The roles of geography and environment in divergence within and between two closely related plant species inhabiting an island-like habitat

Aim: In island-like habitats, geographic isolation facilitates population and species divergence by constraining gene flow, while environmental isolation can enhance divergence. We tested the relative contribution of geographic and environmental isolation in genetic and phenotypic divergence within and between two species of the figwort Ameroglossum (Scrophulariaceae) inhabiting spatially isolated habitats, known as inselbergs. Location: Borborema Plateau, north-eastern Brazil. Methods: Multivariate models of redundancy (RDAs) and partial redundancy analyses (pRDAs) were used to partition the geographic and climate components of genetic variation in 48 microsatellite alleles, and phenotypic variation in 11 leaf and flower traits. We also used linear mixed-effect models (LMEs) to test independent associations of floral tube length variation among inselbergs with local pollinator phenotypes, climate and geography. In each approach, we analysed the data for each species separately and in pooled models for both species. Results: RDAs revealed that genetic variation within and between the species of Ameroglossum was associated similarly with geography and climate. Phenotypic variation within A. manoel-felixii and between species was also associated similarly with geography and climate but, within A. pernambucense, phenotype was more strongly associated with climate. Linear mixed-effect models revealed that flower divergence in A. manoel-felixii was associated only with the bill length of local hummingbirds, whereas floral variation in A. pernambucense was associated with geography, bill length and climate. Only climate was associated with flower divergence between species. Main conclusions: Genetic and phenotypic variation in Ameroglossum are associated with geographic and environmental isolation. These findings indicate a significant role of ecological factors shaping plant divergence among inselbergs, irrespective of their spatial distances.