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Diversification of the Core Bromelioids with a focus on the genus Aechmea: phylogeny, morphology, and climate tolerance


Understanding patterns of speciation is of fundamental importance to evolutionary biologists. Species diversity can be maintained and generated through processes originating from interactions with climate, biota, and geography. In order to investigate the role of these factors on diversity in a species-rich group of plants, we developed a phylogeny of the core Bromelioideae including Aechmea and related genera with the specific goals of investigating monophyly of genera, biogeographic history, morphological trait evolution, and environmental niche diversification. The phylogenetic estimation encompassed 20 genera and 4 genomic regions including chloroplast, low-copy, and ribosomal nuclear genes. Subsequently, morphologies were re-explored to understand species relationships that had not been previously recognized and to determine if morphological synapomorphies could define well supported clades. Species were coded as present or absent in 16 geographic regions to reconstruct ancestral areas. Finally, to understand how species within this group divided climatic niche space, research was focused on two clades that diversified independently in Central America. Niche models were generated for each species and for the clades as a whole and were compared for overlap. The distribution of each environmental variable for all occurrence records was compared at each node of divergence within each clade to understand divergence in environmental conditions in a method free from model uncertainty. From these analyses we found: (1) Lack of monophyly in many genera and subgenera (2) Closely related species often were found in geographic sympatry (at the scale of co-occurrence in the same country or state) such that taxonomic groups that had been thought to include geographically disjunct species, actually were representative of distinct evolutionary histories (3) Morphological traits that had been used to assign species to genera were often homoplastic (through convergence or reversals of plesiomorphic states) (4) Although a single unique synapomorphy for each clade was not found, morphological traits were often consistent within clades, providing independent support for the molecular phylogenetic estimation and a basis for future taxonomic work (5) Most well supported clades diversified within one of eight geographic areas, although the ancestral areas for several larger clades remains uncertain (6) Both clades native to Central America are found in a unique environmental niche (i.e. environmental niche models of each clade do not overlap and there is significant divergence of climatic variables between the clades) (7) Species within the clade composed of species with greater morphological variation had more environmental niche overlap than the clade with little morphological variation. These findings add to the evolutionary knowledge of the Bromeliaceae and suggest that geographic range should be taken into account when assigning species to genera. Centers of diversity of the core Bromelioids are found within eastern and Amazonian Brazil, northern south America and Central America and each radiation could have been influenced by climate in a variety of ways. In the two studied examples, species divergence is coincident with divergence in environmental niche occupation. However, environmental niche evolution was limited such that each clade as a whole was not modeled in the niche of the other clade, suggesting either some degree of niche conservatism or competition preventing parapatric niche expansion.

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