Hypothesis-driven biogeography to estimate rates and dates when fossil evidence is scarce: a case study using an ancient, Holarctic genus of spider (Araneae, Antrodiaetidae, Antrodiaetus)

Understanding when lineages diverged is a common inquiry within the field of biogeography and often requires a rate of evolution and a means to calibrate the phylogeny to absolute time. While fossil evidence is the most common means regarding calibration, estimated molecular rates specific to the group in question are not as widespread. Furthermore, geological events can also be employed to calibrate a phylogeny, especially when lineages have a poor fossil record. In this paper, the effectiveness of employing biogeographic hypotheses to overcome a depauperate fossil record is tested in a group of spiders whose evolution is strongly affected by abiotic forces. Using a known biogeographic break, the fit of two competing biogeographic hypotheses (Ancient Vicariance vs. Inland Dispersal) is tested as calibration points for estimating an evolutionary rate and dates elsewhere across the tree. An optimized dataset of ultraconserved element (UCE) phylogenomic loci, which have been shown to be highly informative in mygalomorph spider systematics (i.e., tarantulas and trapdoor spiders), is employed. Hypothesis testing strongly supports Ancient Vicariance over Inland Dispersal, and phylogenetic inferences recover topological patterns predicted by Ancient Vicariance. Significantly, subsequent modeling proposes a novel estimate of ancient vicariance older than that of any other system observed across this biogeographic break. Finally, the modeling proposes divergence estimates among other Antrodiaetus species that correlate with previous hypotheses. This research broadens knowledge of mygalomorph evolutionary rates, answers questions about the evolutionary history of a relatively old lineage of folding-door spiders, Antrodiaetus (Antrodiaetidae, Mygalomorphae), and proposes a novel hypothesis for divergence times in the Pacific Northwest. The authors estimate a novel evolutionary rate for atypoid spiders. The authors demonstrate the use of a biogeographic calibration point in a testable fashion for divergence dating. The authors identify a surprising ancient divergence in the Pacific Northwest that is older than other divergences observed across a known biogeographic break. The authors find additional evidence of the impact of emerging grassland habitat on promoting the divergence of Nearctic lineages.