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.