Abstract
The sky islands of southeastern Arizona (AZ) mark a major transition zone between tropical and temperate biota and are considered a neglected biodiversity hotspot. Dispersal ability and host plant specificity are thought to impact the history and diversity of insect populations across the sky islands. We aimed to investigate the population structure and phylogeography of two pine‐feeding pierid butterflies, the pine white (Neophasia menapia) and the Mexican pine white (Neophasia terlooii), restricted to these “islands” at this transition zone. Given their dependence on pines as the larval hosts, we hypothesized that habitat connectivity affects population structure and is at least in part responsible for their allopatry. We sampled DNA from freshly collected butterflies from 17 sites in the sky islands and adjacent high‐elevation habitats and sequenced these samples using ddRADSeq. Up to 15,399 SNPs were discovered and analyzed in population genetic and phylogenetic contexts with Stacks and pyRAD pipelines. Low genetic differentiation in N. menapia suggests that it is panmictic. Conversely, there is strong evidence for population structure within N. terlooii. Each sky island likely contains a population of N. terlooii, and clustering is hierarchical, with populations on proximal mountains being more related to each other. The N. menapia habitat, which is largely contiguous, facilitates panmixia, while the N. terlooii habitat, restricted to the higher elevations on each sky island, creates distinct population structure. Phylogenetic results corroborate those from population genetic analyses. The historical climate‐driven fluxes in forest habitat connectivity have implications for understanding the biodiversity of fragmented habitats.
Highlights
The isolated mountain habitats of western North America provide a natural laboratory for investigating the evolutionary processes at work in taxa restricted to naturally or anthropogenically fragmented habitats
There is some overlap in the voltinism of these two species in AZ; N. menapia is univoltine, adults are active from mid‐July through August, and N. terlooii is bivoltine and is active in low numbers from June through early September with peak adult activity in October
Given the close and abrupt range disjunction in Neophasia, our main objective was to determine whether Neophasia species resid‐ ing in forested habitats of different ecoregions of the sky islands and surrounding area have similar concordant population structure
Summary
The isolated mountain habitats of western North America provide a natural laboratory for investigating the evolutionary processes at work in taxa restricted to naturally or anthropogenically fragmented habitats. Neophasia butterflies are an excellent system for investigating how the sky islands have shaped species history. These butterflies are coniferous forest specialists and their evolutionary history likely follows that of their forest habitats, which have been hypothesized to fluctuate in area and connectivity in association with climate changes (Thompson & Anderson, 2000). There is some overlap in the voltinism of these two species in AZ; N. menapia is univoltine, adults are active from mid‐July through August, and N. terlooii is bivoltine and is active in low numbers from June through early September with peak adult activity in October. Given the close and abrupt range disjunction in Neophasia, our main objective was to determine whether Neophasia species resid‐ ing in forested habitats of different ecoregions of the sky islands and surrounding area have similar concordant population structure. In addition to physiological constraints and climate differences be‐ tween habitats (Halbritter, Teets, Williams, & Daniels, 2018), the desert and grassland matrices separating the two species' mountain habitats are likely to be dispersal barriers contributing to allopatry
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