Clines, the gradual variation in traits along a geographic axis, can be formed by both neutral and selective processes. Takahashi et al. accessed genetic and morphological variation of the series Sakawanum (genus Asarum, Aristolochiaceae), which shows interspecific clinal variation in the calyx lobe lengths.
A total of 27 populations covering the natural geographic distribution of Sakawanum were sampled and six nuclear microsatellite markers were used to investigate genetic structure and genetic diversity. Calyx lobe lengths were significantly differentiated among the taxa, while genetic differentiation was low, implying interspecific gene flow within the series. Genetic differentiation was significantly lower than phenotypic differentiation. This study elucidates the roles of population history and divergent selection in shaping the cline of a flower trait.