Species delimitation and hybridisation history of a hazel species complex
Abstract Background and Aims Hybridisation increases species adaptation and biodiversity but also obscures species boundaries. In this study, species delimitation and hybridisation history were examined within one Chinese hazel species complex (Corylus chinensis–Corylus fargesii). Two species including four varieties have already been described for this complex, with overlapping distributions. Methods A total of 322 trees from 44 populations of these four varieties across their ranges were sampled for morphological and molecular analyses. Climatic datasets based on 108 geographical locations were used to evaluate their niche differentiations. Flowering phenology was also observed for two co-occurring species or varieties. Key Results Four statistically different phenotypic clusters were revealed, but these clusters were highly inconsistent with the traditional taxonomic groups. All the clusters showed statistically distinct niches, with complete or partial geographic isolation. Only two clusters displayed a distributional overlap, but they had distinct flowering phenologies at the site where they co-occurred. Population-level evidence based on the genotypes of 10 simple sequence repeat (SSR) loci supported four phenotypic clusters. In addition, one cluster was shown to have an admixed genetic composition derived from the other three clusters through repeated historical hybridisations. Conclusions Based on our new evidence, it is better to treat the four clusters identified here as four independent species. One of them was shown to have an admixed genetic composition derived from the other threes through repeated historical hybridisations. This study highlights the importance of applying integrative and statistical methods to infer species delimitations and hybridisation history. Such a protocol should be adopted widely for future taxonomic studies.