Diversification and the patterns of introgression in a clade of Neotropical trees

Abstract

The evolutionary processes that led to the assembly of the diverse Neotropical tree flora are complex and poorly understood. It remains unclear at what pace this superlative diversity was accumulated, as do the causes of poor phylogenetic resolution and gene tree incongruence, which are commonly found when reconstructing the phylogeny of rainforest tree species. In addition, the role that introgression has played in the diversification of Neotropical trees and its effect on phylogeny remains enigmatic. The Brownea clade (Leguminosae) are a group of tree species characteristic of Neotropical rainforests, and so serve as an excellent study group with which to approach questions about the diversification of Neotropical trees. This investigation aimed to infer the rates of diversification and biogeographical history of the Brownea clade following the estimation of a phylogenetic tree for the group. The relationships between species within Brownea were then examined using phylogenomic methods to quantify gene tree incongruence, and phylogenetic model testing was used to ascertain whether this incongruence was better explained by incomplete lineage sorting (ILS) or introgression. Finally, two Brownea species found in sympatry were used to examine the impact of introgression on divergence, and whether species boundaries are maintained despite gene flow. More specifically, this study assessed whether genomic loci differed in the degree to which they introgress when compared to the genome-wide average, which would be expected if species boundaries were being maintained by selection. Furthermore, loci which were candidate regions for selection, and those associated with different aspects of the environment and morphology, were inferred in order to determine possible factors maintaining the divergence between the two species.