# The Evolutionary History of the Extinct Baltic Sea Harp Seal Population

## TL;DR

Ancient DNA analysis reveals the evolutionary history of an extinct Baltic Sea harp seal population, showing its genetic distinctiveness and possible causes of its extinction.

## Contribution

The study provides new insights into the genetic differentiation and evolutionary dynamics of an extinct harp seal population using ancient DNA.

## Key findings

- Ancient Baltic harp seals were genetically distinct from contemporary populations and retained their genetic composition over time.
- The population's decline and extinction are hypothesized to result from climate change, reduced salinity, and human harvest.
- Ancient and modern mitogenome analyses suggest limited gene flow and a late Pleistocene range expansion from a common refugial population.

## Abstract

The now‐extinct harp seal population that inhabited the Baltic Sea from the Mesolithic to the Iron Age is an enigma. It occurred outside the species' contemporary Arctic range, likely deviated from typical harp seal migratory behaviour, and experienced body size reductions and dramatic population fluctuations leading up to its extinction. Here we use ancient DNA analyses to shed more light on the evolutionary history of the Baltic Sea harp seal population, including its origin, timing of colonisation, diversity and factors contributing to its demise. We generated 49 ancient Baltic and eight ancient Arctic harp seal mitogenomes, which we analysed together with 53 contemporary Arctic harp seal mitogenomes. We detected limited phylogeographic resolution among ancient and contemporary populations, which we interpret as a late Pleistocene range expansion from a common refugial population with subsequent gene flow. Ancient Baltic harp seals were significantly genetically differentiated from contemporary harp seal populations and retained their own genetic composition throughout time. The genetic diversity of Baltic harp seals decreased over time, yet was comparable to that of contemporary populations. This suggests that Baltic harp seals formed a distinct breeding population, which may occasionally have received immigrants from the Arctic but was itself confined in the Baltic Sea until the end. We hypothesise that loss of genetic diversity and the ultimate extinction of the Baltic harp seal population was a consequence of population fluctuations caused by climatic change, reduced salinity and biological productivity, and periodic intense human harvest.

The figure exhibits the spatiotemporal distribution of harp seal mitogenome clades, showing temporal changes in ancient Baltic Sea populations and spatial differences among modern populations from Newfoundland and the White Sea. Key localities analysed include ancient and modern samples from the Baltic Sea, Finnmark, White Sea, Greenland Sea, and Newfoundland, highlighting variations in mitogenome clades over time and across regions.