Directed evolution (DE) is a method used to produce enzymes for industrial or medical purposes.
The method is protein engineering which mimics natural selection.
The basic idea is to put a gene through repeated rounds of mutation, to make a library of variants. Selection isolates genes with the desired function. They are a template for the next round.
This can be done in vivo (in living cells of bacteria or yeast), or in vitro (free in solution or microdroplets).
Testing more mutants increases the chances of finding one with the desired properties.
During in vivo evolution, each cell (usually bacteria or yeast) is transformed with a plasmid containing a different member of the variant library. Only the gene of interest differs between the cells, with all other genes being kept the same.
The cells express the protein either in their cytoplasm or surface where its function can be tested. This format has the advantage of selecting for properties in a cellular environment, which is useful when the evolved protein or RNA is to be used in living organisms.

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