Vertebrate evolution by interspecific hybridisation--are we polyploid?

For the growing fraction of human genes with identified functions there are often homologues known from invertebrates such as Drosophila. A survey of well established gene families from aldolases to zinc finger transcription factors reveals that usually a single invertebrate gene corresponds to up to four equally related vertebrate genes on different chromosomes. This pattern was before widely noticed for the Hox gene clusters but appears to be more general. Genome quadruplication by two rounds of hybridisation is discussed as a simple biological mechanism that could have provided the necessary raw material for the success of vertebrate evolution.