Parallel speciation with gene flow: snapshots from Amazonia. Geonoma

Differences across the ecological landscape can cause population segregation and drive speciation of new lineages. Further, similar ecological factors may affect a species in different parts of its distribution, causing parallel divergence and speciation. Parallel ecological speciation occurs when the evolution of reproductive isolation is iterated across independent populations but driven by similar types of differential selection. Parallel speciation has been recorded in several animal species, but is poorly evidenced in plants. The western Amazonian palm Geonoma macrostachys presents distinct morphotypes defined by their leaf morphology repeatedly occurring in sympatry. Here, we tested for parallel speciation under gene flow by the combined analysis of target sequence capture data, morphology, climate, soil chemistry, and reproductive biology of independent populations. Using demographic models, we tested whether morphotypes evolved either through a single origin, achieving secondary contact at some sites, or whether morphotypes resulted from a series of parallel events occurring at different localities. Morphometric analysis identified individuals presenting intermediate morphologies, suggesting gene flow contributes to speciation in this system. The multispecies coalescent tree did not infer monophyletic clades for the morphotypes, although geographic clustering was identified, indicating the repeated evolution of leaf type across space. We identified a strong function of sampling scale on our results, where niche segregation is nearly complete at the local scale but is less evident at the regional scale, as expected under a parallel evolution scenario. Reproductive isolation amongst morphotypes has been found at other local sites, but in our study, it was not complete, suggesting it has evolved multiple times and at different levels across the species distribution. Taken together our findings suggest that parallel ecological speciation is driving diversification in G. macrostachys, further supporting a model of speciation under gene flow.