Speciation in mountain refugia: phylogeography and demographic history of the pine siskin and black-capped siskin complex

Following Pleistocene glacial maxima, species that adapted to temperate climates in low-latitude refugia had to modify their ranges as climate changed, expanding either latitudinally towards the poles, or altitudinally to higher elevations in mountainous regions. Within just a few thousand years, populations taking alternative routes during interglacials became isolated from each other and subjected to different selection pressures, often leading to lineage divergence and speciation. The pine siskin Spinus pinus is a common and widespread songbird showing relative phenotypic uniformity across the North American continent. One exception is the subspecies found in the highlands of northern Central America (S. p. perplexus), which shows marked differentiation in plumage color and shares some traits with the endemic and partly sympatric black-capped siskin S. atriceps, suggesting potential introgression or even a hybrid origin of perplexus. Relationships and species limits among pinus, perp...