The influence of environment on adaptive radiation of diatoms in East African Rift lakes

Adaptive radiation is considered to play an important role in the diversification of life on Earth. This is especially true in isolated long-term environments where the largest adaptive radiations have been found and where the adaptive nature of diversification has been best studied. However, the environmental conditions that influence rapid diversification during adaptive radiations and potentially lead to differences in evolutionary trajectories and species richness across the tree of life are still unclear, primarily because there few, if any, fossil records for some of the most iconic examples of vertebrate radiations. Here, we use two diverse groups of diatoms (Diploneis and Afrocymbella) with different lifestyles and great fossilization potential to test the role of environment in adaptive radiation and its impact on evolutionary trajectories between different diatom clades in East African Rift lakes that are home to the world's largest radiations – that of cichlid fishes. We constructed a time-calibrated molecular phylogeny of extant and extinct species, as well as a trait matrix, and show that the two diatom groups evolved within the rift from a common ancestor over a relatively short time, with accelerated diversification leading to much higher species richness in the genus Diploneis. We then correlate the inferred diversification rates and trajectories of trait evolution with biological and environmental variables to determine the influence of the environment on the progression of adaptive radiation. This integration of genetic, morphological, and paleoenvironmental information allowed us to demonstrate the influence of the environment on a key process that has produced much of Earth's biological diversity.