The specialisation/speciation continuum: diversification linked to larval host plant in the butterfly Eumedonia eumedon

It is widely accepted that the relationship between phytophagous insects and their host plants influences insect diversity. However, this subject is mainly addressed through macroevolutionary approaches, which focus on the outcome of speciation but are blind to population-level processes. An alternative fine-scale approach along the speciation process is required to shed light on the mechanisms and dynamics of host plant-driven diversification. With this aim, we investigated the speciation continuum of the geranium argus butterfly (Eumedonia eumedon). Genomic data revealed an unexpected array of host-specific lineages in Iberia, each feeding on a distinct plant species. Three of these clades were found nearby in a single mountain range. These results suggested that E. eumedon can locally specialise on a plant, resulting in some degree of reproductive isolation and genetic differentiation. These results show that the speciation continuum is shaped by local adaptations, a concept we define as the specialisation/speciation continuum: the gradual ecological differentiation and specialisation of populations that is linked to genetic differentiation and increased reproductive isolation. The unexpected eco-evolutionary variety discovered highlights the need to consider the spatial dimension at high resolution, as we show that, locally, the specialisation/speciation may be more advanced compared to nearby populations.