Directed evolution: new parts and optimized function.

Constructing novel biological systems that function in a robust and predictable manner requires better methods for discovering new functional molecules and for optimizing their assembly in novel biological contexts. By enabling functional diversification and optimization in the absence of detailed mechanistic understanding, directed evolution is a powerful complement to 'rational' engineering approaches. Aided by clever selection schemes, directed evolution has generated new parts for genetic circuits, cell-cell communication systems, and non-natural metabolic pathways in bacteria.