Effects of the interaction between Linnean and Darwinian shortfalls on diversification gradients

It is now widely recognized that broad-scale patterns in species richness, particularly the Latitudinal Diversity Gradients (LDGs), are driven by complex interactions among ecological, evolutionary, and historical processes. However, even if it is now possible to better evaluate evolutionary explanations for LDGs based on speciation and diversification rates estimated from phylogenies, a subtle interaction between such estimates and the geographic structure of the Linnean shortfall, forming the Latitudinal Taxonomy Gradient (LTG), was recently recognized. Here, we expand on previous simulation results and show that a relatively small geographical bias in the Linnean shortfall, adding less than 20% of new species phylogenetically correlated with previously described ones in the southern (richer) region, would be enough to change the patterns in diversification rates, based on different methods (tip rates DR and GeoSSE). Further investigations of the magnitude of LTG and new empirical modeling of problems in species delimitation are thus necessary to evaluate the robustness of the estimates of the diversification gradients to biased knowledge and taxonomic uncertainty, allowing a better understanding of the evolutionary dynamics underlying LDGs. Geographical patterns in the Linnean shortfall, creating Latitudinal Taxonomy Gradients (LTGs), may jeopardize robust estimates of diversification gradients and our understanding of processes underlying diversity patterns. Simulations show that a relatively small geographic bias in the Linnean shortfall, adding new undiscovered species phylogenetically close to the known ones, is sufficient to invert patterns in diversification rates estimated by two distinct methods. A more comprehensive assessment of the magnitude of diversification gradients in respect to the LTGs is necessary to evaluate the robustness of our understanding of LDGs. Empirical modeling of species delimitation quality and simulations adding small branches in the phylogenies, mitigating simultaneously both Linnean and Darwinian shortfalls, may provide more consistent evidence for drivers of LDGs accounting for taxonomic uncertainty.