Predictive Comparative Phylogeography of PNW Rainforest Taxa
Comparative phylogeographic studies have addressed disjunct mesic forest taxa in the PNW of North America for decades. These have largely focused on the rather coarse phylogeographic hypotheses synthesized by Brunsfeld et al. (2001). Several taxa, primarily amphibians (Ascaphus truei /A. montanus , Nielson et al, 2001; Plethodon vandykei /P. idahoensis , Carstens et al, 2005; Dicamptodon copei /D. aterrimus , Carstens et al. 2005), show evidence of an ancient vicariance and persistence of populations in inland refugia throughout the Pleistocene, with no evidence of gene flow. Red alder (Alnus rubra ) genomics (Ruffley et al. 2018) show evidence of ancient vicariance but with consistent migration between coastal and inland populations through the Pleistocene. Still other disjunct rainforest endemics, including water voles (Microtus richardsoni , Carstens et al 2005) and several disjunct taildropper slugs (Prophysaon andersoni , P. dubium , P. coeruleum , andP. vanattae ; Smith et al., 2018), show no evidence of Pleistocene persistence in ITR refugia, but rather have attained the disjunct distribution via post-Pleistocene dispersal.
Because of this collection of complex evolutionary histories for mesic forest disjunct taxa, and the conservation and evolutionary implications, EspĂ­ndola et al. (2016) and Sullivan et al. (2019) have developed a framework for predicting the presence or absence of cryptic divergence in this system. Our data on the two mesic forest climax community dominant tree species represent a critical refinement to this predictive framework. This is especially true because western hemlock and western redcedar show strong evidence for a pre-Pleistocene divergence as well as evidence of post-Pleistocene gene flow through the non-zero estimation of migration rates between the populations (Table 3), a pattern thus far seen in only one other disjunct taxon (Alnus rubra ); this should increase our ability to identify other inland rainforest taxa that may demonstrate a similar evolutionary history, especially those with wind-dispersed pollen and/or seeds/spores.