What a Long Strange Trip It's Been: Miocene Salinian Block Migration and the Biogeography of Megahexura Spiders

Aim In west‐central California, the landscapes west of the San Andreas fault include the arc plutons of the Salinian block which form the geologic backbone of several South Coast mountain ranges. In the early Miocene Salinia was adjacent to the southern Sierra Nevada, but was subsequently captured by the Pacific Plate, with right‐lateral slip migration displacing terranes northwestwards since the formation of the San Andreas fault. We hypothesise that a migrating Salinia has played an important role in the biogeographic history of dispersal‐limited Megahexura mygalomorph spiders. Location Central and southern California Floristic Province, including Salinia (Santa Lucia, Sierra de Salinas, Gabilan, and Santa Cruz mountains), southern Sierra Nevada, and Transverse Ranges. Taxon Megahexura , Family Megahexuridae, Infraorder Mygalomorphae, Order Araneae. Methods Analyses of ultraconserved element (UCE) nuclear loci and single nucleotide polymorphism (SNP) data are used to estimate ingroup relationships and genetic population structuring, root placement and divergence timing, and a Bayesian‐estimated biogeographic history of Megahexura . Results Phylogenomics strongly supports three primary phylogenetic lineages in Megahexura , with a common ancestor that diverged in the early to mid‐Miocene, depending upon calibration parameters. The southern Sierra Nevada is reconstructed as the centre of origin for Megahexura , and one Megahexura lineage now resides mostly on ranges that include Salinian basement rocks. In the Santa Lucia Range, phylogenetic directionality is structured from north to south. The southern Sierra Nevada is also a more recent ancestral area for Megahexura populations currently occupying the western and southern Transverse Ranges, some of which have expanded northwestwards to secondarily contact Salinian populations. Main Conclusions We uncover a unique biogeographic history where instead of animal populations dispersing recently over an old landscape, phylogenomic patterns are best explained by the landscape itself moving, carrying animal populations. Our results are consistent with a compelling Salinian biogeography and represent a possible case of Neogene divergence driven principally by plate tectonics.