Two Closely Related Butterflies Show Pseudo-Concordant Patterns of Geographic Differentiation in a Heterogeneous Mountain Region.

Sympatric species are often found either to broadly match or to differ remarkably in their phylogeographic patterns. However, sometimes they show 'phylogeographic pseudo-concordance', that is, they share the same geographic arrangement of population differentiation, but differ in the level of differentiation between populations. Especially in heterogeneous regions like mountains, cases of pseudo-concordance are particularly insightful to gauge the relative role of landscape features, species traits and colonisation history in shaping evolutionary differentiation. Yet, such cases have been rarely documented so far. We investigated the differentiation patterns of two closely related butterfly species, Erebia euryale and E. ligea, in a highly heterogeneous mountain region in the south-eastern Alps. We applied a fine-scale geographical sampling and complemented genomic analyses (SNPs from ddRAD sequencing) with an analysis of phenotypic differentiation (morphological characters of the wings and male copulatory structures). Our findings provide strong evidence for pseudo-concordance between the two species, which are both differentiated into two groups of populations: one occurring in the western part of the study area and the other in the eastern part, with the geographic boundary separating the two groups being coincident in the two species. However, while in E. euryale the two population groups correspond to lineages that appear differentiated at the species level, with negligible gene flow and significant phenotypic divergence, the two groups within E. ligea only show a weak genetic divergence, with substantial gene flow and no detectable phenotypic differentiation. Our case study shows how sampling populations at an adequate spatial resolution and addressing evolutionary differentiation encompassing both genomic and phenotypic components may succeed in detecting and contributing strong evidence for phylogeographic pseudo-concordance between sympatric species in highly heterogeneous regions like mountains.