Investigating the Evolutionary History of the Pacific Northwest Mesic Forest Ecosystem: Hypothesis Testing Within a Comparative Phylogeographic Framework

Abstract

We examine the evolution of mesic forest ecosystems in the Pacific Northwest of North America using a statistical phylogeography approach in four animal and two plant lineages. Three a priori hypotheses, which explain the disjunction in the mesic forest ecosystem with either recent dispersal or ancient vicariance, are tested with phylogenetic and coalescent methods. We find strong support in three amphibian lineages (Ascaphus spp., and Dicampton spp., and Plethodon vandykei and P. idahoensis) for deep divergence between coastal and inland populations, as predicted by the ancient vicariance hypothesis. Unlike the amphibians, the disjunction in other Pacific Northwest lineages is likely due to recent dispersal along a northern route. Topological and population divergence tests support the northern dispersal hypothesis in the water vole (Microtus richardsoni) and northern dispersal has some support in both the dusky willow (Salix melanopsis) and whitebark pine (Pinus albicaulis). These analyses demonstrate that genetic data sampled from across an ecosystem can provide insight into the evolution of ecological communities and suggest that the advantages of a statistical phylogeographic approach are most pronounced in comparisons across multiple taxa in a particular ecosystem. Genetic patterns in organisms as diverse as willows and salamanders can be used to test general regional hypotheses, providing a consistent metric for comparison among members of an ecosystem with disparate life-history traits.

Identifying cryptic diversity with predictive phylogeography

“…The alternative hypotheses deny the persistence of inland Pleistocene refugia, instead positing the inland dispersal of rainforest taxa after the Pleistocene (electronic supplementary material, table S1). Because the latter invoke the recent ( post-Pleistocene) establishment of inland taxa, these recent dispersal models predict a lack of cryptic diversity, which has been shown in some studied taxa [33]. (ii) Arid lands of Southwestern North America…”

“…This ecosystem is rich in endemic species, and includes at least 150 plant, animal and fungal species that are disjunct with over 300 km of arid Columbia Basin shrub-steppe between the coastal and inland portions of its distribution [30 -32]. Because of the degree of isolation between these disjunct regions, conspecific populations from coastal and inland habitats have received much attention [33,34], with studies demonstrating that the system harbours substantial cryptic diversity (e.g. [35], reviewed in [36]).…”

“…The first hypothesis predicts high cryptic diversity, because of the hypothesized old age of the disjunction [37], and indeed, phylogeographic investigations have revealed cryptic diversity in taxa that were originally described as single, disjunct species (e.g. [33,38]; see electronic supplementary material, table S1). The alternative hypotheses deny the persistence of inland Pleistocene refugia, instead positing the inland dispersal of rainforest taxa after the Pleistocene (electronic supplementary material, table S1).…”

Glacial refugia and modern genetic diversity of 22 western North American tree species

“…Modern genetic population structure can be screened comprehensively with a moderate research effort, but evolutionary events inferred from genetic markers cannot be linked to a specific time and location in the past. Nevertheless, phylogeographic studies allow formulating and testing hypotheses about refugia and migration routes, summarized in several review papers for North America [5][6][7][8].…”

“…Pinus contorta (lodgepole pine), Pinus ponderosa (ponderosa pine), Pseudotsuga menziesii (Douglas fir). These could have emerged through east-west separation of populations by the coastal Cascade Mountains throughout cycles of glaciations and de-glaciation [8][9][10]. On the other hand, many temperate tree species of the Pacific northwest with disjunct coastal and interior populations show little or no genetic differentiation, e.g.…”