Abstract

not-yet-known not-yet-known not-yet-known unknown Spatiotemporal environmental heterogeneity is a major evolutionary driver, which can cause profound phylogeographic complexity, particularly at the periphery of species ranges. Ringed seals display a highly disjoint distribution with an abundant subspecies occurring throughout the circumpolar Arctic, as well as three relict subspecies in Fennoscandia; the Baltic Sea, Lake Saimaa and Lake Ladoga. Traditionally regarded as originating from a single colonisation event from the paleo-Skagerrak-Kattegat region after the Last Glacial Maximum (LGM), recent studies have challenged this perception. Here, we analyse 246 mitogenomes and 180 skulls to unravel the diversity and spatiotemporal pattern of diversification in Fennoscandian ringed seals. Contrary to previous assumptions, our results reveal a complex evolutionary history characterised by several Fennoscandian colonisation events and pre-LGM diversification from Arctic ringed seals. We hypothesis that Lake Saimaa seals originate directly from the Arctic, possibly via the Karelia seaway, Ladoga via a similar route as well as from paleo-Skagerrak-Kattegat-Baltic, while the Baltic ringed seal have mixed evolutionary origins, which may be traced to distinct European glacial refugia, as well as ongoing gene-flow with the Arctic. Lake Saimaa and to some extent Lake Ladoga ringed seals have experienced a loss of diversity and evolved divergent skull morphologies as a result of colonisation bottlenecks, isolation and dietary specialisation, while Baltic Sea ringed seals have retained remarkably high levels of diversity. Our study supports the current classification of Lake Saimaa, Lake Ladoga and Baltic Sea ringed seals as distinct subspecies, and highlights the need for management and conservation efforts to mitigate cumulative impacts of human activities and climate change on Fennoscandian ringed seals.