This dissertation investigates the forces at work in the formation of a tightly knit but ultimately non-genetic dialect group. The Iranian languages, a genetic sub-branch of the larger Indo-European language family, are a group whose development has been profoundly affected by millennia of internal contact. This work is concerned with aspects of the diversification and disparification (i.e., the development of different versus near-identical features across languages) of this group of languages, namely issues pertaining to the development of the so-called West Iranian group, whose status as a legitimate genetic subgroup has long remained unclear. To address the phenomena under study, I combine a traditional comparative-historical approach with existing quantitative methods as well as newly developed quantitative methods designed to deal with the sort of linguistic situation that Iranian typifies. The studies I undertake support the idea that West Iranian is not a genetic subgroup, as sometimes assumed; instead, similarities between West Iranian languages that give the impression of close genetic relatedness have come about due to interactions between contact and parallel driftlike tendencies. I develop new methodologies which make it possible to demonstrate which similarities are due to contact and which are due to drift.