DNA sequence variation of the mitochondrial control region among geographically and morphologically remote European brown trout Saltno trutta populations

Throughout its natural range, the brown trout Salmo trutta L. exhibits a complex pattern of morphological and life-history variation. This has led to considerable taxonomic confusion, hampering the understanding of the evolutionary history of the species. To document the phylogenetic relationships among morphologically and geographically remote brown trout populations across western Europe, we determined the DNA sequence variation in segments of the mitochondrial control region for 151 individuals representing 24 populations. DNA was prepared for double-stranded sequencing by the polymerase chain reaction (PCR). Twenty-one variable nucleotide positions within a 640-bp fragment surveyed defined 12 genotypes differing by a mean of 7 nucleotide substitutions (range 1-12). Five major phylogenetic assemblages differing by mean sequence divergence estimates of 0.96 to 1.44% were identified. These groupings exhibited a strong spatial partitioning but lacked congruence with either ecological or morphological differentiation. Complete mitochondrial DNA (mtDNA) monomorphism across all Atlantic basin populations contrasted with the high interdrainage genetic diversity observed in more southerly populations. This study exemplified the usefulness of mitochondrial DNA sequence analysis for estimating phylogenetic relationships within S. trutta populations.     

High levels of population subdivision in a morphologically conserved Mediterranean toad (Alytes cisternasii) result from recent,multiple refugia: evidence from mtDNA,microsatellites and nuclear genealogies

Pleistocene glaciations often resulted in differentiation of taxa in southern European peninsulas, producing the high levels of endemism characteristic of these regions (e.g. the Iberian Peninsula). Despite their small ranges, endemic species often exhibit high levels of intraspecific differentiation as a result of a complex evolutionary history dominated by successive cycles of fragmentation, expansion and subsequent admixture of populations. Most evidence so far has come from the study of species with an Atlantic distribution in northwestern Iberia, and taxa restricted to Mediterranean‐type habitats remain poorly studied. The Iberian Midwife toad (Alytes cisternasii) is a morphologically conserved species endemic to southwestern and central Iberia and a typical inhabitant of Mediterranean habitats. Applying highly variable genetic markers from both mitochondrial and nuclear genomes to samples collected across the species’ range, we found evidence of high population subdivision within A. cisternasii. Mitochondrial haplotypes and microsatellites show geographically concordant patterns of genetic diversity, suggesting population fragmentation into several refugia during Pleistocene glaciations followed by subsequent events of geographical and demographic expansions with secondary contact. In addition, the absence of variation at the nuclear β‐fibint7 and Ppp3caint4 gene fragments suggests that populations of A. cisternasii have been recurrently affected by episodes of extinction and recolonization, and that documented patterns of population subdivision are the outcome of recent and multiple refugia. We discuss the evolutionary history of the species with particular interest in the increasing relevance of Mediterranean refugia for the survival of genetically differentiated populations during the Pleistocene glaciations as revealed by studies in co‐distributed taxa.     

Evaluation of experimental genetic management in reintroduced bighorn sheep

Positive demographic responses have been reported in several species where the immigration or supplementation of genetically distinct individuals into wild populations has resulted in a genetic rescue effect. However, rarely have researchers incorporated what could be considerable risk of outbreeding depression into planning for genetic management programs. We assess the genetic effects of an experiment in genetic management involving replicate populations of California bighorn sheep (Ovis canadensis californiana) in Oregon, USA, which previously experienced poor productivity and numerical declines. In the experiment, two declining populations were supplemented with ewes from a more genetically diverse population of California bighorn sheep in Nevada. We incorporated analysis of genetic samples representing both experimental populations prior to supplementation, samples from the supplemented individuals, and samples collected from both experimental populations approximately one generation after supplementation. We used genetic analyses to assess the integration of supplemented and resident populations by identifying interpopulation hybrids. Further, we incorporated demographic simulations to assess the risk of outbreeding depression as a result of the experimental augmentation. Finally, we used data from microsatellites and mitochondrial sequences to determine if genetic management increased genetic diversity in the experimental populations. Our analyses demonstrated the success of genetic management by documenting interpopulation hybrids, identifying no evidence for outbreeding depression as a result of contact between the genetically distinct supplemented and resident populations, and by identifying increased population-level metrics of genetic diversity in postsupplementation populations compared with presupplementation levels.     

Complex pattern of genetic structuring in the Atlantic salmon (Salmo salar L.) of the River Foyle system in northwest Ireland: disentangling the evolutionary signal from population stochasticity

Little is known about the microevolutionary processes shaping within river population genetic structure of aquatic organisms characterized by high levels of homing and spawning site fidelity. Using a microsatellite panel, we observed complex and highly significant levels of intrariver population genetic substructure and Isolation-by-Distance, in the Atlantic salmon stock of a large river system. Two evolutionary models have been considered explaining mechanisms promoting genetic substructuring in Atlantic salmon, the member-vagrant and metapopulation models. We show that both models can be simultaneously used to explain patterns and levels of population structuring within the Foyle system. We show that anthropogenic factors have had a large influence on contemporary population structure observed. In an analytical development, we found that the frequently used estimator of genetic differentiation, F(ST), routinely underestimated genetic differentiation by a factor three to four compared to the equivalent statistic Jost's D(est) (Jost 2008). These statistics also showed a near-perfect correlation. Despite ongoing discussions regarding the usefulness of "adjusted"F(ST) statistics, we argue that these could be useful to identify and quantify qualitative differences between populations, which are important from management and conservation perspectives as an indicator of existence of biologically significant variation among tributary populations or a warning of critical environmental damage.     

A TEST OF THE GLACIAL REFUGIUM HYPOTHESIS USING PATTERNS OF MITOCHONDRIAL AND NUCLEAR DNA SEQUENCE VARIATION IN ROCK PTARMIGAN (LAGOPUS MUTUS)

The glacial refugium hypothesis (GRH) proposes that glaciers promoted differentiation and generation of intraspecific diversity by isolating populations in ice-free refugia. We tested three predictions of this hypothesis for the evolutionary divergence of rock ptarmigan (Lagopus mutus) during the Wisconsin glaciation of the late Pleistocene. To do this, we examined subspecies distributions, population genetic structure, and phylogenetic relationships in 26 populations across North America and the Bering Sea region. First, we analyzed sequence variation in the mitochondrial control region, in a nuclear intron (Gapdh), and in an internal transcribed spacer (ITS1). Control region sequences of 154 rock ptarmigan revealed strong population and phylogeographic structure. Variation in intron sequences of 114 rock ptarmigan also revealed significant population structure compatible with results for the control region. Rock ptarmigan were invariant for ITS1. Second, we show that five known Nearctic refugia and an Icelandic refugium are concordant with the current distribution of morphologically distinct subspecies; five of these six refugia are geographically concordant with the distribution of closely related control region haplotypes. Third, our estimates of the time since phylogenetic lineages diverged predated the last glacial maximum for all but two lineages. In addition, all lines of evidence suggest that two unknown refugia in the Bering Sea region supported rock ptarmigan during the Wisconsin glaciation. Overall, our results are most consistent with the hypothesis that isolated populations of rock ptarmigan diverged in multiple refugia during the Wisconsin and that geographic variation reflects patterns of recolonization of the Nearctic after the ice receded. The GRH may therefore offer the most plausible explanation for similar biogeographic patterns in a variety of Nearctic vertebrates.     

Limited differentiation in microsatellite DNA variation among northern populations of the yellow warbler: evidence for male‐biased gene flow?

Comparisons of the patterns of differentiation among genetic markers with different modes of inheritance can provide insights into patterns of sex-biased dispersal and gene flow. Here, we compare the patterns of differentiation in six microsatellite loci among eight northern breeding populations of the yellow warbler (Dendroica petechia) with results obtained with mitochondrial DNA. Significant but low levels of differentiation (overall FST = 0.014; overall RST = 0.015) were present across all populations. The level of differentiation is substantially less than that observed in the same samples based on mitochondrial DNA control region variation. The presence of low population imbalance index values and significant isolation-by-distance relationships for both FST and RST suggests that these populations are at evolutionary equilibrium and that the high degree of similarity between populations may be due to high levels of male-biased gene flow. This suggests that there may be significant but previously unappreciated differences in the long-distance and/or episodic dispersal behaviour of males and females in these birds.     

Genetic structure among populations in the endemic Hawaiian Plantago lineage: insights from microsatellite variation

Endemic Hawaiian species in the genus Plantago show considerable morphological and ecological diversity. Despite their variation, a recent phylogenetic analysis based on DNA sequence data showed that the group is monophyletic and that sequence variation among species and morphotypes is low. This lack of sequence polymorphisms resulted in an inability to resolve species and population affinities within the most recently derived clade of this lineage. To assess species boundaries, population genetic structure and interpopulation connectivity among the morphologically and ecologically distinct populations within this clade, genetic variation was examined using eight microsatellite loci. Within‐population genetic diversity was found to be lowest in the Maunaiu, Hawai'i population of the endangered P. hawaiensis, and highest in the large P. pachyphylla population from 'Eke, West Maui. Isolation by distance across the range of populations was detected and indicated restricted dispersal. This result is likely to be attributable to few interisland dispersal events in the evolutionary history of this lineage. Genetic differentiation within islands tended to be higher among populations occurring in contrasting bog and woodland habitats, suggesting ecological barriers to gene flow and the potential role of ecological divergence in population diversification. Overall, these results are consistent with findings from phylogenetic analysis of the entire lineage. Our data bring new insights regarding patterns of dispersal and population genetic structure to this endemic and endangered group of island taxa. As island environments become increasingly fragmented, information of this type has important implications for the successful management of these fragile populations and habitats.     

Mitochondrial DNA under siege in avian phylogeography

Mitochondrial DNA (mtDNA) has been the workhorse of research in phylogeography for almost two decades. However, concerns with basing evolutionary interpretations on mtDNA results alone have been voiced since the inception of such studies. Recently, some authors have suggested that the potential problems with mtDNA are so great that inferences about population structure and species limits are unwarranted unless corroborated by other evidence, usually in the form of nuclear gene data. Here we review the relative merits of mitochondrial and nuclear phylogeographical studies, using birds as an exemplar class of organisms. A review of population demographic and genetic theory indicates that mitochondrial and nuclear phylogeographical results ought to concur for both geographically unstructured populations and for populations that have long histories of isolation. However, a relatively common occurrence will be shallow, but geographically structured mtDNA trees--without nuclear gene corroboration--for populations with relatively shorter periods of isolation. This is expected because of the longer coalescence times of nuclear genes (approximately four times that of mtDNA); such cases do not contradict the mtDNA inference of recent isolation and evolutionary divergence. Rather, the nuclear markers are more lagging indicators of changes in population structure. A review of the recent literature on birds reveals the existence of relatively few cases in which nuclear markers contradict mitochondrial markers in a fashion not consistent with coalescent theory. Preliminary information from nuclear genes suggests that mtDNA patterns will prove to be robust indicators of patterns of population history and species limits. At equilibrium, mitochondrial loci are generally a more sensitive indicator of population structure than are nuclear loci, and mitochondrial estimates of F(ST)-like statistics are generally expected to exceed nuclear ones. Hence, invoking behavioural or ecological explanations of such differences is not parsimonious. Nuclear genes will prove important for quantitative estimates of the depths of haplotype trees, rates of population growth and values of gene flow.     

Genetic structure of desert ground squirrels over a 20-degree-latitude transect from Oregon through the Baja California peninsula

The genetic structure of populations over a wide geographical area should reflect the demographic and evolutionary processes that have shaped a species across its range. We examined the population genetic structure of antelope ground squirrels (Ammospermophilus leucurus) across the complex of North American deserts from the Great Basin of Oregon to the cape region of the Baja California peninsula. We sampled 73 individuals from 13 major localities over this 2500-km transect, from 43 to 22 degrees north. Our molecular phylogeographical analysis of 555 bp of the mitochondrial cytochrome b gene and 510 bp of the control region revealed great genetic uniformity in a single clade that extends from Oregon to central Baja California. A second distinct clade occupies the southern half of the peninsula. The minimal geographical structure of the northern clade, its low haplotype diversity and the distribution of pairwise differences between haplotypes suggest a rapid northward expansion of the population that must have followed a northward desert habitat shift associated with the most recent Quaternary climate warming and glacial retreat. The higher haplotype diversity within the southern clade and distribution of pairwise differences between haplotypes suggest that the southern clade has a longer, more stable history associated with a southern peninsular refugium. This system, as observed, reflects both historical and contemporary ecological and evolutionary responses to physical environmental gradients within genetically homogeneous populations.     

Postglacial recolonization patterns and genetic relationships among whitefish (Coregonus sp.) populations in Denmark, inferred from mitochondrial DNA and microsatellite markers

The genetic relationships among morphologically and geographically divergent populations of whitefish (genus: Coregonus ) from Denmark and the Baltic Sea region were studied by analysis of microsatellites and polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) analysis of mitochondrial DNA (mtDNA) segments. The endangered North Sea houting (classified as C. oxyrhynchus ) differs morphologically and physiologically from other Danish whitefish ( C. lavaretus ). However, limited divergence of North Sea houting was observed both at the level of mtDNA and microsatellites. The implications of these results for the conservation status of North Sea houting are discussed in the light of current definitions of evolutionary significant units. Both mtDNA and microsatellite data indicated that postglacial recolonization by C. lavaretus in Denmark was less likely to have taken place from the Baltic Sea. Instead, the data suggested a recent common origin of all Danish whitefish populations, including North Sea houting, probably by recolonization via the postglacial Elbe River system. Estimates of genetic differentiation among populations based on mtDNA and microsatellites were qualitatively different. In addition, for both classes of markers analyses of genetic differentiation yielded different results, depending on whether molecular distances between alleles or haplotypes were included.     

Molecular and morphological assessment of Australia’s most endangered snake, <Emphasis Type="Italic">Hoplocephalus bungaroides</Emphasis>, reveals two evolutionarily significant units for conservation

The Broad-headed snake Hoplocephalus bungaroides is one of Australia’s most endangered vertebrates. Extant populations of H. bungaroides are restricted to several geographically isolated reserves to the north, west, and south of Sydney. We analysed mitochondrial DNA from 184 specimens drawn from across the geographic range of the Broad-headed snake. Phylogenetic analysis demonstrated that H. bungaroides comprises two divergent mitochondrial lineages with a “northern” clade comprising populations west and north of Sydney and a “southern” clade comprising animals in Morton National Park. The two clades differ by an uncorrected genetic distance of 1.7%, which implies a divergence dating to approximately 755,000–850,000 years ago. We complemented our molecular data set with a detailed analysis of morphological variation both between and within the genetic clades. The two H. bungaroides genetic clades are morphologically indistinguishable and show little sexual dimorphism. Our results demonstrate that the populations north and south of this biogeographic split function as two distinct populations with no recent gene flow. There is no reason for separate taxonomic recognition of these two clades, but they do represent distinct evolutionarily significant units (ESUs) that require separate conservation management. In addition, within the northern ESU, populations from Royal National Park, Blue Mountains National Park, Wollemi National Park, and the Sydney Water Catchment supply areas should be considered as separate management units to conserve both evolutionary and ecological processes.     

Niche shifts and range expansions along cordilleras drove diversification in a high‐elevation endemic plant genus in the tropical Andes

The tropical Andes represent one of the world's biodiversity hot spots, but the evolutionary drivers generating their striking species diversity still remain poorly understood. In the treeless high‐elevation Andean environments, Pleistocene glacial oscillations and niche differentiation are frequently hypothesized diversification mechanisms; however, sufficiently densely sampled population genetic data supporting this are still lacking. Here, we reconstruct the evolutionary history of Loricaria (Asteraceae), a plant genus endemic to the Andean treeless alpine zone, based on comprehensive population‐level sampling of 289 individuals from 67 populations across the entire distribution ranges of its northern Andean species. Partly incongruent AFLP and plastid DNA markers reveal that the distinct genetic structure was shaped by a complex interplay of biogeography (spread along and across the cordilleras), history (Pleistocene glacial oscillations) and local ecological conditions. While plastid variation documents an early split or colonization of the northern Andes by at least two lineages, one of which further diversified, a major split in the AFLP data correlate with altitudinal ecological differentiation. This suggests that niche shifts may be important drivers of Andean diversification not only in forest–alpine transitions, but also within the treeless alpine zone itself. The patterns of genetic differentiation at the intraspecific level reject the hypothesized separation in spatially isolated cordilleras and instead suggest extensive gene flow among populations from distinct mountain chains. Our study highlights that leveraging highly variable markers against extensive population‐level sampling is a promising approach to address mechanisms of rapid species diversifications.     

Divergence with gene flow and fine-scale phylogeographical structure in the wedge-billed woodcreeper, Glyphorynchus spirurus, a Neotropical rainforest bird

Determining the relative roles of vicariance and selection in restricting gene flow between populations is of central importance to the evolutionary process of population divergence and speciation. Here we use molecular and morphological data to contrast the effect of isolation (by mountains and geographical distance) with that of ecological factors (altitudinal gradients) in promoting differentiation in the wedge-billed woodcreeper, Glyphorynchus spirurus , a tropical forest bird, in Ecuador. Tarsus length and beak size increased relative to body size with altitude on both sides of the Andes, and were correlated with the amount of moss on tree trunks, suggesting the role of selection in driving adaptive divergence. In contrast, molecular data revealed a considerable degree of admixture along these altitudinal gradients, suggesting that adaptive divergence in morphological traits has occurred in the presence of gene flow. As suggested by mitochondrial DNA sequence data, the Andes act as a barrier to gene flow between ancient subspecific lineages. Genome-wide amplified fragment length polymorphism markers reflected more recent patterns of gene flow and revealed fine-scale patterns of population differentiation that were not detectable with mitochondrial DNA, including the differentiation of isolated coastal populations west of the Andes. Our results support the predominant role of geographical isolation in driving genetic differentiation in G. spirurus , yet suggest the role of selection in driving parallel morphological divergence along ecological gradients.     

Genetic structuring of Patagonian toothfish populations in the Southwest Atlantic Ocean: the effect of the Antarctic Polar Front and deep-water troughs as barriers to genetic exchange

The environmental and/or life history factors affecting genetic exchange in marine species with potential for high dispersal are of great interest, not only from an evolutionary standpoint but also with regard to effective management. Previous genetic studies have demonstrated substantial differentiation among populations of the Patagonian toothfish around the Southern Ocean, indicating breakdown of gene flow across large distances between inhabited shelf areas. The present study examined genetic structuring through analysis of microsatellite loci and restriction fragment length polymorphism (RFLP) of the mitochondrial ND2 gene and control region of the toothfish population in the SW Atlantic, allowing examination of the relative effects of the Antarctic Polar Front (APF), deep-water troughs and distance between sites. Mitochondrial DNA (mtDNA) data indicated a sharp genetic division between the Patagonian Shelf/North Scotia Ridge and the Shag Rocks/South Georgia samples, whereas microsatellite data showed much less distinct structuring and an intermediate position of the North Scotia Ridge samples. We suggest these data indicate that the APF, as a barrier to larval dispersal, is the major inhibitor of genetic exchange between toothfish populations, with deep-water troughs and distance between sites contributing to genetic differentiation by inhibiting migration of relatively sedentary adults. We also suggest that differences between mtDNA and nuclear DNA population patterns may reflect either genome population size effects or (putative) male-biased dispersal.     

Intraspecific genetic analysis of the summer tanager Piranga rubra: implications for species limits and conservation

The summer tanager Piranga rubra is a Neotropical migrant that has experienced noted declines in the southwestern United States caused by extensive habitat loss of native riparian woodlands. This species is composed of two morphologically and behaviorally distinct taxa that traditionally have been recognized as subspecies, each occupying unique habitats in the southern part of North America. Genetic analyses of intraspecific variation are important in studies of threatened or endangered species because they can indicate whether smaller management units exist below the species level and they also provide estimates of within population variability. Using a mitochondrial DNA marker, the intraspecific genetic variation of this species is explored to determine whether the morphologically and behaviorally distinct subspecies are also genetically unique. By using traditional phylogenetic methods and building haplotype networks, results from this study indicate that the subspecies represent two phylogenetic species and should be managed as separate units. In addition, the level of gene flow among geographically isolated populations of the western subspecies is explored using Nested Clade Phylogeographic Analysis and population genetic tests. These analyses show that populations are genetically diverse and that haplotypes are shared across populations. Newly colonized populations are as diverse as older populations. This suggests that as habitat degrades in traditional breeding areas of the summer tanager, if suitable habitat elsewhere becomes available for new populations, these new colonies should be genetically diverse.     

Human Races: A Genetic and Evolutionary Perspective

Race is generally used as a synonym for subspecies , which traditionally is a geographically circumscribed, genetically differentiated population. Sometimes traits show independent patterns of geographical variation such that some combination will distinguish most populations from all others. To avoid making "race" the equivalent of a local population, minimal thresholds of differentiation are imposed. Human "races" are below the thresholds used in other species, so valid traditional subspecies do not exist in humans. A "subspecies" can also be defined as a distinct evolutionary lineage within a species. Genetic surveys and the analyses of DNA haplotype trees show that human "races" are not distinct lineages, and that this is not due to recent admixture; human "races" are not and never were "pure." Instead, human evolution has been and is characterized by many locally differentiated populations coexisting at any given time, but with sufficient genetic contact to make all of humanity a single lineage sharing a common evolutionary fate.     

Evolutionary history of Scinax treefrogs on land‐bridge islands in south‐eastern Brazil

Aim We investigated how Pleistocene refugia and recent (c. 12,000 years ago) sea level incursions shaped genetic differentiation in mainland and island populations of the Scinax perpusillus treefrog group. Location Brazilian Atlantic Forest, São Paulo state, south‐eastern Brazil. Methods Using mitochondrial and microsatellite loci, we examined population structure and genetic diversity in three species from the S. perpusillus group, sampled from three land‐bridge islands and five mainland populations, in order to understand the roles of Pleistocene forest fragmentation and sea level incursions on genetic differentiation. We calculated metrics of relatedness and genetic diversity to assess whether island populations exhibit signatures of genetic drift and isolation. Two of the three island populations in this study have previously been described as new species based on a combination of distinct morphological and behavioural characters, thus we used the molecular datasets to determine whether phenotypic change is consistent with genetic differentiation. Results Our analyses recovered three distinct lineages or demes composed of northern mainland São Paulo populations, southern mainland São Paulo populations, and one divergent island population. The two remaining island populations clustered with samples from adjacent mainland populations. Estimates of allelic richness were significantly lower, and estimates of relatedness were significantly higher, in island populations relative to their mainland counterparts. Main conclusions Fine‐scale genetic structure across mainland populations indicates the possible existence of local refugia within São Paulo state, underscoring the small geographic scale at which populations diverge in this species‐rich region of the Atlantic Coastal Forest. Variation in genetic signatures across the three islands indicates that the populations experienced different demographic processes after marine incursions fragmented the distribution of the S. perpusillus group. Genetic signatures of inbreeding and drift in some island populations indicate that small population sizes, coupled with strong ecological selection, may be important evolutionary forces driving speciation on land‐bridge islands.     

Inter‐island divergence within Drosophila mauritiana,a species of the D. simulans complex: Past history and/or speciation in progress?

Speciation with gene flow may be more common than generally thought, which makes detailed understanding of the extent and pattern of genetic divergence between geographically isolated populations useful. Species of the Drosophila simulans complex provide a good model for speciation and evolutionary studies, and hence understanding their population genetic structure will increase our understanding of the context in which speciation has occurred. Here, we describe genetic diversity and genetic differentiation of two distant populations of D. mauritiana (Mauritius and Rodrigues Islands) at mitochondrial and nuclear loci. We surveyed the two populations for their mitochondrial haplotypes, eight nuclear genes and 18 microsatellite loci. A new mitochondrial type is fixed in the Rodrigues population of D. mauritiana. The two populations are highly differentiated, their divergence appears relatively ancient (100,000 years) compared to the origin of the species, around 0.25MYA, and they exhibit very limited gene flow. However, they have similar levels of divergence from their sibling, D. simulans. Both nuclear genes and microsatellites revealed contrasting demographic histories between the two populations, expansion for the Mauritius population and stable population size for the Rodrigues Island population. The discovery of pronounced geographic structure within D. mauritiana combined to genetic structuring and low gene flow between the two island populations illuminates the evolutionary history of the species and clearly merits further attention in the broad context of speciation.     

Genetic differentiation among populations of a Hispaniolan trunk anole that exhibit geographical variation in dewlap colour

Long neglected by classic island biogeographical theory, speciation within and among islands is increasingly recognized as a major contributor to insular diversity. Although the factors responsible for island speciation remain poorly understood, this process appears critically dependent on geographical variation and speciation in allopatry or parapatry. Here, we investigate geographical variation and speciation in a complex of Hispaniolan trunk anoles (Anolis distichus), where populations with strikingly distinct dewlap colours and patterns correspond with deeply divergent mtDNA structure. Using a multilocus, population‐level analysis, we investigate whether these phenotypically and mitochondrially distinct populations exhibit the type of nuclear differentiation expected among species or incipient species. Along a transect that extends across a recently recessed marine barrier, our results are consistent with the persistence of an abrupt phenotypic and mitochondrial transition following secondary contact, in spite of little or no evidence for a reduction in nuclear gene flow. Along a second transect extending across a steep environmental gradient, our phenotypic and microsatellite data suggest a sharp genetic break with little or no admixture, whereas mtDNA recovers a signature of extensive unidirectional introgression. Together, these results are consistent with previous studies of Lesser Antillean anoles, suggesting that allopatric divergence alone is insufficient for speciation, whereas reduced gene flow and partial reproductive isolation may accumulate in the presence of ecological gradients.     

Multiple axes of ecological vulnerability to climate change

Observed ecological responses to climate change are highly individualistic across species and locations, and understanding the drivers of this variability is essential for management and conservation efforts. While it is clear that differences in exposure, sensitivity, and adaptive capacity all contribute to heterogeneity in climate change vulnerability, predicting these features at macroecological scales remains a critical challenge. We explore multiple drivers of heterogeneous vulnerability across the distributions of 96 vegetation types of the ecologically diverse western US, using data on observed climate trends from 1948 to 2014 to highlight emerging patterns of change. We ask three novel questions about factors potentially shaping vulnerability across the region: (a) How does sensitivity to different climate variables vary geographically and across vegetation classes? (b) How do multivariate climate exposure patterns interact with these sensitivities to shape vulnerability patterns? (c) How different are these vulnerability patterns according to three widely implemented vulnerability paradigms—niche novelty (decline in modeled suitability), temporal novelty (standardized anomaly), and spatial novelty (inbound climate velocity)—each of which uses a distinct frame of reference to quantify climate departure? We propose that considering these three novelty paradigms in combination could help improve our understanding and prediction of heterogeneous climate change responses, and we discuss the distinct climate adaptation strategies connected with different combinations of high and low novelty across the three metrics. Our results reveal a diverse mosaic of climate change vulnerability signatures across the region's plant communities. Each of the above factors contributes strongly to this heterogeneity: climate variable sensitivity exhibits clear patterns across vegetation types, multivariate climate change data reveal highly diverse exposure signatures across locations, and the three novelty paradigms diverge widely in their climate change vulnerability predictions. Together, these results shed light on potential drivers of individualistic climate change responses and may help to inform effective management strategies.