Why do taxa differ in species richness? A multifactorial evolutionary view at species richness disparities in microgastropods

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The striking differences in species richness across phylogenetic clades of organisms has been attributed to differences in clade age or to variation in diversification rates (speciation minus extinction), which is driven by extrinsic (e.g. habitat type, geographic distribution, and species co-occurrence) and/or intrinsic (e.g. morphological features) factors. Whereas clade age appears to be a straight-forward criterion for richness differences, the intrinsic and extrinsic drivers of diversification remain not well understood. Previous studies have usually focused on the influence of single factors on diversification rate. However, the effect of various factors on diversification rates may be complexly dependent, which suggests a need for multifactorial analyses. Moreover, our current evolutionary understanding of richness differences is strongly biased towards marine or terrestrial systems, vertebrates, and/or small taxonomic groups. The main goal of this project is to test for drivers of species richness disparities using a taxonomically and ecologically diverse model taxon (the aquatic gastropod family Hydrobiidae) and a novel multifactorial phylogenetic comparative approach that enables estimation of the joint effect of these extrinsic and intrinsic factors. A robust time calibrated phylogeny will be constructed (based on multi locus DNA sequence data for >400 species) to infer speciation, extinction, and net diversification rates over time and among clades. The effects of the above mentioned extrinsic and intrinsic factors on diversification rates will be studied individually using phylogenetic comparative analyses and the joint influence of these variables will be assessed utilizing a multifactorial phylogenetic model. This novel approach that we will utilize will increase our knowledge of the drivers of diversification in freshwater organisms and may also serve more broadly as a new conceptual and methodological basis for comparing patterns and causes of diversification within the tree of life and among different realms. Additionally, our findings may contribute to the development of a more rigorous framework for the prioritization of conservation policies in inland aquatic ecosystems.