Orthologs and paralogs
Genes showing a high sequence similarity are likely to be functionally related and may share a common evolutionary origin. Similar sequences are typically clustered into gene families of potentially homologous genes.
Orthologs are genes in different species evolved from a common ancestral gene by a speciation (lineage-splitting) event.
While orthologous genes keep the same function, paralogous genes often develop different functions due to missing selective pressure on one copy of the duplicated gene.