The invention describes a method for isolating one or more genetic elements encoding a gene product having a desired activity, comprising of the steps of: compartmentalizing genetic elements into microcapsules; expressing the genetic elements to produce their respective gene products within the microcapsules; sorting the genetic elements which produce the gene product having a desired activity. The invention enables the in vitro evolution of nucleic acids by repeated mutagenesis and iterative applications of the method of the invention.