Sky Island Populations Research Articles

Regional differences in stem form between southern and northern red spruce (Picea rubens Sarg.) populations

Abstract Red spruce (Picea rubens Sarg.) is a commercially and ecologically important conifer species that primarily exists at northern latitudes of eastern North America and extends southward following the Appalachian Mountains into North Carolina and Tennessee. Due to a warming climate and human-caused disturbances, only fragmented, sky-island populations remain at the highest peaks of the southern Appalachians where their habitat continues to be threatened. While they have been recognized for the rare wildlife habitat they provide in the region, these populations remain understudied. This work aimed to examine differences in stem form between the northern and southern populations of red spruce and to provide additional quantitative methods for managing red spruce stands through providing regionally fitted stem taper models. First, we examined differences in stem form using two methods: a sectional rate of change in diameter and a region variable added to the Kozak (2004) Model 02 taper model. The sectional taper comparison showed significant differences (P < .05) in taper rate throughout the stem that were most pronounced below breast height and above the midpoint. The nested model comparison also showed a significant difference after performing a likelihood ratio test. These results agreed that significant differences in stem form between the two populations exist and supported the idea that localized taper models would provide the best results. Next, we evaluated four stem taper models for their ability to predict upper stem diameters and total volume in southern Appalachian red spruce: a quadratic polynomial, a segmented, a variable exponent, and a geometric model. Fitting data came from a 1992 study across three southern Appalachian spruce-fir sites. Validation data came from a new dataset collected in 2022 at Unaka Mountain, Tennessee. Based on past studies and our results, we found that the Kozak (My last words on taper equations. For Chron 2004;80:507–15. https://doi.org/10.5558/tfc80507-4.) Model 02 variable exponent and the Max and Burkhart (Segmented polynomial regression applied to taper equations. For Sci 1976;22:283–9.) segmented polynomial models performed equally well. The choice of the final model should depend on the users’ objectives and practical limitations (i.e. programming ability, availability of fitting data, error tolerance).

Reference genome of the rubber boa, Charina bottae (Serpentes: Boidae).

The rubber boa, Charina bottae is a semi-fossorial, cold-temperature adapted snake that ranges across the wetter and cooler ecoregions of the California Floristic Province. The rubber boa is 1 of 2 species in the family Boidae native to California and currently has 2 recognized subspecies, the Northern rubber boa C. bottae bottae and the Southern rubber boa C. bottae umbratica. Recent genomic work on C. bottae indicates that these 2 subspecies are collectively composed of 4 divergent lineages that separated during the late Miocene. Analysis of habitat suitability indicates that C. bottae umbratica montane sky-island populations from southern California will lose the majority of their habit over the next 70 yr, and is listed as Threatened under the California Endangered Species Act. Here, we report a new, chromosome-level assembly of C. bottae bottae as part of the California Conservation Genomics Project (CCGP). Consistent with the reference genome strategy of the CCGP, we used Pacific Biosciences HiFi long reads and Hi-C chromatin-proximity sequencing technology to produce a de novo assembled genome. The assembly comprises 289 scaffolds covering 1,804,944,895 bp, has a contig N50 of 37.3 Mb, a scaffold N50 of 97 Mb, and BUSCO completeness score of 96.3%, and represents the first reference genome for the Boidae snake family. This genome will enable studies of genetic differentiation and connectivity among C. bottae bottae and C. bottae umbratica populations across California and help manage locally endemic lineages as they confront challenges from human-induced climate warming, droughts, and wildfires across California.

Genomic data reveal local endemism in Southern California Rubber Boas (Serpentes: Boidae, Charina) and the critical need for enhanced conservation actions

The mountains of southern California represent unique, isolated ecosystems that support distinct high-elevation habitats found nowhere else in the area. Analyses of several moisture-dependent species across these sky-islands indicate they exist as locally endemic lineages that occur across these fragmented mountains ranges. The Rubber Boa is a semi-fossorial snake species that is widely distributed in the cooler and more moist ecoregions regions of western North America, including isolated populations across southern California mountain ranges. We developed a genomic and ecological dataset to examine genetic diversity within Rubber Boas and to determine if the endemic Southern Rubber Boa represents a distinct lineage. We quantified current and future habitat suitability under a range of climate change scenarios, and discuss the possible environmental threats facing these unique montane isolates. Our results support four major lineages within Rubber Boas, with genetic breaks that are consistent with biogeographic boundaries observed in other co-distributed, cool-temperature, moisture adapted species. Our data support previous studies that the Southern Rubber Boa is an independent evolutionary unit and now includes multiple locally endemic sky-island populations, restricted to isolated mountain tops and ranges across southern California. Analyses of future habitat suitability indicate that many of these sky-island populations will lose most of their suitable habitat over the next 70 years given predicted increases in drought, rising temperatures, and wildfires. Collectively these data emphasize the critical conservation needs of these montane ecosystems in southern California under current and projected climate change conditions.

Strict allopatric speciation of sky island Pyrrhula erythaca species complex

Increasing evidence of post-divergence gene flow between taxa is shifting our understanding on the mode of speciation. A fundamental question arises concerning the circumstances under which strict allopatric speciation still holds true. Sky island populations might undergo reduced gene flow by niche conservatism to highland habitats and follow divergence in an allopatric manner. In this study, we tested this hypothesis in the sky island Grey-headed Bullfinch (Pyrrhula erythaca) species complex via statistical analyses of both genetic and ecological data. Results of coalescent-based analysis of multiple nuclear loci suggested that P. e. owstoni likely colonized Taiwan island during the severe mid-Pleistocene glacial climate followed by strictly allopatric divergence from P. e. erythaca distributed in Himalayas-Hengduan mountains and central North China. Results of ecological niche modeling suggested that their speciation may be attributed to the niche conservatism of these birds and the lack of a suitable ecological corridor during subsequent milder glacial episodes. In addition, we delimited the traditionally defined P. erythaca into two full species, P. erythaca in the Asian mainland and P. owstoni on the island of Taiwan, based on both genetic and behavioural evidences. These results suggest that ecology can have a dynamic role in allowing highland populations to expand their ranges and isolated by habitat barriers to diversify in a strictly allopatric manner.

Isolation by elevation: mitochondrial divergence among sky island populations of Sacramento Mountain salamander (Aneides hardii)

The Sacramento Mountain salamander (Aneides hardii) is a fully terrestrial plethodontid endemic to mountains in south-central New Mexico. This species is of conservation concern but there is scant knowledge regarding the degree of genetic divergence among populations. This information is vital for developing an appropriate species management strategy. To address this issue, we generated sequence data from the mitochondrial cytochrome b gene and four nuclear loci to explore population demographic history. Cytochrome b data revealed three deeply divergent (2.3–2.8%) mitochondrial lineages corresponding to distinct mountains (Capitan, White, and Sacramento). Divergence dates suggest separation since the early Pleistocene, and signatures of population expansion in the mid to late Pleistocene. Of 25 haplotypes identified, none were shared among mountains, but genetic diversity differed among mitochondrial lineages and suggested different demographic histories within lineages. A single microsatellite locus identified private alleles in each of the three lineages. Other nuclear loci screened were invariant within and among populations, which corresponds to a lower mutation rate compared with mtDNA. Together these results indicate that each mountain population of A. hardii represents a demographically and genetically distinct management unit.

Genomic phylogeography of the endemic Mountain Black‐eye of Borneo (Chlorocharis emiliae): montane and lowland populations differ in patterns of Pleistocene diversification

AbstractAimOur understanding of population diversification in the lowlands of Sundaland has improved substantially over the last 20 years through phylogeographical study, but we know almost nothing about population diversification in the mountains of the region. Here, we apply genomic analysis and habitat modelling to the phylogeography of Chlorocharis emiliae, the Mountain Black‐eye, an endemic montane bird of Borneo with a sky‐island distribution, to investigate the structure and interconnectivity of its populations. In the process, we consider factors driving population diversification in the mountains versus lowlands on the island, and how population structure of C. emiliae compares with structure of previously studied lowland species.LocationBorneo.MethodsUsing RAD‐seq, we produced thousands of SNPs in 25 individuals from five sky‐island populations of C. emiliae. These populations represent the species entire range across Borneo. Genetic structure and species tree analyses were applied to measure population relationships and connectivity. Ecological niche modelling was used to estimate habitat distributions during current and LGM time periods.ResultsWe identified slight to moderate genetic distinctiveness among all populations. Based on demographic models, isolation with migration was the main pattern of divergence, and the most admixture occurred among three populations in north‐eastern Borneo: Kinabalu, Trus Madi and Murud. The most divergent population, Pueh, in north‐western Borneo, has low genetic diversity and a small effective population size, is geographically isolated and has diverged in isolation without notable gene flow. These results, based on a large genomic dataset, contradict the evolutionary relationships identified in an earlier mitochondrial DNA study.Main conclusionsGenomic phylogeographical comparisons indicate that diversification among sky‐island populations of C. emiliae was driven largely by distance and historical habitat distribution, resulting in isolation but also some inter‐population gene flow. During Pleistocene glacial events, cooler temperatures would have caused montane forest to descend and spread, thereby increasing connectivity among sky‐island populations. However, despite increased montane habitat, the north‐western population of Pueh remained isolated. This montane pattern contrasts with phylogeographical patterns in the Bornean lowlands, where populations often display evidence of vicariance followed by secondary contact.

Low genetic variation and high differentiation across sky island populations of Lupinus alopecuroides (Fabaceae) in the northern Andes

The tropical alpine flora in the northern Andes has caught the attention of evolutionary biologists and conservationists because of the extent of its diversity and its vulnerability. Although population genetics studies are essential to understand how diversity arises and how it can be maintained, plant populations occurring above 4100 m a.s.l. in the so-called super-paramo have rarely been studied at the molecular level. Here, we use 11 microsatellite DNA markers to examine genetic structure in populations of Lupinus alopecuroides, a long-lived semelparous giant rosette known from only 10 geographically isolated populations. Each population is located on a different mountain top, of which three are in Colombia and seven in Ecuador. We analysed 220 individuals from all the ten known populations. We find low genetic variation in all but one of the populations. Four populations are completely monomorphic, and another five show only one polymorphic locus each. On the other hand, we find extremely high genetic differentiation between populations. We discuss the mechanisms that might cause this pattern, and we suggest that it is related to founder effects, lack of gene flow, and autogamy. The genetic relationships among the populations, and the lack of correlation between the genetic and geographic distances also point to the importance of founder effects and colonization history in driving differentiation among the populations.

A genomic investigation of the putative contact zone between divergent Brown Creeper (Certhia americana) lineages: chromosomal patterns of genetic differentiation

Sky islands, or montane forest separated by different lowland habitats, are highly fragmented regions that potentially limit gene flow between isolated populations. In the sky islands of the Madrean Archipelago (Arizona, USA), various taxa display different phylogeographic patterns, from unrestricted gene flow among sky islands to complex patterns with multiple distinct lineages. Using genomic-level approaches allows the investigation of differential patterns of gene flow, selection, and genetic differentiation among chromosomes and specific genomic regions between sky island populations. Here, we used thousands of SNPs to investigate the putative contact zone of divergent Brown Creeper (Certhia americana) lineages in the Madrean Archipelago sky islands. We found the two lineages to be completely allopatric (during the breeding season) with a lack of hybridization and gene flow between lineages and no genetic structure among sky islands within lineages. Additionally, the two lineages inhabit different climatic and ecosystem conditions and have many local primary song dialects in the southern Arizona mountain ranges. We identified a positive relationship between genetic differentiation and chromosome size, but the sex chromosome (Z) was not found to be an outlier. Differential patterns of genetic differentiation per chromosome may be explained by genetic drift--possibly in conjunction with non-random mating and non-random gene flow--due to variance in recombination rates among chromosomes.

Isolation by environment in White-breasted Nuthatches (Sitta carolinensis) of the Madrean Archipelago sky islands: a landscape genomics approach.

Understanding landscape processes driving patterns of population genetic differentiation and diversity has been a long-standing focus of ecology and evolutionary biology. Gene flow may be reduced by historical, ecological or geographic factors, resulting in patterns of isolation by distance (IBD) or isolation by environment (IBE). Although IBE has been found in many natural systems, most studies investigating patterns of IBD and IBE in nature have used anonymous neutral genetic markers, precluding inference of selection mechanisms or identification of genes potentially under selection. Using landscape genomics, the simultaneous study of genomic and ecological landscapes, we investigated the processes driving population genetic patterns of White-breasted Nuthatches (Sitta carolinensis) in sky islands (montane forest habitat islands) of the Madrean Archipelago. Using more than 4000 single nucleotide polymorphisms and multiple tests to investigate the relationship between genetic differentiation and geographic or ecological distance, we identified IBE, and a lack of IBD, among sky island populations of S.carolinensis. Using three tests to identify selection, we found 79 loci putatively under selection; of these, seven matched CDS regions in the Zebra Finch. The loci under selection were highly associated with climate extremes (maximum temperature of warmest month and minimum precipitation of driest month). These results provide evidence for IBE - disentangled from IBD - in sky island vertebrates and identify potential adaptive genetic variation.

Population genetic structure of Iris ensata on sky-islands and its implications for assisted migration

Due to global warming since the Last Glacial Maximum, many plant populations have retreated to mountain tops, i.e., sky-islands, which are isolated by low-elevation barriers and inhospitable habitats. Under projected environmental changes, these populations may decline and face local extinction. Iris ensata Thunb. populations at the species’ most southern distribution range are in this position. We used eight polymorphic nuclear microsatellite markers and three fragments of chloroplast genome to analyze population structure of six extant sky-island populations of I. ensata. A total of 83 alleles were found across 192 individuals from six populations. High levels of intra-population genetic diversity (HE = 0.578, HO = 0.608) of I. ensata were detected. Moderate but significant levels of genetic differentiation were also found among the populations (FST = 0.133, P < 0.001). Mantel test showed no isolation-by-distance pattern (r = 0.339, P = 0.161). Assignment analysis classified all individuals into five groups, and four populations were dominated by only one group. Five chloroplast DNA (cpDNA) haplotypes were found, and one was shared by all populations. Two populations contained a private haplotype. Long-term fragmentation, but relatively large population sizes and restricted gene flow among populations, contributed to the above patterns. Under projected habitat changes, the studied populations are at risk of local extinction. We identified three populations that had a high priority of ex situ conservation to be the source populations for the future assisted migration. Number of individuals, how to select individuals, and a potential recipient site were discussed.

Continent-scale genetic structure in a boreal forest migrant, the Blackpoll Warbler (Setophaga striata)

ABSTRACT. We describe the range-wide phylogeography of Blackpoll Warblers (Setophaga striata), a migratory passerine with a broad breeding range in North America's boreal forest that encompasses several possible biogeographic barriers but shows no phenotypic geographic variation. We used mitochondrial control-region sequences from 304 individuals in combination with ecological niche models and coalescent simulations to test alternative historical hypotheses about the number of Pleistocene refugial populations and divergences among modern populations. Population pairwise FSTand spatial analyses of molecular variance suggested significant genetic structure among western, eastern, and Newfoundland populations, but no structure among sky-island populations at the southeastern periphery of the breeding range. Inferred gene flow fits a model of isolation-by-distance. Coalescent simulations rejected all multiple-refugia hypotheses in favor of a single refugium. Paleodistribution models and modern migratory pathw...

The evolution and age of populations of Scaphinotus petersi Roeschke on Arizona Sky Islands (Coleoptera, Carabidae, Cychrini).

Populations of the ground beetle Scaphinotus petersi are isolated in subalpine conifer forest habitats on mountain ranges or Sky Islands in southeastern Arizona. Previous work on this species has suggested these populations have been isolated since the last post-glacial maximum times as warming caused this cool adapted species to retreat to high elevations. To test this hypothesis, we inferred the phylogeny from mitochondrial DNA sequence data from several Arizona Sky Island populations of Scaphinotus petersi and estimated the divergence time of the currently isolated populations. We found two major clades of Scaphinotus petersi, an eastern clade and a western group. Our results indicated most mountain ranges form clades except the Huachucas, which are polyphyletic and the Santa Catalinas, which are paraphyletic. We estimated the Pinaleño population is much older than the last glacial maximum, but the Huachuca and Pinal populations may have been fragmented from the Santa Catalina population since the post-glacial maximum times.

Integrating paleoecology and genetics of bird populations in two sky island archipelagos

BackgroundGenetic tests of paleoecological hypotheses have been rare, partly because recent genetic divergence is difficult to detect and time. According to fossil plant data, continuous woodland in the southwestern USA and northern Mexico became fragmented during the last 10,000 years, as warming caused cool-adapted species to retreat to high elevations. Most genetic studies of resulting 'sky islands' have either failed to detect recent divergence or have found discordant evidence for ancient divergence. We test this paleoecological hypothesis for the region with intraspecific mitochondrial DNA and microsatellite data from sky-island populations of a sedentary bird, the Mexican jay (Aphelocoma ultramarina). We predicted that populations on different sky islands would share common, ancestral alleles that existed during the last glaciation, but that populations on each sky island, owing to their isolation, would contain unique variants of postglacial origin. We also predicted that divergence times estimated from corrected genetic distance and a coalescence model would post-date the last glacial maximum.ResultsOur results provide multiple independent lines of support for postglacial divergence, with the predicted pattern of shared and unique mitochondrial DNA haplotypes appearing in two independent sky-island archipelagos, and most estimates of divergence time based on corrected genetic distance post-dating the last glacial maximum. Likewise, an isolation model based on multilocus gene coalescence indicated postglacial divergence of five pairs of sky islands. In contrast to their similar recent histories, the two archipelagos had dissimilar historical patterns in that sky islands in Arizona showed evidence for older divergence, suggesting different responses to the last glaciation.ConclusionThis study is one of the first to provide explicit support from genetic data for a postglacial divergence scenario predicted by one of the best paleoecological records in the world. Our results demonstrate that sky islands act as generators of genetic diversity at both recent and historical timescales and underscore the importance of thorough sampling and the use of loci with fast mutation rates to studies that test hypotheses concerning recent genetic divergence.

ESTIMATING A GEOGRAPHICALLY EXPLICIT MODEL OF POPULATION DIVERGENCE

Patterns of genetic variation can provide valuable insights for deciphering the relative roles of different evolutionary processes in species differentiation. However, population-genetic models for studying divergence in geographically structured species are generally lacking. Since these are the biogeographic settings where genetic drift is expected to predominate, not only are population-genetic tests of hypotheses in geographically structured species constrained, but generalizations about the evolutionary processes that promote species divergence may also be potentially biased. Here we estimate a population-divergence model in montane grasshoppers from the sky islands of the Rocky Mountains. Because this region was directly impacted by Pleistocene glaciation, both the displacement into glacial refugia and recolonization of montane habitats may contribute to differentiation. Building on the tradition of using information from the genealogical relationships of alleles to infer the geography of divergence, here the additional consideration of the process of gene-lineage sorting is used to obtain a quantitative estimate of population relationships and historical associations (i.e., a population tree) from the gene trees of five anonymous nuclear loci and one mitochondrial locus in the broadly distributed species Melanoplus oregonensis. Three different approaches are used to estimate a model of population divergence; this comparison allows us to evaluate specific methodological assumptions that influence the estimated history of divergence. A model of population divergence was identified that significantly fits the data better compared to the other approaches, based on per-site likelihood scores of the multiple loci, and that provides clues about how divergence proceeded in M. oregonensis during the dynamic Pleistocene. Unlike the approaches that either considered only the most recent coalescence (i.e., information from a single individual per population) or did not consider the pattern of coalescence in the gene genealogies, the population-divergence model that best fits the data was estimated by considering the pattern of gene lineage coalescence across multiple individuals, as well as loci. These results indicate that sampling of multiple individuals per population is critical to obtaining an accurate estimate of the history of divergence so that the signal of common ancestry can be separated from the confounding influence of gene flow-even though estimates suggest that gene flow is not a predominant factor structuring patterns of genetic variation across these sky island populations. They also suggest that the gene genealogies contain information about population relationships, despite the lack of complete sorting of gene lineages. What emerges from the analyses is a model of population divergence that incorporates both contemporary distributions and historical associations, and shows a latitudinal and regional structuring of populations reminiscent of population displacements into multiple glacial refugia. Because the population-divergence model itself is built upon the specific events shaping the history of M. oregonensis, it provides a framework for estimating additional population-genetic parameters relevant to understanding the processes governing differentiation in geographically structured species and avoids the problems of relying on overly simplified and inaccurate divergence models. The utility of these approaches, as well as the caveats and future improvements, for estimating population relationships and historical associations relevant to genetic analyses of geographically structured species are discussed.

REGIONAL SEISMIC SONG DIFFERENCES IN SKY ISLAND POPULATIONS OF THE JUMPING SPIDER HABRONATTUS PUGILLIS GRISWOLD (ARANEAE, SALTICIDAE)

Jumping spiders have long been used as model organisms to study visual communication. However, recent studies documenting the presence of intricate multicomponent seismic songs during court- ship displays suggest an important role of seismic communication as well. Given the relatively recent focus on seismic communication, the extent to which seismic songs vary among jumping spider species or even among populations remains poorly understood. Here, we use the extensively studied Habronattus pugillis Griswold 1987 complex to explore putative seismic song diversity among males from isolated populations. H. pugillis populations have been studied extensively because of the tremendous diversifi- cation of male visual secondary sexual ornaments observed among adjacent mountain-top populations in southeastern Arizona (''sky islands''). Here, we aim to explore putative parallel patterns of diversification in seismic courtship songs between different sky island populations. Using laser vibrometry, we examined in detail the songs of three mountaintop populations (Atascosa (AT), Santa Rita (SR), and Santa Catalina (SC)) and observed an extraordinary diversity of songs and song types among these three populations. Large differences were seen in both the temporal and spectral properties of male seismic songs. In addition, we observed differences in song complexity between populations with some populations having ''simple'' songs (1 component) and others having ''complex'' songs (3 components). We also present preliminary data from four additional populations (Galiuro (GA), Huachuca (HU), Mule (MU), and Patagonia (PA)). Results from this study suggest that the diversification of male visual signals observed among populations of H. pugillis finds a counterpart in male seismic songs

Geographic isolation, genetic divergence, and ecological non-exchangeability define ESUs in a threatened sky-island rattlesnake

We utilized nine microsatellite loci and 865 bases from two mtDNA genes to estimate demographic parameters and visualize historic/contemporary connectivity among populations of a sky-island rattlesnake (New Mexico ridge-nosed rattlesnake, Crotalus willardi obscurus). This taxon is listed as threatened under the United States Endangered Species Act (ESA) and is distributed patchily within three borderland mountain ranges [Animas (ANM), Peloncillo (PEL), Sierra San Luis (SSL)] of southeastern Arizona, southwestern New Mexico and north-central México. Molecular data support a hypothesis of northward range expansion from México, with subsequent isolation on sky-islands through vicariant desertification that transformed intervening wooded valleys and low elevation passes into inhospitable habitat. Historic and recent movements have been within rather than among mountains. Although populations are linked via ancestral polymorphism, they do not represent a single mtDNA gene pool. All are genetically bottlenecked, with the Peloncillo reflecting deepest/sharpest declines and fewest captures per unit effort. Most recent population declines occurred ∼4 kybp (thousands of years before present). Thus, population reductions are historic and environmental rather than contemporary and anthropogenic. Our data demonstrate that PEL is ecologically non-exchangable with other sky-island populations, and thus comprises one ‘evolutionary significant unit’ (ESU), while SSL and ANM comprise ‘management units’ (MUs) within a second ESU. All three meet legal criteria for recognition as ‘distinct population segments’ (DPS) under the ESA.

EVOLUTIONARY DYNAMICS OF A SPATIALLY STRUCTURED HOST-PARASITE ASSOCIATION: DROSOPHILA INNUBILA AND MALE-KILLING WOLBACHIA

The mode and tempo of host-parasite evolution depend on population structure and history and the strength of selection that the species exert on each other. Here we genetically and epidemiologically characterize populations of the mycophagous fly Drosophila innubila and its male-killing Wolbachia endosymbiont, with the aim of integrating the local through global nature of this association. Drosophila innubila inhabit the forested "sky island" regions of the of the southwestern United States and northern Mexico, where its distribution is highly fragmented. We examine geographically isolated sky island populations of D. innubila, surveying the frequency and expression of Wolbachia infection as well as the distribution of genetic variation within and among populations of the host and parasite. In all populations, Wolbachia infection is associated with virtually complete male-killing, thus providing no evidence for the evolution of population-specific interaction phenotypes or local resistance. Although Wolbachia infection occurs in each of the main populations, there is variation among populations in the prevalence of infection and the resulting population-level sex ratio of D. innubila. Among these populations, the nuclear genes of D. innubila show moderate, though significant, differentiation. In contrast, the host mitochondrial DNA (mtDNA), which shares transmission with Wolbachia, exhibits substantially greater geographic differentiation, even after accounting for differences in transmission between nuclear and mitochondrial genes. We suggest that this pattern is caused by local Wolbachia--but not D. innubila--fluctuations in prevalence that increase the severity of drift experienced only by the mtDNA. Overall, our data suggest that the association between D. innubila and male-killing Wolbachia is ecologically dynamic within local populations, but evolutionarily coherent across the species as a whole.

Did the pleistocene glaciations promote divergence? Tests of explicit refugial models in montane grasshopprers.

There is a long-standing debate over whether or not the Pleistocene glaciations promoted speciation. While some models predict that extensive mixing of populations during interglacial expansion would have inhibited divergence, others postulate that divergence among allopatric glacial refuges or founder events during recolonization of previously glaciated areas would have promoted differentiation. Using a combination of traditional and coalescent based population genetic approaches, this study finds that the glaciations did not inhibit divergence among populations of the grasshopper Melanoplus oregonensis. Instead, drift associated with recolonization of previously glaciated areas, as well as divergence among multiple allopatric glacial refugia, have both contributed to differentiation in this montane grasshopper from the 'sky islands' of the northern Rocky Mountains. Significant population structure was detected by phylogenetic and FST analyses, including significant FST values among individual pairs of sky-island populations. In addition to clustering of haplotypes within populations, there is some evidence of regional phylogeographic structure, although none of the 'regional groups' form a monophyletic clade and there is a lack of concordance between the genealogical and geographical positions of some haplotypes. However, coalescent simulations confirm there is significant regional phylogeographic structure that most likely reflects divergence among multiple ancestral refugial populations, and indicate that it is very unlikely that the observed gene tree could have been produced by the fragmentation of a single widespread ancestral population. Thus, rather than inhibiting differentiation, the glaciations appear to have promoted population divergence in M. oregonensis, suggesting that they may have contributed to the radiation of Melanoplus species during the Pleistocene.