Secondary contact after divergence in allopatry explains current lack of ecogeographical isolation in two hybridizing alpine plant species

Aim Allopatric speciation plays an important role in generating the high diversity of alpine biomes, but in cases of species with largely overlapping distribution, allopatric divergence is usually rather assumed than explicitly tested. The hypothesis of allopatric divergence in currently sympatric and hybridizing species is here tested using phylogeographical tools. Location European high mountain ranges (Alps, Pyrenees). Methods We gathered amplified fragment length polymorphism (AFLP) data and plastid DNA sequences from (1–)2–7 individuals of each of 38 populations of Androsace helvetica and of nine populations of A. pubescens (Primulaceae), spanning their entire distributional areas. AFLP data were analysed with Bayesian clustering approaches, PopGraphs analysis and Bayesian tree estimation methods, the latter permitting ancestral area reconstruction including estimation of Bayes factor support for specific regions. Relationships among plastid haplotypes were inferred using statistical parsimony. Results Androsace helvetica and A. pubescens originated in allopatry and attained their overlapping distributional areas after range expansion. Gene flow between species did occur (evidenced by haplotype sharing in areas of sympatry), but apparently was not sufficient to erode species boundaries (no signature for gene flow from AFLP data even in syntopic populations of the two Androsace species). Distributional patterns of gene pools and of plastid haplotypes suggest that the largely centrifugal pattern in A. helvetica and the unidirectional pattern of range expansion in A. pubescens were modulated by range contractions during Pleistocene climate oscillations into peripheral and interior refugia. Main conclusions The hypothesis of allopatric divergence in closely related and ecologically similar species can be readily tested using ancestral area reconstruction at the intraspecific level. Phylogeographical patterns in the two Androsace species emphasize the relevance of both peripheral and interior refugia for Pleistocene range formation.