The missing branches of the bee Tree of Life: addressing global Darwinian shortfalls and their drivers

Understanding the Darwinian shortfall (i.e., the lack of knowledge about phylogenetic relationships) can help us to guide future biodiversity research and conservation efforts. Overcoming this shortfall is essential to develop robust strategies to preserve the Tree of Life while facing the ongoing biodiversity crisis. Here, we present the first global assessment of Darwinian shortfalls and their drivers in one of the main groups of pollinators, the bees. We built phylogenies for over 12000 bee species, combining the most comprehensive phylogeny and an algorithm with random solutions to insert missing lineages. The Darwinian shortfall was quantified as the Phylogenetic Diversity (PD) deficit, the ratio of inserted branch lengths, at the assemblage level. The highest shortfalls were identified in the Southern Hemisphere. Mean species range size and species richness were the strongest drivers, as smaller ranges and higher richness were associated with higher deficits. Per capita GDP was negatively associated with PD deficits, while population and road densities showed positive but weak effects. Sample completeness had a weaker effect, limited by missing occurrence data in many regions. Our findings underscore the need for integrative efforts combining taxonomy, data digitization, adequate research investments, and targeted sampling, especially in the Global South.