GENERATION OF VARIATION. “Gene duplication may also allow partitioning of function among genotypes within a species, leading to genetic differentiation/intraspecific variation. This process is contingent upon genetic neo-functionalization and would suggest that duplicated genes should show higher levels of intraspecific variation in gene expression than “unique” genes.”
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0001838