Turning a hobby into a job: How duplicated genes find new functions

Natural selection uses duplicated genes as raw material for functional innovation, co-opting their existing features to new functions.Understanding genetic innovation requires two questions to be addressed: which gene was involved in the duplication; and how has natural selection acted on that duplication to optimize the novel function?Genes with functions such as enzymes, transporters and transcription factors often survive in duplicate. However, the mechanism of duplication is important: genes that are part of complex cellular networks are more easily duplicated by whole-genome duplication (WGD) than by small-scale duplication (SSD).In order to have the potential to acquire a new function, a duplicate gene must come under the protection of natural selection so that it is not eliminated by degenerative mutations. At least three mechanisms can allow natural selection to preserve a duplicate gene pair: neofunctionalization, subfunctionalization and selection for gene dosage.Strikingly, all three of the above mechanisms have been involved in the appearance of novel functions. For instance, dosage selection can maintain a gene duplication in order to provide sufficient expression of a gene product with a weak but beneficial new activity.Such existing minor activities in genes might or might not be related to the gene's evolved function. Examples include enzymes with minor activities for substrates related to their primary substrate, and receptors with affinities for several ligands.Subfunctionalization can also be involved in the process of generating novelty. An example is the GAL1–GAL3 gene duplication in Saccharomyces cerevisiae, in which a single gene first gained a novel function that was then optimized by duplication and adaptive subfunctionalization.