Fig. 1

A schematic representation of the evolutionary relations among genes of multiple organisms. The leaf nodes of the tree represent the genes and the internal nodes correspond to evolutionary events. X1 has two ancestral nodes associated with speciation events; one is the last common ancestor with Y1, and the other is common with Z1. Thus, Y1 and Z1 are orthologs of X1. On the other hand, X2 and Y2 are paralogs to X1, since their last common ancestor has a duplication event associated. Likewise, all the pairwise orthology/paralogy relations can be defined according to the strucutre of the given tree