Throughout the Quaternary Period, the Pacific Northwest of North America (PNW) experienced dramatic shifts in climate due to the repeated expansion and retreat of glaciers (Shafer et al., 2010). At the peak of the most recent glacial expanse, known as the Last Glacial Maximum (LGM), average temperatures in the PNW were substantially colder than present-day (Otto-Bliesner & Brady, 2006), and massive ice sheets rendered large regions of land inhospitable for many species (Pielou, 2008). After the LGM, conditions in the PNW

Table 1. Species used in analysis. For each species, the scientific name, type of organism, type of data, number of sequences, and reference of original publication is shown. SpeciesBroad Taxon Type of Data # sequences Original publication Bryopsis sp. Green Algae cpDNA 66 Krellwitz et al. (2001) Gracilaria tikvahiae Red Algae cpDNA 20 Gurgel et al.(2004) Xerula furfuracea Fungi nuDNA 41 Yang et al.(2009) & Petersen and Hughes (2010) & Hao et al.(2016) Sphagnum bartlettianum Bryophyta cpDNA + nuDNA 12 Shaw et al.(2005) Acer rubrum Angiosperm cpDNA 38 McLachlan et al.(2005) Apios americana Angiosperm nuDNA 18 Joly & Bruneau (2004) Dicerandra spp Angiosperm cpDNA 30 Oliveira et al.(2007) Fagus grandifolia Angiosperm cpDNA 23 McLachlan et al.(2005) Liquidambar styraciflua Angiosperm cpDNA 109 Morris et al.(2008) Prunus spp Angiosperm cpDNA 226 Shaw & Small (2005) Tilia americana Angiosperm cpDNA 297 McCarthy and Mason-Gamer (2016) Trillium cuneatum Angiosperm cpDNA 281 Gonzales et al.(2008) Uniola paniculata Angiosperm cpDNA 131 Hodel & Gonzales (2013) Bugula neritina Bryozoa mtDNA 30 McGovern & Hellberg (2003) Daphnia obtusa Crustacean mtDNA 36 Penton et al.(2004) Emerita talpoida Crustacean mtDNA 4 Tam et al.(1996) Farfantepenaeus aztecus Crustacean mtDNA 76 McMillen-Jackson and Bert (2003) Litopenaeus setiferus Crustacean mtDNA 92 McMillen-Jackson and Bert (2003) & Maggioni et al. (2001) &Vazquez-Bader et al.(2004) & Bremer et al.(2010) Pagarus longicarpus Crustacean mtDNA 67 Young et al.(2002) Pagarus pollicaris Crustacean mtDNA 13 Young et al.(2002) Busycon sinistrum Gastropod mtDNA 31 Wise et al.(2004) Lampsilis altilis Mollusk mtDNA 5 Roe et al.(2001) Lampsilis australis Mollusk mtDNA 5 Roe et al.(2001) Lampsilis ovata Mollusk mtDNA 2 Roe et al.(2001) & Campbell et al.(2005) Lampsilis perovalis Mollusk mtDNA 5 Roe et al.(2001) Lampsilis teres Mollusk mtDNA 2 Roe et al.(2001) & Lydeard et al.(2000) Spisula solidissima Mollusk mtDNA 52 Hare and Weinberg (2005) Ambystoma tigrinum Amphibian mtDNA 56 Church et al.(2003) Desmognathus wrightii Amphibian mtDNA 29 Crespi et al.(2003) Eumeces fasciatus Amphibian mtDNA 82 Howes et al.(2006) Eurycea bislineata Amphibian mtDNA 56 Kozak et al.(2006) Eurycea cirrigera Amphibian mtDNA 251 Kozak et al.(2006) Eurycea junaluska Amphibian mtDNA 6 Kozak et al.(2006) Eurycea multiplicata Amphibian mtDNA 46 Bonett & Chippindale (2004) Eurycea tymerensis Amphibian mtDNA 16 Bonett & Chippindale (2004) Eurycea wilderae Amphibian mtDNA 129 Kozak et al.(2006)

Premise: Hybridization is increasingly being identified in the genomes of species across the tree of life, leading to a general recognition that hybridization plays an important role in the generation of species diversity. While hybridization may increase species diversity directly via the formation of new taxa through hybrid speciation, it may also act indirectly via the exchange of phenotypic and genetic variance between species, which may in turn stimulate future speciation events. Methods: Using high-throughput sequence data, we resolved phylogenetic relationships and investigated the role of hybridization as a diversification mechanism in the shrubby beardtongues (Penstemon subgenus Dasanthera), a group of North American wildflowers that has undergone a recent and rapid adaptive radiation. Specifically, we tested four hypotheses of hybrid taxon formation resulting from hybridization between P. davidsonii and P. fruticosus. Results: Species tree inference supports the monophyly of subgenus Dasanthera and elucidates relationships between taxa distributed in the Cascades and Sierra Nevada Mountains. Results also provide evidence of gene flow between P. davidsonii and P. fruticosus and support at least one hybrid origin hypothesis (P. davidsonii var. menziesii) in a region of contemporary distributional overlap. Hybridization may have also been facilitated by historical overlap in geographic distribution caused by species' responses to climatic changes during the Pleistocene. Conclusions: Our results support a history of hybridization between focal taxa in a rapidly radiating clade of plants and more broadly contribute to our growing understanding of the role of hybridization as a diversification mechanism in plants.

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