Certain species of fish have recently become important model systems in comparative genomics and in developmental biology, in certain instances because of their small genome sizes (e.g., in the pufferfish) and, in other cases, because of the opportunity they provide to combine an easily accessible and experimentally manipulable embryology with the power of genetic approaches (e.g., in the zebrafish). The resulting accumulation of genomic information indicates that, surprisingly, many gene families of fish consist of more members than in mammals. Most (...) modern fish, including the zebrafish and medakka, are diploid organisms; however, the greater number of genes in fish was possibly caused by additional ancient genome duplications which happened in the lineage leading to modern ray-finned fishes but not along the lineage leading to tetrapods. Since these two lineages shared their last common ancestor (in the Devonian about 360 million years ago) individual duplicated members of gene families were later lost in fish. Interestingly, comparative data indicate that, in some cases, genes in mammals even serve somewhat different functions than their homologues in fish, highlighting that the degree of evolutionary relatedness of genes is not always a reliable predictor of their evolutionary conservation and their similarity of function. Since fish are phenotypically probably not more complex than mammals, it is possible that evolution took alternative paths to the “economics of genomics” through alternative solutions to gene regulation. It is suggested that the more complex genomic architecture of fish permitted them to adapt and speciate quickly in response to changing selective regimes. BioEssays 20:511–515, 1998. © 1998 John Wiley & Sons, Inc. (shrink)