Inverted patterns of genetic diversity in continental and island populations of the heather Erica scoparia s.l.
azores, dispersal, Endemism, island biogeography, isolation by distance, Macaronesia, spatial autocorrelation, spatial genetic structure, speciation
Aim Using the heather Erica scoparia s.l. as a model, this paper aims to test theoretical predictions that island populations are genetically less diverse than continental ones and to determine the extent to which island and continental populations are connected by pollen- and seed-mediated gene flow. Location Macaronesia, Mediterranean, Atlantic fringe of Europe. Methods Patterns of genetic diversity are described based on variation at two chloroplast DNA (cpDNA) loci and one nuclear DNA (nDNA) locus for 109 accessions across the entire distribution range of the species. Global patterns of genetic differentiation were investigated using principal coordinates analysis. Genetic differentiation between island and continental areas, estimations of pollen- and seed-mediated gene flow, and the presence of phylogeographical signal were assessed by means of Fst/NST (continental scale) and Fij/Nij (local scale). Extant and past distribution ranges of the species were inferred from niche modelling using layers describing present and Last Glacial Maximum (LGM) macroclimatic conditions. Results The Azores exhibited a significantly higher genetic diversity than the continent. The lowest levels of genetic differentiation were observed between the Azores and the western Mediterranean, and the diversity observed in the Azores resulted from at least two colonization waves. Within the Azores, kinship coefficients showed a significant and much steeper decrease with geographical distance in the cpDNA than in the nDNA. The distribution predicted by LGM models was markedly different from the current potential distribution, particularly in western Europe, where no suitable areas were predicted by LGM models, and along the Atlantic coast of the African continent, where LGM models predicted highly suitable climatic conditions. Main conclusions The higher diversity observed in Azorean than in continental populations is inconsistent with MacArthur and Wilson’s equilibrium model and derived theoretical population genetic expectations. This inverted pattern may be the result of extinction on the continent coupled with multiple island colonization events and subsequent allopatric diversification and lineage hybridization in the Azores. The results highlight the role of allopatric diversification in explaining diversification on islands and suggest that this process has played a much more significant role in shaping Azorean biodiversity than previously thought.