Gene duplication has long been considered an important force in genome evolution. Families of tandemly duplicated genes show 'microfunctionalization': genes encoding similar proteins with subtly different functions, such as olfactory receptors.

Microfunctionalized gene families seem to arise within gene factories: genomic regions rich in repetitive elements that undergo increased levels of unequal crossing-over.

References

 Hancock JM. Gene factories, microfunctionalization and the evolution of gene families. Trends Genet. 2005 Nov;21(11):591-5. PMID: #16153739#