Linking microevolutionary and macroevolutionary processes in freshwater mollusks from the East African Rift. – EVOLINK

Modern evolutionary theory developed with the main objective to explain and predict changes in populations from one generation to the next, and how species evolve. Largely independently, paleontologists have accrued a body of knowledge informed by patterns of organismal diversity in deep time, but with the limitations of the fossil record. How mechanisms causing differentiation between populations ultimately contribute to macroevolution, however, remains a central question in evolutionary biology. I propose to address this question by studying mechanisms of differentiation at various levels of biological organization in evolutionary radiations of freshwater mollusks of the East African Rift System (EARS), an outstanding emerging model system. The EARS is geographically subdivided in a set of quasi-replicate systems to study evolution at various spatial and taxonomic scales; its freshwater mollusks are diverse, with life-history traits that typically correlate well to ecological niches; diversification is currently ongoing so that processes leading to speciation can be reconstructed accurately; and ancestors can be followed over long periods with substantial phylogenetic and time control. We will develop insight into the link between microevolution and macroevolution in two work packages (WPs): in WP1 we will study how processes of population differentiation lead to speciation and in WP2 we will embed our findings of WP1 into their wider taxonomic and geographic context to provide insight into macroevolution. Our approach will be similar in both WPs in that we will integrate data on genetic diversity, morphological disparity and habitat characteristics to examine how diversity originates (WP1) and is maintained (WP2). Methodologically, we will develop a next-generation sequencing (NGS) pipeline with two steps: 1) transcriptome sequencing to construct a reference transcriptome, and 2) the construction of an NGS library from transcriptomic data for gene-capture approaches (which can be used on existing EtOH-preserved collections). Additionally, morphological disparity will be studied with the same geometric morphometric methods across both WPs, and ecological data will be collected from sampling localities.

In WP1 we will focus on mechanisms of differentiation in two mollusk clades, i.e. Bellamya [4 nominal species] and Nyassunio [3 nominal species] from the Malawi Basin. We aim to determine which processes (ecological or non-ecological) have caused differentiation and how these processes have interacted in space and time in the multiple speciation events that took place or are ongoing. We will obtain insight into mechanisms by comparing the genetic, morphological and environmental bases of differentiation from as many populations in the basin as possible. Specifically, single-nucleotide polymorphism data will be obtained from NGS, geometric morphometric data from shells, and these data will be subjected to population genomic studies together with habitat information. In WP2 we will establish relations in monophyletic clades of Viviparidae and Unionidae (African Bellamyinae and Coelaturini, to which Bellamya and Nyassunio, respectively, belong) on a rift-wide scale from NGS sequence data. These reconstructions will be used to test hypotheses on biogeographic patterns, parallel and iterative evolution, and trait evolution in relation to diversification. Secondly, we will integrate fossil taxa to study patterns of extinction selectivity and to test whether macroevolutionary conclusions based on the modern fauna hold when fossils are included. Finally, results from WPs 1 & 2 will be integrated to examine to what extent microevolutionary patterns affect macroevolution, and thus how drastic differences in diversity even in closely related clades can be explained. Beyond a profound influence on evolutionary biology, this understanding is vital to understand the implications of environmental changes on modern diversity.