Unfinished speciation, hybridisation or intraspecific polymorphism? The debated case of the Onthophagus fracticornis-similis-opacicollis species complex

Species complexes, composed by closely related species of difficult identification for their morphological similarity, provide unique opportunity to study the early phases of morphological and genetic differentiation and the microevolutionary dynamics promoting speciation. Within the genus Onthophagus the presence of many species complexes and sibling species testify to a recent impressive adaptive radiation. We focused on the debated species-complex Onthophagus (Palaeonthophagus) ‘fracticornis-similis-opacicollis’ as a model to study the evolution of morphology, the role of male polyphenism and the microevolutionary dynamics promoting speciation. Even if at present there is a general consensus that O. fracticornis is a good species, greater discrepancies arise on the taxonomic status of O. similis and O. opacicollis: according to some authors these are sibling species with a wide range of sympatry whilst others maintain they are morphotypes of a single, polymorphic species. To investigate the relationships within the complex and to study the evolution of morphology and the dynamics of their differentiation, we studied together allopatric and sympatric populations integrating multiple approaches: the geometric morphometric analysis of the shapes of genital and non-genital morphological structures, the comparison of horn static allometries and the biomolecular analysis of a mitochondrial gene. The results provided a complex and often incongruent pattern of morphological and genetic evolution, consistent with a hypothesis of a still unfinished speciation process in sympatric areas and a fully realised speciation by means of ecological segregation in allopatry.