Genetic redundancies and their evolutionary maintenance.

Genetic redundancy refers to the common phenomenon that deleting or mutating a gene from a genome has minimal or no impact on the phenotype or fitness of the organism because of functional compensation conferred by one or more other genes. Here I summarize studies of functional redundancies between duplicate genes and those among metabolic reactions that respectively represent genetic redundancies at the individual gene level and at the systems level. I discuss the prevalence of genetic redundancies in a genome, evolutionary origins of these redundancies, and mechanisms responsible for their stable maintenance. I show that genetic redundancies are highly abundant. While some of them may be evolutionarily transient, many are stable. The majority of the stable redundancies are likely to have been selectively kept, not because of their potential benefits in regard to future deleterious mutations, but because of their actual benefits at present or in the recent past. The rest are probably preserved by selection on nonredundant pleiotropic functions. The studies summarized here illustrate the utility of systems analysis for understanding evolutionary phenomena and the importance of evolutionary thinking in uncovering the functions and origins of systemic properties.