OLDER VERSUS YOUNGER GENES. “Given that segmental duplications are typically older than speciation events, it is likely that segmentally duplicated genes have fixed different functions and are in essence acting as unique genes. As such, it would be expected that increases in intraspecific variation may only be detected among younger tandem duplicated genes.”
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0001838