Contributions of biogeographical functions to species accumulation may change over time in refugial regions

Abstract

Aim: Elevated biodiversity is the result of the cradle, museum or sink functions. The contributions of these three functions to species accumulation and their changes through time remain unknown for glacial refugia. Additionally, our understanding of the role these functions played during pre-glacial periods is limited. We test for changes in contributions of functions through time leading to the current diversity patterns using a model refugium and taxon. Location: Anatolia, Western Palaearctic. Taxon: Freshwater neritid snails (genus Theodoxus). Methods: Assessments were made to define molecular operational taxonomic units (MOTUs) for Theodoxus and reaffirm the genus as a suitable model taxon with elevated interspecific diversity in noted glacial refugia. Thereafter, we constructed a time-calibrated multilocus Bayesian phylogeny of mtDNA and nDNA by using both fossil data and published substitution rates. Ancestral area estimation was performed on the phylogeny to define the contribution of the functions through time. Results: Accumulation of Theodoxus diversity in Anatolia over the Miocene–Pliocene transition is attributed to the museum function, but its contribution was small as only few divergence events occurred. The cradle function dominated during the Pliocene and Early Pleistocene, when most interspecific diversity built up and extant lineages in Anatolia were established. The sink function acted from the Middle Pleistocene to present-day, but with only a small contribution to the total extant Anatolian interspecific diversity. Main conclusion: Our results do not entirely mitigate the role glacial cycles played in species accumulation, but highlight Ice Ages may have been less effective in forcing temperate aquatic interspecific diversity into more opportune areas. The elevated diversity in refugia may rather be the result of earlier in situ diversification. Elevated interspecific diversity attributed to the legacy of glacially forced retreats may need to be re-evaluated in cases where refugia have long and complex geological histories such as Anatolia. These results highlight the importance of considering species accumulation through a temporal perspective to adequately explain present-day biodiversity patterns.