Habitat and reproductive isolation as factors in speciation between Lilium longiflorum thunb. and L. formosanum Wallace

We clarified the isolation status between Lilium longiflorum and L. formosanum which are endemic to the subtropical archipelago extended from the Ryukyu Archipelago to Taiwan. From field observations of natural populations, covering almost the entire native distribution of the species in their natural habitats and recording their flowering times, stem leaf morphology, the three populations from the mainland and an islet of Taiwan and all those of the Ryukyu Archipelago were readily classified as L. longiflorum, whereas the remaining Taiwanese populations were grouped as L. formosanum. This indicates that an interspecific isolation was never established by island separation, but probably by the combination of relatively distinct differentiations in adaptability to their natural habitats and in flowering time. The evolutionary trends seem to have severely restricted the gene flow between the species, and, thereby, promoted speciation.