Hox genes

Vertebrae types directly correlate with the prior gene expression domains of Hox genes across different vertebrate species, thus changes in expression domains could generate diverse morphologies. First expressed in the CNS, somites and unpaired fins; Paired fins evolved after unpaired fins Later in evolution Hox gene codes are coopted to the paired fins and are used to subdivide them molecularly; Cooption of a molecular programme from one body region to another!