Paralogous genes arise due to:
Paralogy describes homology relationship between genes within same species that diverged after duplication event. Duplicate loci initially identical reside in different chromosomal positions; over time independent mutations lead to sequence divergence. Functional fate includes retention of similar function, diversification toward new substrate specificity or loss of function. This contrasts orthology, where genes separate via speciation, retaining similar function across species. Human alpha and beta globin genes, HOX clusters and olfactory receptors are classic paralog examples originating from ancient tandem duplications, expanding genome coding potential beyond original template.
Ref: Koonin EV, Annual Review Genetics 2005: Orthologs, Paralogs and Evolutionary Genomics, Gene Duplication