Neotropical forests have the highest tree diversity on earth, with an estimated 22,000 species. In contrast temperate North America, Europe and Asia combined support only 1,166 tree species. In the western Amazon rainforest, complex patterns of edaphic heterogeneity have been invoked as potential drivers of plant diversity through local. Numerous studies have demonstrated that physiological trade offs associated with adaptation to different habitat types leads to ecological sorting which in turn drives the spatial distribution of tropical trees along environmental gradients. If populations that are locally adapted to different environments experience a reduction in gene flow they may continue to differentiate potentially leading to new species formation.
Ecological speciation, whereby divergent natural selection results in reproductive isolation occurs as a direct result of adaptation to ecological conditions, is thought to be an important driver of plant diversification. Ecological speciation is known to occur in allopatry and a number of studies have demonstrated that it may be possible in parapatry and sympatry. However, the latter scenario, in which divergence and speciation occurs in the face of gene flow remains a point of contention. In the absence of geographic barriers to gene flow, disruptive selection must be strong enough to overcome genetic `dilution' from neighboring populations. Environmental factors commonly associated with ecologically based disruptive selection include variation in topography, microenvironmental differences, soil heterogeneity, herbivore pressure, and pollinator differentiation, all of which can reduce gene flow, potentially leading to the evolution of reproductive barriers.
Reproductive barriers can act to isolate diverging lineages before or after fertilization. Prezygotic isolating mechanisms in plants can be intrinsic, usually entailing pollen incompatibility with the stigma or ovule, or extrinsic, where isolation is ecologically driven, as with pollinator-mediated barriers or changes in flowering phenology. Postzygotic isolating mechanisms can be intrinsic, in the case of hybrid sterility and inviability, or extrinsic, where isolation is enforced through ecologically based natural selection where hybrid offspring are less suited for survival in either parental habitat type. To date, only a handful of studies have comprehensively quantified the strength of individual reproductive isolating mechanisms among closely related plant species, the majority of which are herbaceous and found in temperate zones. Even fewer studies have focused on tropical plants and those that have focused on herbaceous lineages.
This dissertation aimed to investigate the broader role of divergent natural selection as an agent in generating and maintaining Amazonian tree diversity using edaphically divergent populations of Protium subserratum. Using a combination of molecular population genetics field observations and hand-pollination experiments I: (1) Developed nuclear microsatellite markers in order to evaluate population level differentiation and gene flow between populations of P. subserratum associated with different soil types. (2) Inferred the role of divergent natural selection in driving the genetic structure of P. subserratum populations found on clay, brown-sand, and white-sand soils distributed more than 100km across the Peruvian Amazon. (3) Investigated the role of habitat and distance in driving turnover in stingless bee communities, the putative effective pollinators for P. subserratum. (4) Systematically evaluated the role of multiple pre- and post-zygotic barriers to reproduction in maintaining population integrity between parapatric populations of white-sand and brown-sand P. subserratum.
I successfully developed 17 polymorphic nuclear microsattelite markers, which were subsequently used to assess population variation across populations of P. subserratum, found on different soil types. I found evidence that suggests that edaphic specialization has occurred multiple times in P. subserratum and that natural selection may be driving divergence across edaphic boundaries. I found that location and soil type play a significant role in structuring stingless bee communities in white-sand and non-white sand forest and that community turnover may be more strongly influenced by distance in white-sand habitats than non-white sand habitats. I was able to identify four active barriers to reproduction between parapatric white-sand and brown-sand populations of P. subserratum including ecogeographic isolation, differential pollen adhesion, differences in pollinator assemblages, and low levels of hybrid seed development. I demonstrated that a combination of pre-zygotic and post-zygotic barriers to reproduction act to maintain near complete reproductive isolation between edaphically divergent populations of the tropical tree, P. subserratum.