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Abstract
One important mechanism for functional innovation during evolution is the duplication
of genes and entire genomes. Evidence is accumulating that during the evolution of
vertebrates from early deuterostome ancestors entire genomes were duplicated through
two rounds of duplications (the 'one-to-two-to-four' rule). The first genome duplication
in chordate evolution might predate the Cambrian explosion. The second genome duplication
possibly dates back to the early Devonian. Recent data suggest that later in the Devonian,
the fish genome was duplicated for a third time to produce up to eight copies of the
original deuterostome genome. This last duplication took place after the two major
radiations of jawed vertebrate life, the ray-finned fish (Actinopterygia) and the
sarcopterygian lineage, diverged. Therefore the sarcopterygian fish, which includes
the coelacanth, lungfish and all land vertebrates such as amphibians, reptiles, birds
and mammals, tend to have only half the number of genes compared with actinopterygian
fish. Although many duplicated genes turned into pseudogenes, or even 'junk' DNA,
many others evolved new functions particularly during development. The increased genetic
complexity of fish might reflect their evolutionary success and diversity.