Phylogeography and diversification of the Pieris napi species group in the Western Palaearctic

The butterflies of the Pieris napi complex, encompassing diverse taxa across Europe, Asia and North Africa, represent an emergent model system for studying climatic adaptation. In this study, we use genomic data for this group to address phylogenetic relationships, population structure and ecological differentiation in the Western Palearctic. Our results reveal two main groups: the napi clade and the bryoniae-balcana clade. We clarify the status and show that the distribution of the debated species P. balcana extends to south Romania and the Peloponnese. This species is shown to be sister to P. bryoniae, which indicates that melanism is an evolutionarily labile trait linked to climatic adaptation. The P. napi clade is divided into three components, each predominating in a different south European peninsula, and the boreal P. n. adalwinda, which is notably differentiated. The endangered Moroccan taxon segonzaci produced conflicting results, probably due to low-quality DNA. Admixture analyses suggest substantial gene flow between taxa in contact areas. Redundancy analyses identify summer temperature and precipitation as key drivers of adaptive genetic variation within the group. This work provides a robust evolutionary framework for future ecological studies of the Pieris napi complex, to forecast eco-evolutionary responses and to address conservation priorities in a rapidly changing climate.