Tree species must adapt or migrate in response to anthropogenic climate change, but there are concerns that some species may not be able to keep pace and risk extinction. There is a significant discrepancy between migration rates inferred from paleorecords and those observed in species tracking the modern climate. Recent studies have emphasized the potential for disjunct refugial populations to help explain this gap, but adaptation during migration may have also played a role. As the leading edge of a range shift expands, traits such as seed weight and wing size undergo “spatial selection” where trees with better migration potential are more likely to populate the leading edge and seed the continued advance. Selection favoring the first to arrive and populate new terrain may have the effect of progressively concentrating adaptive alleles as migration continues. As a consequence, this may increase the rate of migration as better colonization ability is selected. We conducted Genotyping-by-Sequencing on the conifer species western redcedar using RADseq to infer its post-glacial migration, and paired these data with common-garden seed phenotype measurements to analyze patterns of selection during migration.
The Pacific Northwest offers an ideal biogeographical model as mesic species in this system underwent substantial post-glacial expansion over a fragmented matrix of suitable habitat to reach their modern species distribution. The refugial origin and post-glacial expansion of species genetic lineages must be considered to infer evidence of selection with migration. Sequencing of 154 range-wide individuals produced 40,701 polymorphic loci for western redcedar, which divide into three distinct groups of coastal population structure: a southern group in California and Oregon, and central group in Washington and southern British Columbia, and a northern group in Haida Gwaii and northern British Columbia. We compared refugial versus recolonized regions and observed patterns of decreasing seed weight and increasing seed wing size with distance from the refugial origins. We also found an inverse trend for wing loading with latitude, suggesting generally increased dispersal capabilities in the northern populations. In sum, these patterns suggest that the modern distribution of seed traits are a result of post-glacial migration selection. The northern refugia may have experienced greater selection pressure for dispersal capabilities as they recolonized a larger portion of the modern distribution during post-glacial expansion.