Homologs are genes/proteins that look &/or act similar because they share an evolutionary origin
They can further be classified as:
orthologs: speciation after the evolution of a gene/protein leads there to be different versions of it in different species
paralogs: genetic duplication leads there to be multiple copies of a gene that can then evolve separately – can be in the same species or in different species
Homologous genes/proteins may have various degrees of functional similarity – sometimes they do very similar things and can compensate for one another; other times they do really different things. What defines them is having shared ancestry, not having similar functions.
