What evolutionary process creates an extra copy of a gene that may later gain a new function?

Gene duplication creates an extra copy of a gene that may later gain a new function.

Duplications can occur when DNA is copied unevenly during meiosis, when unequal crossing-over affects homologous chromosomes, or when larger chromosome segments or whole genomes are duplicated. Afterward, one copy can continue performing the original job while the other accumulates mutations.

Most extra copies are lost or become inactive, but some are preserved by natural selection. A duplicate may divide the original job, acquire a new function, or develop a changed pattern of expression. Gene duplication is therefore an important source of evolutionary novelty.

It is distinct from gene conversion, in which one DNA sequence is overwritten by a similar sequence. It is also different from alternative RNA splicing, which produces different transcripts from one gene without creating another DNA copy.