English: Novel gene formation from ancestral genes. Novel genes can be formed from ancestral genes through a variety of mechanisms. (A) Duplication and divergence. Following duplication, one copy experiences relaxed selection and gradually acquires novel function(s). (B) Gene fusion. A hybrid gene formed from some or all of two previously separate genes. Gene fusions can occur by different mechanisms; shown here is an interstitial deletion. (C) Gene fission. A single gene separates to form two distinct genes, such as by duplication and differential degeneration of the two copies. (D) Horizontal gene transfer. Genes acquired from other species by horizontal transfer undergo divergence and neofunctionalization. (E) Retroposition. Transcripts may be reverse transcribed and integrated as an intronless gene elsewhere in the genome. This new gene may then undergo divergence.