gene product, the method comprising: (a) introducing host cells into a lagoon, wherein the host cell comprises a low selection stringency plasmid and a high selection stringency plasmid, wherein the low selection stringency plasmid comprises a viral gene required to package a selection viral vector into an infectious viral particle controlled by a small-molecule inducible promoter that allows evolutionary drift to occur, wherein at least one gene required to package the selection viral vector into an infectious viral particle is expressed in response to a concentration of a small molecule, and wherein the high selection stringency plasmid comprises a second copy of the viral gene required to package the selection viral vector into the infectious viral particle controlled by an activity-dependent promoter, wherein at least one viral gene required to package the selection viral vector into an infectious viral particle is expressed in response to a desired activity property of a gene product encoded by the gene to be evolved or an evolution product thereof; (b) introducing a selection viral vector comprising a gene to be evolved into a flow of host cells through a lagoon, wherein the gene to be evolved produces an active gene product or a weakly active or inactive gene product, wherein the active gene product has an activity that drives the expression of the viral gene required to package the selection viral vector into infectious viral particles in the high selection stringency plasmid and wherein the weakly active or inactive gene product has a relatively lower activity than the activity of the active gene product; and (c) mutating the gene to be evolved within the flow of host cells, wherein the host cells are introduced through the lagoon at a flow rate that is faster than the replication rate of the host cells and slower than the replication rate of the virus thereby permitting replication of the selection viral vector in the lagoon. (a) introducing host cells into a lagoon, wherein the host cell comprises a low selection stringency plasmid and a high selection stringency plasmid, wherein the low selection stringency plasmid comprises a viral gene required to package a selection viral vector into an infectious viral particle controlled by a small-molecule inducible promoter that allows evolutionary drift to occur, wherein at least one gene required to package the selection viral vector into an infectious viral particle is expressed in response to a concentration of a small molecule, and wherein the high selection stringency plasmid comprises a second copy of the viral gene required to package the selection viral vector into the infectious viral particle controlled by an activity-dependent promoter, wherein at least one viral gene required to package the selection viral vector into an infectious viral particle is expressed in response to a desired activity property of a gene product encoded by the gene to be evolved or an evolution product thereof; (b) introducing a selection viral vector comprising a gene to be evolved into a flow of host cells through a lagoon, wherein the gene to be evolved produces an active gene product or a weakly active or inactive gene product, wherein the active gene product has an activity that drives the expression of the viral gene required to package the selection viral vector into infectious viral particles in the high selection stringency plasmid and wherein the weakly active or inactive gene product has a relatively lower activity than the activity of the active gene product; and (c) mutating the gene to be evolved within the flow of host cells, wherein the host cells are introduced through the lagoon at a flow rate that is faster than the replication rate of the host cells and slower than the replication rate of the virus thereby permitting replication of the selection viral vector in the lagoon. <EOS>