The invention provides methods for controlling the density of different molecular species on the surface of a solid support. A first mixture of different molecular species is attached to a solid support under conditions to attach each species at a desired density, thereby producing a derivatized support having attached capture molecules. The derivatized support is treated with a second mixture of different molecular species, wherein different molecular species in the second mixture bind specifically to the different capture molecules attached to the solid support. One or more of the capture molecules can be reversibly modified such that the capture molecules have a different activity before and after the second mixture of molecular species are attached. In particular embodiments, the different molecular species are nucleic acids that are reversibly modified to have different activity in an amplification reaction.