this invention relates to methods and reagents for selecting a desired protein or nucleic acid molecule by linking mrna , with known or unknown sequences , to its translated protein to form a cognate pair . the cognate pair is selected based upon desired properties of the protein or the nucleic acid . this method also includes the evolution of a desired protein or nucleic acid molecule by amplifying the nucleic acid portion of the selected cognate pair , introducing variation into the nucleic acid , translating the nucleic acid , attaching the nucleic acid to its protein to form a second cognate pair , and re - selecting this cognate pair based upon desired properties .