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 dependent 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 morphological 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, biologically meaningful conservation strategies.