Data from: Evidence for ecological divergence across a mosaic of soil types in an Amazonian tropical tree: Protium subserratum (Burseraceae)

Soil gradients are known to be an important driver of divergent natural selection in plant populations. Neotropical trees have the highest diversity on earth and it is not uncommon to find soil specialist congeners distributed parapatrically. Nevertheless very little is known about the role that edaphic heterogeneity plays in the origin and maintenance of tropical tree diversity. We predict that the mosaic of different soils in the lowland Amazon rainforest play a major role in generating and maintaining genetic variation in tropical tree populations through ecological divergence. This study examines genetic and morphological differentiation in the tropical tree Protium subserratum populations growing on the mosaic of white-sand, brown-sand and clay soil types distributed parapatrically throughout the western Amazon basin. We use nuclear microsatellites and vegetative characters to answer four major questions: (1) Are populations phenotypically and genetically differentiated across all three habitat types? (2) Can we detect a signature of natural selection over drift divergence among populations on different soil types? (3) What is the extent of hybridization and introgression across habitat types? (4) What is the relative importance of spatial distance and soil type in structuring P. subserratum tree populations and influencing migration rates among populations? We found significant morphological variation in populations across the three soil types. Higher levels of genetic differentiation and lower levels of gene flow were observed between adjacent populations found on different soil types than between geographically distant populations on the same soil type. Comparisons of phenotypic and neutral genetic variation among soil specialist populations suggest a role for natural selection in population divergence and a small number of hybrid individuals were detected between soil ecotypes showing that gene flow among populations may be occurring at a low frequency. Our results suggest that edaphic specialization in P. subserratum populations has occurred across multiple soil boundaries. While we cannot discern which factors caused initial divergence our results do suggest that natural selection is plays an important role in maintaining the integrity of populations across ecological boundaries and that edaphic specialization may be a general mechanism that promotes and maintains Amazonian tree diversity