Throughout evolution of life on Earth, random nucleic acid combinations can result in a new gene that might carry out a novel function of importance to survival of a given species. Once a successful gene arises, functionally similar orthologs of that gene evolve in other species following speciation, and paralogs may arise from duplication of the ancestral gene; two or more paralogs can be grouped into a gene family. These paralogs could acquire new function(s) through neofunctionalization, or share the functions of the ancestor between copies through subfunctionalization, or accumulate mutations and become a pseudogene through non-functionalization1.
Intriguingly, some gene family members can become non-functional (pseudogenized) for millions of years and then be mutated back to an active gene that carries out a novel, yet related, function; this is called repurposing. Whereas some gene families have evolved slowly, others have undergone evolutionary blooms with very rapid expansion and evolution of paralogs; these processes are only beginning to be understood.
1. Dornburg A, Mallik R, Wang Z, Bernal MA, Thompson B, Bruford EA, Nebert DW, Vasiliou V, Yohe LR, Yoder JA, Townsend JP. Placing human gene families into their evolutionary context. Hum Genomics. 2022 Nov 11;16(1):56. doi: 10.1186/s40246-022-00429-5. PMID: 36369063