Phylogeographic structure in long-tailed voles (Rodentia: Arvicolinae) belies the complex Pleistocene history of isolation, divergence, and recolonization of Northwest North America's fauna

A precarious future for distinctive peripheral populations of meadow voles (Microtus pennsylvanicus)

Conservation efforts rely on robust taxonomic assessments that should be based on critical assessment of interspecific boundaries, infraspecific variation, and potentially distinctive peripheral populations. The meadow vole (Microtus pennsylvanicus) is widely distributed across North America, including 28 morphologically defined subspecies and numerous isolated populations. Because some subspecies are of high conservation concern, we examined geographic variation across the range of the species to test existing infraspecific taxonomy in terms of local and regional diversification. We sequenced mitochondrial DNA (mtDNA) from 20 subspecies of M. pennsylvanicus and contextualized infraspecific variation through comparison of pairwise genetic distances derived from an extended data set of 63 species of Microtus. We found strong support for at least three divergent clades within M. pennsylvanicus, with observed intraspecific clade divergence exceeding that between several pairwise comparisons of sister species within Microtus. Six nuclear genes were then sequenced to test the validity of mtDNA structure and to further evaluate the possibility of cryptic, species-level diversity using Bayes factor species delimitation (BFD) analyses. BFD consistently and decisively supported multiple species based on the multilocus approach. We propose that taxonomic revision of the meadow vole is required, with the eastern clade now identified as M. pennsylvanicus (Ord 1815), the western clade as M. drummondii (Audubon and Bachman 1853), and the coastal Florida clade as M. dukecampbelli (Woods, Post, and Kilpatrick 1982). We suggest that such an arrangement would more closely reflect evolutionary history and provide critical context for further examination of distinctive southern peripheral populations that harbor novel evolutionary legacies and adaptive potential.

Living on the edge: Exploring the role of coastal refugia in the Alexander Archipelago of Alaska

Although islands are of long-standing interest to biologists, only a handful of studies have investigated the role of climatic history in shaping evolutionary diversification in high-latitude archipelagos. In this study of the Alexander Archipelago (AA) of Southeast Alaska, we address the impact of glacial cycles on geographic genetic structure for three mammals co-distributed along the North Pacific Coast. We examined variation in mitochondrial and nuclear loci for long-tailed voles (Microtus longicaudus), northwestern deermice (Peromyscus keeni), and dusky shrews (Sorex monticola), and then tested hypotheses derived from Species Distribution Models, reconstructions of paleoshorelines, and island area and isolation. In all three species, we identified paleoendemic clades that likely originated in coastal refugia, a finding consistent with other paleoendemic lineages identified in the region such as ermine. Although there is spatial concordance at the regional level for endemism, finer scale spatial and temporal patterns are less clearly defined. Demographic expansion across the region for these distinctive clades is also evident and highlights the dynamic history of Late Quaternary contraction and expansion that characterizes high-latitude species.

Whole-genome analysis of Mustela erminea finds that pulsed hybridization impacts evolution at high latitudes

At high latitudes, climatic shifts hypothetically initiate recurrent episodes of divergence by isolating populations in glacial refugia—ice-free regions that enable terrestrial species persistence. Upon glacial recession, populations subsequently expand and often come into contact with other independently diverging populations, resulting in gene flow. To understand how recurrent periods of isolation and contact may have impacted evolution at high latitudes, we investigated introgression dynamics in the stoat (Mustela erminea), a Holarctic mammalian carnivore, using whole-genome sequences. We identify two spatio-temporally distinct episodes of introgression coincident with large-scale climatic shifts: contemporary introgression in a mainland contact zone and ancient contact ~200 km south of the contemporary zone, in the archipelagos along North America’s North Pacific Coast. Repeated episodes of gene flow highlight the central role of cyclic climates in structuring high-latitude diversity, through refugial divergence and introgressive hybridization. When introgression is followed by allopatric isolation (e.g., insularization) it may ultimately expedite divergence.

Jocelyn Colella et al. report whole-genome sequences of 10 stoats (Mustela erminea) from four regions of glacial refugia. They find evidence for two past introgressive events between lineages that coincide with interglacial periods, a pattern that may extend to other high–latitude species.