Directed evolution

Directed evolution (DE) is a method used in protein engineering that mimics the process of natural selection to steer proteins or nucleic acids toward a user-defined goal. It consists of subjecting a gene to iterative rounds of mutagenesis (creating a library of variants), selection (expressing those variants and isolating members with the desired function) and amplification (generating a template for the next round). It can be performed in vivo (in living organisms), or in vitro (in cells or free in solution). Directed evolution is used both for protein engineering as an alternative to rationally designing modified proteins, as well as for experimental evolution studies of fundamental evolutionary principles in a controlled, laboratory environment. Directed evolution has its origins in the 1960s with the evolution of RNA molecules in the "Spiegelman's Monster" experiment. The concept was extended to protein evolution via evolution of bacteria under selection pressures that favoured the evolution of a single gene in its genome. Early phage display techniques in the 1980s allowed targeting of mutations and selection to a single protein. This enabled selection of enhanced binding proteins, but was not yet compatible with selection for catalytic activity of enzymes. Methods to evolve enzymes were developed in the 1990s and brought the technique to a wider scientific audience. The field rapidly expanded with new methods for making libraries of gene variants and for screening their activity. The development of directed evolution methods was honored in 2018 with the awarding of the Nobel Prize in Chemistry to Frances Arnold for evolution of enzymes, and George Smith and Gregory Winter for phage display. Directed evolution is a mimic of the natural evolution cycle in a laboratory setting. Evolution requires three things to happen: variation between replicators, that the variation causes fitness differences upon which selection acts, and that this variation is heritable.