Constant diversification rates of endemic gastropods in ancient Lake Ohrid

12 Ancient lakes represent key ecosystems for endemic freshwater species. This high endemic 13 biodiversity has been shown to be mainly the result of intra-lacustrine diversification. Whereas the 14 principle role of this mode of diversification is generally acknowledged, actual diversification rates in 15 ancient lakes remain little understood. At least four modes are conceivable. Diversification rates may 16 be constant over time, they may fluctuate, rates may be higher in the initial phase of diversification, or 17 there may be a pronounced lag phase between colonization and subsequent diversification. As 18 understanding the tempo of diversification in ancient lake environments may help unrevealing the 19 underlying processes that drive speciation and extinction, we here use the Balkan Lake Ohrid as a 20 model system and the largest species flock in the lake, the non-pyrgulinid Hydrobiidae, as a model 21 taxon to study changes in diversification rates over time together with the respective drivers. 22 Based on phylogenetic, molecular-clock, lineage-through-time plot and diversification-rate 23 analyses we found that this monophyletic group is comparatively old and that it most likely evolved 24 with a constant diversification rate. Preliminary data of the SCOPSCO deep-drilling program do 25 indicate signatures of severe environmental/climatic perturbations in Lake Ohrid. However, so far 26 there is no evidence for the occurrence of catastrophic environmental events. We therefore propose 27 that the constant diversification rate observed in endemic gastropods has been caused by two factors: 28 i) a potential lack of catastrophic environmental events in Lake Ohrid and/or ii) a probably high 29 ecosystem resilience, buffering environmental changes. Parameters potentially contributing to the 30 lake’s high ecosystem resilience are its distinct bathymetry, ongoing tectonic activities, and karst 31 hydrology. 32 The current study not only contributes to one of the overall goals of the SCOPSCO deep33 drilling program – inferring the driving forces for biotic evolution in Lake Ohrid. It might also enhance 34 our understanding of how ecosystem resilience, in general, may promote relative constant 35 diversification rates in isolated ecosystems. However, we encourage future studies testing hypotheses 36 about the lack of catastrophic events in Lake Ohrid. These studies should be based on high-resolution 37 data for the entire geological history of the lake, and potentially involving information from the 38 sediment fossil record, not only for gastropods but also for other groups with a high share of endemic 39 taxa. 40 41