Orthologous genes

Orthologous genes (or orthologs) are genes that are homologous (share a common ancestry) but were separated into different genomes by a speciation event. For instance, if species S1 has gene G1, and species S1 diverges into species S2 and S3, each with genes G2 and G3, then genes G2 and G3 are orthologs of one another: