Fine-scale population structure despite recent colonization in Western Fence Lizards (Sceloporus occidentalis) at their range edge

Population dynamics within species at the edge of their range, including the formation of genetic structure during range expansion, is difficult to study at recent timescales. Western Fence Lizards (Sceloporus occidentalis) have a patchy distribution at the northern edge of their range on islands and coastal habitats in the Puget Sound, Washington, which was covered by Pleistocene glaciation as recently as 16,000 years ago. To test for population differentiation across this recently colonized landscape we collect genome-wide SNP data and morphological data. Time-calibrated species tree inferences support the hypothesis of recent, post-glacial population expansion into the region. Despite the strong signal for fine-scale population genetic structure with as many as 8 - 10 distinct populations supported by the SNP data, there is minimal evidence for morphological differentiation. Historical demographic analyses suggest that these populations expanded into the region recently. Population isolation, lack of dispersal corridors, and strict habitat requirements are the key drivers of population differentiation in this system.