Pattern, Tempo, and Mode of Evolutionary and Biogeographic Divergence in Oxystylis and Wislizenia (Capparaceae)

Six populations of Oxystylis lutea and nine populations representing three subspecies of Wislizenia refracta were analyzed for electrophoretic variation at 18 loci. Several marker alleles differentiate O. lutea from W. refracta, although mean pair‐wise genetic identity values (range, 0.53–0.64) are comparable to those found among many congeners. Oxystylis and Wislizenia share two duplicated gene loci that are not present in outgroup species in Cleome and Cleomella. Thus, they appear to be monophyletic sister taxa within a larger monophyletic group. Based on allozyme differentiation patterns, we infer a pre‐Pleistocene divergence prior to differentiation within W. refracta. Subsequent anagenesis in the O. lutea lineage and cladogenesis in the W. refracta lineage have resulted in extensive morphological differentiation between Oxystylis and Wislizenia. Therefore, Oxystylis and Wislizenia are phenetically and phylogenetically distinct, and we recognize them as genera. Geographic modes of speciation predominate in the group. Variation patterns within W. refracta are consistent with recognition of three subspecies, and indicate that subsp. californica is not a historical introduction in the Central Valley of California.