Multi-disciplinary evidence illuminates the speciation history of a monophyletic yet dimorphic lily group

Species boundaries are dynamic and constantly challenged by gene flow. Understanding the strategies different lineages adopt to maintain ecological and genetic distinctiveness requires employing an integrative species concept that incorporates data from a variety of sources. In this study, we incorporated genetic, ecological, and environmental evidence to assess the extent of speciation or evolutionary divergence within a monophyletic yet dimorphic group (i.e., clade Leucolirion consisting of six species) within the genus Lilium. This clade consists of two lineages that exhibit unexpectedly distinct perianth appearances: whitish trumpet (funnel form, encompassing four species) and orange recurved (reflex form, including two species), respectively, which are separated by completely different pollination syndromes. Transcriptome-based nuclear and plastome datasets indicate that these two lineages are isolated, with only weak ancient gene flow between them. Within each lineage, several taxa with incomplete isolation have diverged, as indicated by weak genetic structure, strong gene flow, and conflicts between nuclear and chloroplast phylogenies, especially in the trumpet lineage. Although these taxa are not entirely independent, our evidence indicates that they are diverging, with recent gene flow disappearing and multiple isolation strategies emerging, such as differences in flowering time and niche specialization. Taken together, our findings suggest that species divergence and maintenance in Lilium are driven by a combination of adaptive and non-adaptive processes, highlighting the complex interplay of historical climate changes, ecological adaptation, and gene flow in shaping biodiversity within this genus.