The genomes of extant vertebrates have been shaped by a series of whole genome and individual gene duplication events. The 2R hypothesis, which postulates that two whole genome duplications occurred in relatively rapid succession very early in chordate evolution, is gaining increasing acceptance. A further entire genome duplication is believed to have occurred in the ancestral fish lineage approximately 320-350 Myr ago, as well as more recent independent tetraploidization events, mostly but not exclusively, in particular teleost and amphibian lineages. Superimposed upon these whole genome duplications are tandem or segmental duplications of individual genes or groups of genes that have taken place at different rates in the various vertebrate lineages. The majority of duplicated genes become pseudogenes or are deleted but some may evolve to encode components with new functional roles. Genes encoding members of neuropeptide Y- and tachykinin-families are associated with the HOX-bearing chromosomes and these systems provide examples of duplication events that have led to rapid evolution of the duplicated gene which has occasionally produced peptides, such as pancreatic polypeptide, seminalplasmin and hemokinin-1, with new biological functions.