Causes of the genetic architecture of south-west European high mountain disjuncts

Abstract

Postglacial climatic warming in south-western Europe and the retreat of cold-adapted species into higher elevations, starting in the Sierra Nevada and proceeding northwards to the Pyrenees and Alps, should have resulted in a pattern of ‘successive vicariance’. Alternatively, long-distance dispersal might explain the extant distribution pattern of mountain species in this region. Aims: Here, we report an investigation of two alpine plants, Saxifraga oppositifolia and S. stellaris, which co-occur in the Sierra Nevada, the Pyrenees, and the south-western Alps/Massif Central. Our aim was to distinguish between (successive) vicariance and long-distance dispersal as alternative explanations for their present-day geographical distribution across these high mountain ranges. The patterns found in these two species were compared to those of four other mountain species previously studied by us. Methods: Samples of 378 individuals from 21 populations were surveyed for amplified fragment length polymorphism (AFLP) variation. Results: We found that vicariance best explains the phylogeographic pattern of S. oppositifolia, while in S. stellaris, Sierra Nevada samples fell within the group of non-monophyletic Pyrenean populations. Together with genetic diversity evidence, this latter pattern is hypothesised to reflect incomplete lineage sorting following range fragmentation rather than long-distance dispersal from the Pyrenees to the Sierra Nevada. Additionally, in S. stellaris, a distinct genetic cluster from the Massif Central was observed. Levels of among-region and among-population differentiation were higher in S. stellaris than in S. oppositifolia. This may be taken as evidence of a more strongly disrupted glacial distribution of the former species, and also accords with its preference for more specialised and patchily distributed habitats when compared to those of the ‘pioneering’ S. oppositifolia. Conclusions: In conjunction with our previous investigation, we conclude that vicariance is the predominant, but not sole process shaping the genetic architecture of south-west European high mountain disjuncts. Furthermore, ‘successive vicariance’ was only observed in three previously studied species but not in the two Saxifrages analysed here, possibly due to the persistence of past patterns of population substructuring. © 2008 Botanical Society of Scotland and Taylor & Francis.