Gene Duplication: The Genomic Trade in Spare Parts

If necessity is the mother of invention, then its father is an inveterate tinkerer, with a large garage full of spare parts. Innovation (like homicide) requires motive and opportunity. Clearly, the predominant ‘motive’ during the evolution of a novel gene function is to gain a selective advantage. To understand why gene duplications represent the major ‘opportunities’ from which new genes evolve, we must fi rst consider what constrains genic evolution. The vast majority of genes in every genome are selectively constrained, in that most nucleotide changes that alter the fi tness of the organism are deleterious. How do we know this? Comparisons between genomes clearly demonstrate that coding sequences diverge at slower rates than non-coding regions, largely due to a defi cit of mutations at positions where a base change would cause an amino-acid change. Gene duplication provides opportunities to explore this forbidden evolutionary space more widely by generating duplicates of a gene that can ‘wander’ more freely, on condition that between them they continue to supply the original function. Susumu Ohno was the fi rst to comprehensively elucidate the potential of gene duplication, in his book Evolution by Gene Duplication, published more than 30 years ago (Ohno 1970). The prescience of Ohno’s book is highlighted by the fact that his book has almost certainly been cited more times in the past fi ve years than in the fi rst fi ve years after its publication.