Genetic variability and structure of the water vole Arvicola amphibius across four metapopulations in northern Norway

Water vole Arvicola amphibius populations have recently experienced severe decline in several European countries as a consequence of both reduction in suitable habitat and the establishment of the alien predator American mink Neovison vison. We used DNA microsatellite markers to describe the genetic structure of 14 island populations of water vole off the coast of northern Norway. We looked at intra‐ and inter‐population levels of genetic variation and examined the effect of distance among pairs of populations on genetic differentiation (isolation by distance). We found a high level of genetic differentiation (measured by F_ST) among populations overall as well as between all pairs of populations. The genetic differentiation between populations was positively correlated with geographic distance between them. A clustering analysis grouped individuals into 7 distinct clusters and showed the presence of 3 immigrants among them. Our results suggest a small geographic scale for evolutionary and population dynamic processes in our water vole populations.     

History,geography and host use shape genomewide patterns of genetic variation in the redheaded pine sawfly (Neodiprion lecontei)

Divergent host use has long been suspected to drive population differentiation and speciation in plant‐feeding insects. Evaluating the contribution of divergent host use to genetic differentiation can be difficult, however, as dispersal limitation and population structure may also influence patterns of genetic variation. In this study, we use double‐digest restriction‐associated DNA (ddRAD) sequencing to test the hypothesis that divergent host use contributes to genetic differentiation among populations of the redheaded pine sawfly (Neodiprion lecontei), a widespread pest that uses multiple Pinus hosts throughout its range in eastern North America. Because this species has a broad range and specializes on host plants known to have migrated extensively during the Pleistocene, we first assess overall genetic structure using model‐based and model‐free clustering methods and identify three geographically distinct genetic clusters. Next, using a composite‐likelihood approach based on the site frequency spectrum and a novel strategy for maximizing the utility of linked RAD markers, we infer the population topology and date divergence to the Pleistocene. Based on existing knowledge of Pinus refugia, estimated demographic parameters and patterns of diversity among sawfly populations, we propose a Pleistocene divergence scenario for N. lecontei. Finally, using Mantel and partial Mantel tests, we identify a significant relationship between genetic distance and geography in all clusters, and between genetic distance and host use in two of three clusters. Overall, our results indicate that Pleistocene isolation, dispersal limitation and ecological divergence all contribute to genomewide differentiation in this species and support the hypothesis that host use is a common driver of population divergence in host‐specialized insects.     

Quantifying population structure on short timescales

Quantifying the contribution of the various processes that influence population genetic structure is important, but difficult. One of the reasons is that no single measure appropriately quantifies all aspects of genetic structure. An increasing number of studies is analysing population structure using the statistic D, which measures genetic differentiation, next to G_ST, which quantifies the standardized variance in allele frequencies among populations. Few studies have evaluated which statistic is most appropriate in particular situations. In this study, we evaluated which index is more suitable in quantifying postglacial divergence between three‐spined stickleback (Gasterosteus aculeatus) populations from Western Europe. Population structure on this short timescale (10 000 generations) is probably shaped by colonization history, followed by migration and drift. Using microsatellite markers and anticipating that D and G_ST might have different capacities to reveal these processes, we evaluated population structure at two levels: (i) between lowland and upland populations, aiming to infer historical processes; and (ii) among upland populations, aiming to quantify contemporary processes. In the first case, only D revealed clear clusters of populations, putatively indicative of population ancestry. In the second case, only G_ST was indicative for the balance between migration and drift. Simulations of colonization and subsequent divergence in a hierarchical stepping stone model confirmed this discrepancy, which becomes particularly strong for markers with moderate to high mutation rates. We conclude that on short timescales, and across strong clines in population size and connectivity, D is useful to infer colonization history, whereas G_ST is sensitive to more recent demographic events.     

Genetic population structure in prickly sculpin (Cottus asper) reflects isolation‐by‐environment between two life‐history ecotypes

Life‐history transitions have evolved repeatedly in numerous taxa, although the ecological and evolutionary conditions favouring such transitions in the presence of gene flow remain poorly understood. The present study aimed to disentangle the effects of isolation‐by‐distance and isolation‐by‐environment on genetic differentiation between two sympatric life‐history ecotypes. Using 14 microsatellite loci, we first characterized amphidromous and freshwater groups of Cottus asper in a high gene flow setting in the Lower Fraser River system (south‐western British Columbia, Canada) to test for the effects of habitat and geographical distance on the distribution of life‐history ecotypes. Within the main river channel, no genetic differentiation was found, whereas tributaries even close to the estuary were genetically differentiated. Partial mantel tests confirmed that genetic differentiation between river tributaries and the main channel was independent from geographical distance, with distance‐scaled migration rates indicating reduced gene flow from the main channel into the tributaries. Our results suggest that isolation‐by‐environment can play an important role for the early stage of life‐history transitions, and may promote differentiation among life‐history ecotypes despite the presence of gene flow. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 943–957.     

Living in two worlds: Evolutionary mechanisms act differently in the native and introduced ranges of an invasive plant

Identifying the factors that influence spatial genetic structure among populations can provide insights into the evolution of invasive plants. In this study, we used the common reed (Phragmites australis), a grass native in Europe and invading North America, to examine the relative importance of geographic, environmental (represented by climate here), and human effects on population genetic structure and its changes during invasion. We collected samples of P. australis from both the invaded North American and native European ranges and used molecular markers to investigate the population genetic structure within and between ranges. We used path analysis to identify the contributions of each of the three factors—geographic, environmental, and human‐related—to the formation of spatial genetic patterns. Genetic differentiation was observed between the introduced and native populations, and their genetic structure in the native and introduced ranges was different. There were strong effects of geography and environment on the genetic structure of populations in the native range, but the human‐related factors manifested through colonization of anthropogenic habitats in the introduced range counteracted the effects of environment. The between‐range genetic differences among populations were mainly explained by the heterogeneous environment between the ranges, with the coefficient 2.6 times higher for the environment than that explained by the geographic distance. Human activities were the primary contributor to the genetic structure of the introduced populations. The significant environmental divergence between ranges and the strong contribution of human activities to the genetic structure in the introduced range suggest that invasive populations of P. australis have evolved to adapt to a different climate and to human‐made habitats in North America.     

Living on the edge: the role of geography and environment in structuring genetic variation in the southernmost populations of a tropical oak

Understanding the factors determining genetic diversity and structure in peripheral populations is a long‐standing goal of evolutionary biogeography, yet little empirical information is available for tropical species. In this study, we combine information from nuclear microsatellite markers and niche modelling to analyse the factors structuring genetic variation across the southernmost populations of the tropical oak Quercus segoviensis. First, we tested the hypothesis that genetic variability decreases with population isolation and increases with local habitat suitability and stability since the Last Glacial Maximum (LGM). Second, we employed a recently developed multiple matrix regression with randomisation (MMRR) approach to study the factors associated with genetic divergence among the studied populations and test the relative contribution of environmental and geographic isolation to contemporary patterns of genetic differentiation. We found that genetic diversity was negatively correlated with average genetic differentiation with other populations, indicating that isolation and limited gene flow have contributed to erode genetic variability in some populations. Considering the relatively small size of the study area (<120 km), analyses of genetic structure indicate a remarkable inter‐population genetic differentiation. Environmental dissimilarity and differences in current and past climate niche suitability and their additive effects were not associated with genetic differentiation after controlling for geographic distance, indicating that local climate does not contribute to explain spatial patterns of genetic structure. Overall, our data indicate that geographic isolation, but not current or past climate, is the main factor determining contemporary patterns of genetic diversity and structure within the southernmost peripheral populations of this tropical oak.     

Influence of geography and environment on patterns of genetic differentiation in a widespread submerged macrophyte,Eurasian watermilfoil (Myriophyllum spicatum L., Haloragaceae)

The effects of geographic and environmental variables on the pattern of genetic differentiation have been thoroughly studied, whereas empirical studies on aquatic plants are rare. We examined the spatial genetic differentiation of 58 Myriophyllum spicatum populations distributed throughout China with 12 microsatellite loci, and we analyzed its association with geographic distance, geographic barriers, and environmental dissimilarity using causal modeling and multiple matrix regression with randomization (MMRR) analysis. Two genetic clusters were identified, and their geographic distribution suggested mountain ranges as a barrier to gene flow. The causal modeling revealed that both climate and geographic barriers significantly influenced genetic divergence among M. spicatum populations and that climate had the highest regression coefficient according to the MMRR analysis. This study showed that geography and environment together played roles in shaping the genetic structure of M. spicatum and that the influence of environment was greater. Our findings emphasized the potential importance of the environment in producing population genetic differentiation in aquatic plants at a large geographic scale.     

Genetic population structure of spotted seatrout Cynoscion nebulosus along the south‐eastern U.S.A.

Analyses of the genetic population structure of spotted seatrout Cynoscion nebulosus along the south‐eastern U.S. coast using 13 microsatellites suggest significant population differentiation between fish in North Carolina (NC) compared with South Carolina (SC) and Georgia (GA), with New River, NC, serving as an area of integration between northern and southern C. nebulosus. Although there is a significant break in gene flow between these areas, the overall pattern throughout the sampling range represents a gradient in genetic diversification with the degree of geographic separation. Latitudinal distance and estuarine density appear to be main drivers in the genetic differentiation of C. nebulosus along the south‐eastern U.S. coast. The isolation‐by‐distance gene‐flow pattern creates fine‐scale differences in the genetic composition of proximal estuaries and dictates that stocking must be confined to within 100 km of the location of broodstock collection in order to maintain the natural gradient of genetic variation along the south‐eastern U.S. coast.     

Evolutionary insights into Rhinolophus episcopus (Chiroptera,Rhinolophidae) in China: Isolation by distance,environment, or sensory system?

Evolutionary processes can be influenced by several factors, such as geographic isolation, environmental selection, and sensory variation. For most nocturnal bats, echolocation is the primary sensory system used to prey and communicate, and plays important roles in chiropteran diversification and evolution. Understanding the relative contribution of geography, the environment, and this sensory system to population genetic divergence can elucidate the processes involved in bat incipient speciation and evolution. In this study, we collected spatial and environmental information, echolocation calls, as well as the previously published genetic data (six microsatellite loci and the mitochondrial cytochrome b gene) of widely distributed Rhinolophus episcopus populations to test three hypotheses for nuclear and mitochondrial divergence (isolation by distance, isolation by environment, and isolation by sensory variation) and unveil the factors that drive intraspecific genetic differentiation. The moderate level of nuclear differentiation was correlated with geographic/spatial distance and acoustic variation, whereas the relatively high level of mitochondrial differentiation was mainly associated with acoustic divergence. No significant correlation was observed between genetic divergence and environmental variables. Among the three factors, acoustic divergence explained the highest percentage of both nuclear and mitochondrial divergence. Thus, our results indicate that sensory variation may have played important roles in driving population isolation early in bat speciation, which is consistent with the hypothesis of isolation by sensory variation. Our study emphasizes the need to consider more factors, especially sensory traits, and combine multiple statistical methods in landscape genetic studies to test their potential contributions to driving population divergence.     

Quaternary climate instability is correlated with patterns of population genetic variability in Bombus huntii

Climate oscillations have left a significant impact on the patterns of genetic diversity observed in numerous taxa. In this study, we examine the effect of Quaternary climate instability on population genetic variability of a bumble bee pollinator species, Bombus huntii in western North America. Pleistocene and contemporary B. huntii habitat suitability (HS) was estimated with an environmental niche model (ENM) by associating 1,035 locality records with 10 bioclimatic variables. To estimate genetic variability, we genotyped 380 individuals from 33 localities at 13 microsatellite loci. Bayesian inference was used to examine population structure with and without a priori specification of geographic locality. We compared isolation by distance (IBD) and isolation by resistance (IBR) models to examine population differentiation within and among the Bayesian inferred genetic clusters. Furthermore, we tested for the effect of environmental niche stability (ENS) on population genetic diversity with linear regression. As predicted, high‐latitude B. huntii habitats exhibit low ENS when compared to low‐latitude habitats. Two major genetic clusters of B. huntii inhabit western North America: (a) a north genetic cluster predominantly distributed north of 28°N and (b) a south genetic cluster distributed south of 28°N. In the south genetic cluser, both IBD and IBR models are significant. However, in the north genetic cluster, IBD is significant but not IBR. Furthermore, the IBR models suggest that low‐latitude montane populations are surrounded by habitat with low HS, possibly limiting dispersal, and ultimately gene flow between populations. Finally, we detected high genetic diversity across populations in regions that have been climatically unstable since the last glacial maximum (LGM), and low genetic diversity across populations in regions that have been climatically stable since the LGM. Understanding how species have responded to climate change has the potential to inform management and conservation decisions of both ecological and economic concerns.     

Genetic variation and structure of house sparrow populations: is there an island effect?

Population genetic structure and intrapopulation levels of genetic variation have important implications for population dynamics and evolutionary processes. Habitat fragmentation is one of the major threats to biodiversity. It leads to smaller population sizes and reduced gene flow between populations and will thus also affect genetic structure. We use a natural system of island and mainland populations of house sparrows along the coast of Norway to characterize the different population genetic properties of fragmented populations. We genotyped 636 individuals distributed across 14 populations at 15 microsatellite loci. The level of genetic differentiation was estimated using F‐statistics and specially designed Mantel tests were conducted to study the influence of population type (i.e. mainland or island) and geographic distance on the genetic population structure. Furthermore, the effects of population type, population size and latitude on the level of genetic variation within populations were examined. Our results suggest that genetic processes on islands and mainland differed in two important ways. First, the intrapopulation level of genetic variation tended to be lower and the occurrence of population bottlenecks more frequent on islands than the mainland. Second, although the general level of genetic differentiation was low to moderate, it was higher between island populations than between mainland populations. However, differentiation increased in mainland populations somewhat faster with geographical distance. These results suggest that population bottleneck events and genetic drift have been more important in shaping the genetic composition of island populations compared with populations on the mainland. Such knowledge is relevant for a better understanding of evolutionary processes and conservation of threatened populations.     

Development of genetic diversity,differentiation and structure over 500 years in four ponderosa pine populations

Population history plays an important role in shaping contemporary levels of genetic variation and geographic structure. This is especially true in small, isolated range‐margin populations, where effects of inbreeding, genetic drift and gene flow may be more pronounced than in large continuous populations. Effects of landscape fragmentation and isolation distance may have implications for persistence of range‐margin populations if they are demographic sinks. We studied four small, disjunct populations of ponderosa pine over a 500‐year period. We coupled demographic data obtained through dendroecological methods with microsatellite data to discern how and when contemporary levels of allelic diversity, among and within‐population levels of differentiation, and geographic structure, arose. Alleles accumulated rapidly following initial colonization, demonstrating proportionally high levels of gene flow into the populations. At population sizes of approximately 100 individuals, allele accumulation saturated. Levels of genetic differentiation among populations (F_ST and Jost's D_est) and diversity within populations (F_IS) remained stable through time. There was no evidence of geographic genetic structure at any time in the populations' history. Proportionally, high gene flow in the early stages of population growth resulted in rapid accumulation of alleles and quickly created relatively homogenous genetic patterns among populations. Our study demonstrates that contemporary levels of genetic diversity were formed quickly and early in population development. How contemporary genetic diversity accumulates over time is a key facet of understanding population growth and development. This is especially relevant given the extent and speed at which species ranges are predicted to shift in the coming century.     

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.     

Calls,colours, shape,and genes: a multi‐trait approach to the study of geographic variation in the Amazonian frog Allobates femoralis

Evolutionary divergence in behavioural traits related to mating may represent the initial stage of speciation. Direct selective forces are usually invoked to explain divergence in mate‐recognition traits, often neglecting a role for neutral processes or concomitant differentiation in ecological traits. We adopted a multi‐trait approach to obtain a deeper understanding of the mechanisms behind allopatric divergence in the Amazonian frog, Allobates femoralis. We tested the null hypothesis that geographic distance between populations correlates with genetic and phenotypic divergence, and compared divergence between mate‐recognition (acoustic) and ecological (coloration, body‐shape) traits. We quantified geographic variation in 39 phenotypic traits and a mitochondrial DNA marker among 125 individuals representing eight populations. Geographic variation in acoustic traits was pronounced and tracked the spatial genetic variation, which appeared to be neutral. Thus, the evolution of acoustic traits tracked the shared history of the populations, which is unexpected for pan‐Amazonian taxa or for mate‐recognition traits. Divergence in coloration appeared uncorrelated with genetic distance, and might be partly attributed to local selective pressures, and perhaps to Batesian mimicry. Divergence in body‐shape traits was low. The results obtained depict a complex evolutionary scenario and emphasize the importance of considering multiple traits when disentangling the forces behind allopatric divergence. ©2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98 , 826–838.     

How birds colonize cities: genetic evidence from a common waterbird,the Eurasian coot

Much effort has been devoted to identify ecological and life‐history traits which facilitate urban colonization by wild avian species, but surprisingly little is known about the population‐level mechanisms of urbanization processes. In general, two different patterns of urban colonization have been proposed: 1) the model of independent colonization predicts that birds colonize cities independently in different geographical regions; 2) the model of leapfrog colonization assumes a single colonization event, while additional urban populations are established from the initial urban populations. The aim of this paper was to determine the pattern of urban colonization in a common waterbird, the Eurasian coot Fulica atra. For this purpose, we analysed microsatellite variation in three pairs of urban and rural coot populations from central Poland. We found that a newly‐established urban population was genetically more similar to neighbouring rural populations than to long‐established urban populations, as indicated by the analysis of fixation index, genetic distance and Bayesian assignment of individuals to genetic clusters. These results are consistent with the model of independent colonization, where neighbouring rural populations are a source of individuals that colonize new urban areas. However, our analysis also showed significant differentiation between long‐established urban populations and adjacent rural populations, suggesting that genetic connectivity between two types of habitat decreases with increasing time since urbanization. Our study shows high complexity of urbanization processes in wild animal populations, as well as it underpins utility of molecular tools in studying population‐level mechanisms of urbanization.     

Environmental gradients shape the genetic structure of two medicinal Salvia species in Jordan

• Environmental gradients, and particularly climatic variables, exert a strong influence on plant distribution and, potentially, population genetic diversity and differentiation. Differences in water availability can cause among‐population variation in ecological processes and can thus interrupt populations’ connectivity and isolate them environmentally. The present study examines the effect of environmental heterogeneity on plant populations due to environmental isolation unrelated to geographic distance.
• Using AFLP markers, we analyzed genetic diversity and differentiation among 12 Salvia spinosa populations and 13 Salvia syriaca populations from three phytogeographical regions (Mediterranean, Irano‐Turanian and Saharo‐Arabian) representing the extent of the species’ geographic range in Jordan. Differences in geographic location and climate were considered in the analyses.
• For both species, flowering phenology varied among populations and regions. Irano‐Turanian and Saharo‐Arabian populations had higher genetic diversity than Mediterranean populations, and genetic diversity increased significantly with increasing temperature. Genetic diversity in Salvia syriaca was affected by population size, while genetic diversity responded to drought in S. spinosa. For both species, high levels of genetic differentiation were found as well as two well‐supported phytogeographical groups of populations, with Mediterranean populations clustering in one group and the Irano‐Turanian and Saharo‐Arabian populations in another. Genetic distance was significantly correlated to environmental distance, but not to geographic distance.
• Our data indicate that populations from moist vs. arid environments are environmentally isolated, where environmental gradients affect their flowering phenology, limit gene flow and shape their genetic structure. We conclude that environmental heterogeneity may act as driver for the observed variation in genetic diversity.

     

Landscape genetics reveal broad and fine‐scale population structure due to landscape features and climate history in the northern leopard frog (Rana pipiens) in North Dakota

Prehistoric climate and landscape features play large roles structuring wildlife populations. The amphibians of the northern Great Plains of North America present an opportunity to investigate how these factors affect colonization, migration, and current population genetic structure. This study used 11 microsatellite loci to genotype 1,230 northern leopard frogs (Rana pipiens) from 41 wetlands (30 samples/wetland) across North Dakota. Genetic structure of the sampled frogs was evaluated using Bayesian and multivariate clustering methods. All analyses produced concordant results, identifying a major east–west split between two R. pipiens population clusters separated by the Missouri River. Substructuring within the two major identified population clusters was also found. Spatial principal component analysis (sPCA) and variance partitioning analysis identified distance, river basins, and the Missouri River as the most important landscape factors differentiating R. pipiens populations across the state. Bayesian reconstruction of coalescence times suggested the major east–west split occurred ~13–18 kya during a period of glacial retreat in the northern Great Plains and substructuring largely occurred ~5–11 kya during a period of extreme drought cycles. A range‐wide species distribution model (SDM) for R. pipiens was developed and applied to prehistoric climate conditions during the Last Glacial Maximum (21 kya) and the mid‐Holocene (6 kya) from the CCSM4 climate model to identify potential refugia. The SDM indicated potential refugia existed in South Dakota or further south in Nebraska. The ancestral populations of R. pipiens in North Dakota may have inhabited these refugia, but more sampling outside the state is needed to reconstruct the route of colonization. Using microsatellite genotype data, this study determined that colonization from glacial refugia, drought dynamics in the northern Great Plains, and major rivers acting as barriers to gene flow were the defining forces shaping the regional population structure of R. pipiens in North Dakota.     

Genes mirror geography in Daphnia magna

Identifying the presence and magnitude of population genetic structure remains a major consideration in evolutionary biology as doing so allows one to understand the demographic history of a species as well as make predictions of how the evolutionary process will proceed. Next‐generation sequencing methods allow us to reconsider previous ideas and conclusions concerning the distribution of genetic variation, and what this distribution implies about a given species evolutionary history. A previous phylogeographic study of the crustacean Daphnia magna suggested that, despite strong genetic differentiation among populations at a local scale, the species shows only moderate genetic structure across its European range, with a spatially patchy occurrence of individual lineages. We apply RAD sequencing to a sample of D. magna collected across a wide swath of the species' Eurasian range and analyse the data using principle component analysis (PCA) of genetic variation and Procrustes analytical approaches, to quantify spatial genetic structure. We find remarkable consistency between the first two PCA axes and the geographic coordinates of individual sampling points, suggesting that, on a continent‐wide scale, genetic differentiation is driven to a large extent by geographic distance. The observed pattern is consistent with unimpeded (i.e. no barriers, landscape or otherwise) migration at large spatial scales, despite the fragmented and patchy nature of favourable habitats at local scales. With high‐resolution genetic data similar patterns may be uncovered for other species with wide geographic distributions, allowing an increased understanding of how genetic drift and selection have shaped their evolutionary history.     

Landscape genetic analysis of the tropical freshwater fish Mogurnda mogurnda (Eleotridae) in a monsoonal river basin: importance of hydrographic factors and population history

1. We performed spatial genetic analyses, incorporating landscape genetic methods using microsatellite data and phylogeographic analyses using mtDNA data, to identify the principal factors that determine population heterogeneity of the tropical freshwater fish, Mogurnda mogurnda, in the Daly River, northern Australia. We tested the individual and interactive effects of several environmental variables on spatial genetic patterns, including metrics relating to connectivity (i.e. stream distance, maximum stream gradient and elevation), habitat size (i.e. mean annual discharge) and a categorical variable relating to population history, as determined by mtDNA phylogeographic analyses. The Daly River is geomorphologically and hydrologically complex, and M. mogurnda has life history traits that limit its dispersal potential at river basin scales. Thus, we predicted that variables relating to connectivity would be the most important landscape factors driving population structure of the species. 2. Tree‐based phylogeographic analyses indicated four divergent mtDNA lineages within M. mogurnda in the Daly River, although three of the lineages were sympatric in various combinations and did not correspond with microsatellite groups identified by assignment tests. The allopatric mtDNA lineage detected in the uppermost part of the catchment was also identified as being highly differentiated by the microsatellite data, strongly suggesting that it may be a cryptic species. This site was therefore excluded from subsequent landscape genetic analyses. 3. Analyses of Molecular Variance indicated that M. mogurnda has a hierarchical population structure in the Daly River, thus supporting theoretical expectations that hierarchically arranged river habitats in dendritic systems impose hierarchal population structures on lotic species. 4. All landscape genetic analyses rejected stream distance, and supported stream gradient, as the major determinant of spatial genetic variation in M. mogurnda in the Daly River. Support for elevation as a determinant of spatial genetic patterns differed among the landscape genetic methods. Several of the landscape genetic methods also indicate that population history, including secondary contact between divergent and formerly allopatric genetic lineages, has a strong influence on spatial genetic patterns within M. mogurnda in the Daly River. 5. This study demonstrates the need to consider multiple environmental factors, especially factors relating to connectivity, and their interactions in spatial genetic analysis, rather than just geographic distance. Importantly, it demonstrates the need to account for population history and evolutionary divergences in landscape genetic analyses.     

Secondary contact and changes in coastal habitat availability influence the nonequilibrium population structure of a salmonid (Oncorhynchus keta)

Numerous empirical studies have reported lack of migration–drift equilibrium in wild populations. Determining the causes of nonequilibrium population structure is challenging because different evolutionary processes acting at a variety of spatiotemporal scales can produce similar patterns. Studies of contemporary populations in northern latitudes suggest that nonequilibrium population structure is probably caused by recent colonization of the region after the last Pleistocene ice age ended ~13 000 years ago. The chum salmon's (Oncorhynchus keta) range was fragmented by dramatic environmental changes during the Pleistocene. We investigated the population structure of chum salmon on the North Alaska Peninsula (NAP) and, using both empirical data and simulations, evaluated the effects of colonization timing and founder population heterogeneity on patterns of genetic differentiation. We screened 161 single nucleotide polymorphisms and found evidence of nonequilibrium population structure when the slope of the isolation‐by‐distance relationship was examined at incremental spatial scales. In addition, simulations suggested that this pattern closely matched models of recent colonization of the NAP by secondary contact. Our results agree with geological and archaeological data indicating that the NAP was a dynamic landscape that may have been more recently colonized than during the last deglaciation because of dramatic changes in coastal hydrology over the last several thousand years.