A New Challenge to Species Delimitation: Remarkable Genomic and Ecological Diversity in the Butterfly Melitaea diamina

Evolution relentlessly challenges any concept of species, and human efforts to fit a taxonomic hypothesis to patterns in nature often seem unsatisfactory. Genomics show that a solution does not always come with more data: a fine-scaled view may reveal populations with a more gradual array of differentiation, infer cases of hybrid origin and bring to light the existence of ghost populations. The false heath fritillary, Melitaea diamina, is a Palearctic butterfly with patchy, localized populations highly de- pendent on humid temperate habitats. Despite its extensive geographic range, populations at the southern edge of its European distribution are scarce and poorly studied genetically. Here, we employed genome-wide data to reveal the existence of two major groups in M. diamina: an Iberian and a Eurasian lineage. Within Iberia, we documented a remarkable diversity: the populations in the Pyrenees are of hybrid origin between the Eurasian lineage and a now apparently extinct Iberian population. The ghost population was related to three extant additional Iberian lineages that align with mitochondrial differentiation, distinct mor- phological traits, and/or ecological specialization in the form of host plant association. Gene flow analyses suggested historical admixture despite current isolation, underscoring the complex evolutionary history of these populations. We attempt a revised taxonomic framework for this novel array of evolutionary significant units, adopting an updated subspecies concept—defining subspecies as incompletely separated lineages within a more inclusive lineage. Such an approach allows accurate representation of evolutionary complexity without prematurely elevating these populations to species rank, thereby informing targeted, biolog- ically meaningful conservation strategies.