Allozyme diversity on 13 isozyme loci was investigated for two bulbous species, Lilium longiflorum and L. formosanum, endemic to the subtropical archipelago of continental origin located in East Asia. Degrees of allozyme variability and divergence for L. longiflorum were very high for insular endemic species, indicating relatively longtime persistence of the present widespread distribution across many islands in this phenotypically little-changed species. Lilium formosanum exhibited rather lower variability and divergence than did L. longiflorum and was genetically close to the southern peripheral populations of L. longiflorum with 0.978 as its highest genetic identity value. Combined with other biological and insular geohistorical information, our results suggest that L. longiflorum was established around the end of the Pliocene when the current distribution area was still a continuous part of the ancient Asian continent, and L. formosanum was derived from southern populations of L. longiflorum around the late Pleistocene when the mainland of Taiwan was completely separated from the adjacent islands and the main continent. Depauperization of allozyme variability in some L. longiflorum populations was found on islands with lower altitudes. This reflects bottleneck effects after the complete or almost complete submergence of such low islands during the archipelago's development.

We investigated interspecific isolating mechanisms and population genetic structure for 19 wild populations of Lilium longiflorum and eight of L. formosanum endemic to the Ryukyu Archipelago and Taiwan. Field observation, artificial interpopulation crosses and isozyme analysis were used. Interspecific isolation was never established by island separation, but it was highly likely by habitat differentiation. There appeared to be additional effects by the unilateral cross incompatibility and isolation in the flowering season. There were significant positive correlation between genetic and geographic distance among populations (r=0.791; p<0.001). Degrees of allozyme variability and divergence at the species level in L. longiflorum were definitely higher than those of L. formosanum. They were extremely high as an insular endemic species. Depauperation of genetic diversity in L. longiflorum populations was associated with islands with lower altitude. This reflects severe bottlenecks by large or complete submergence of such low islands during archipelago development or population rebirth by secondary migration from adjacent islands exempted from large submergence. Combined with historical geography in the archipelago, our investigations suggest that 1) L. formosanum was derived from southern peripheral populations of L. longiflorum by ecological and reproductive isolation, presumably around the late Pleistocene when the mainland of Taiwan was completely separated from the adjacent islands and the continent; and 2) L. longiflorum is an old species in continental islands established around the end of the Pliocene when the current distribution area had been a continuous part of the ancient Asian continent.