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

Abstract

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