Phylogenetic biome conservatism underlies the evolution of forest palaeoendemic legume trees in tropical Africa

Abstract

Biome conservatism is prevalent during the evolution of plant lineages. However, studies assessing biome lability, i.e. the capacity to shift biomes and its impact on tropical tree species diversification is currently limited. To address this, we analysed an endemic lineage of African tropical trees to investigate phylogenetic patterns of biome conservatism and lability and their impact on speciation and extinction rates. We reconstructed a time-calibrated phylogeny of the Berlinia clade (16 genera, 201 species) using 140 nuclear genes, 75% of its extant species and two fossil calibrations. We found the forest biome as the ancestral habitat and we inferred nine independent shifts from forest to savanna with no reversals. The forest biome is mostly conserved within the Berlinia clade, while the ability to shift to the savanna biome is randomly distributed across the group. We found five palaeoendemic genera that have persisted solely in the forest biome since the Oligocene. However, the ability to shift among biomes does not seem to influence speciation or extinction rates. Our results suggest that palaeoendemic and forest-restricted lineages are more susceptible to habitat alterations and climate change than lineages with biome lability, due perhaps to an innate limitation to adapt to new habitat types.