Disclosed is a method of generating controlled mutations in a template nucleic acid sequence. Diverse oligonucleotides are hybridized to the template nucleic acid. The diverse oligonucleotides can have defined termini and length, and can be derived from a natural or synthetic source. Hybridization conditions are controlled to favor few or many mismatches between the diverse oligonucleotide and the template. Diversity strands encoding new variants are produced from the diverse oligonucleotides. The new variants that are generated can be screened for an improved property. In some implementations, the diverse oligonucleotides are derived from diverse nucleic acids by cleavage using that is directed by a cleavage-directing oligonucleotide.
