On the scale of whole organisms, of course, all vertebrates are cousins of each other too. The tree of vertebrate evolution is the family tree we are all familiar with, its branch-points representing speciation events—the splitting of species into daughter species. In reverse, they are the rendezvous points that punctuate this pilgrimage. But there is another family tree occupying the same timescale, whose branches represent not speciation events but gene duplication events within genomes.
