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1. Contemporary range position predicts the range‐wide pattern of genetic diversity in balsam poplar ( Populus balsamifera L.)

   https://doi.org/10.1111/jbi.13811  

   Gougherty, Andrew V. ; Chhatre, Vikram E. ; Keller, Stephen R. ; Fitzpatrick, Matthew C. ; Comes, ed., Hans‐Peter ( February 2020 , Journal of Biogeography)

   Patterns of genetic diversity within species’ ranges can reveal important insights into effects of past climate on species’ biogeography and current population dynamics. While numerous biogeographic hypotheses have been proposed to explain patterns of genetic diversity within species’ ranges, formal comparisons and rigorous statistical tests of these hypotheses remain rare. Here, we compared seven hypotheses for their abilities to describe the geographic pattern of two metrics of genetic diversity in balsam poplar (Populus balsamifera), a northern North American tree species.

   North America.

   Balsam poplar (Populus balsamiferaL.).

   We compared seven hypotheses, representing effects of past climate and current range position, for their ability to describe the geographic pattern of expected heterozygosity and per cent polymorphic loci across 85 populations of balsam poplar. We tested each hypothesis using spatial and non‐spatial least‐squares regression to assess the importance of spatial autocorrelation on model performance.

   We found that both expected heterozygosity and per cent polymorphic loci could best be explained by the current range position and genetic structure of populations within the contemporary range. Genetic diversity showed a clear gradient of being highest near the geographic and climatic range centre and lowest near range edges. Hypotheses accounting for the effects of past climate (e.g. past climatic suitability, distance from the southern edge), in contrast, had comparatively little support. Model ranks were similar among spatial and non‐spatial models, but residuals of all non‐spatial models were significantly autocorrelated, violating the assumption of independence in least‐squares regression.

   Our work adds strong support for the “Central‐Periphery Hypothesis” as providing a predictive framework for understanding the forces structuring genetic diversity across species’ ranges, and illustrates the value of applying a robust comparative model selection framework and accounting for spatial autocorrelation when comparing biogeographic models of genetic diversity.

    

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2. Climate structures genetic variation across a species' elevation range: a test of range limits hypotheses

   https://doi.org/10.1111/mec.13528  

   Sexton, Jason P. ; Hufford, Matthew B. ; C.Bateman, Ashley ; Lowry, David B. ; Meimberg, Harald ; Strauss, Sharon Y. ; Rice, Kevin J. ( February 2016 , Molecular Ecology)

   Gene flow may influence the formation of species range limits, and yet little is known about the patterns of gene flow with respect to environmental gradients or proximity to range limits. With rapid environmental change, it is especially important to understand patterns of gene flow to inform conservation efforts. Here we investigate the species range of the selfing, annual plant,Mimulus laciniatus, in the California Sierra Nevada. We assessed genetic variation, gene flow, and population abundance across the entire elevation‐based climate range. Contrary to expectations, within‐population plant density increased towards both climate limits. Mean genetic diversity of edge populations was equivalent to central populations; however, all edge populations exhibited less genetic diversity than neighbouring interior populations. Genetic differentiation was fairly consistent and moderate among all populations, and no directional signals of contemporary gene flow were detected between central and peripheral elevations. Elevation‐driven gene flow (isolation by environment), but not isolation by distance, was found across the species range. These findings were the same towards high‐ and low‐elevation range limits and were inconsistent with two common centre‐edge hypotheses invoked for the formation of species range limits: (i) decreasing habitat quality and population size; (ii) swamping gene flow from large, central populations. This pattern demonstrates that climate, but not centre‐edge dynamics, is an important range‐wide factor structuringM. laciniatuspopulations. To our knowledge, this is the first empirical study to relate environmental patterns of gene flow to range limits hypotheses. Similar investigations across a wide variety of taxa and life histories are needed.

    

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3. Tracing the footprints of a moving hybrid zone under a demographic history of speciation with gene flow

   https://doi.org/10.1111/eva.12795  

   Menon, Mitra ; Landguth, Erin ; Leal‐Saenz, Alejandro ; Bagley, Justin C. ; Schoettle, Anna W. ; Wehenkel, Christian ; Flores‐Renteria, Lluvia ; Cushman, Samuel A. ; Waring, Kristen M. ; Eckert, Andrew J. ( April 2019 , Evolutionary Applications)

   A lack of optimal gene combinations, as well as low levels of genetic diversity, is often associated with the formation of species range margins. Conservation efforts rely on predictive modelling using abiotic variables and assessments of genetic diversity to determine target species and populations for controlled breeding, germplasm conservation and assisted migration. Biotic factors such as interspecific competition and hybridization, however, are largely ignored, despite their prevalence across diverse taxa and their role as key evolutionary forces. Hybridization between species with well‐developed barriers to reproductive isolation often results in the production of offspring with lower fitness. Generation of novel allelic combinations through hybridization, however, can also generate positive fitness consequences. Despite this possibility, hybridization‐mediated introgression is often considered a threat to biodiversity as it can blur species boundaries. The contribution of hybridization towards increasing genetic diversity of populations at range margins has only recently gathered attention in conservation studies. We assessed the extent to which hybridization contributes towards range dynamics by tracking spatio‐temporal changes in the central location of a hybrid zone between two recently diverged species of pines:Pinus strobiformisandP. flexilis. By comparing geographic cline centre estimates for global admixture coefficient with morphological traits associated with reproductive output, we demonstrate a northward shift in the hybrid zone. Using a combination of spatially explicit, individual‐based simulations and linkage disequilibrium variance partitioning, we note a significant contribution of adaptive introgression towards this northward movement, despite the potential for differences in regional population size to aid hybrid zone movement. Overall, our study demonstrates that hybridization between recently diverged species can increase genetic diversity and generate novel allelic combinations. These novel combinations may allow range margin populations to track favourable climatic conditions or facilitate adaptive evolution to ongoing and future climate change.

    

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4. Is the central‐marginal hypothesis a general rule? Evidence from three distributions of an expanding mangrove species, Avicennia germinans (L.) L

   https://doi.org/10.1111/mec.15365  

   Kennedy, John Paul ; Preziosi, Richard F. ; Rowntree, Jennifer K. ; Feller, Ilka C. ( February 2020 , Molecular Ecology)

   The central‐marginal hypothesis (CMH) posits that range margins exhibit less genetic diversity and greater inter‐population genetic differentiation compared to range cores. CMH predictions are based on long‐held “abundant‐centre” assumptions of a decline in ecological conditions and abundances towards range margins. Although much empirical research has confirmed CMH, exceptions remain almost as common. We contend that mangroves provide a model system to test CMH that alleviates common confounding factors and may help clarify this lack of consensus. Here, we document changes in black mangrove (Avicennia germinans) population genetics with 12 nuclear microsatellite loci along three replicate coastlines in the United States (only two of three conform to underlying “abundant‐centre” assumptions). We then test an implicit prediction of CMH (reduced genetic diversity may constrain adaptation at range margins) by measuring functional traits of leaves associated with cold tolerance, the climatic factor that controls these mangrove distributional limits. CMH predictions were confirmed only along the coastlines that conform to “abundant‐centre” assumptions and, in contrast to theory, range marginA. germinansexhibited functional traits consistent with greater cold tolerance compared to range cores. These findings support previous accounts that CMH may not be a general rule across species and that reduced neutral genetic diversity at range margins may not be a constraint to shifts in functional trait variation along climatic gradients.

    

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5. Population genetic structure and demographic history of the lone star tick, Amblyomma americanum (Ixodida: Ixodidae): New evidence supporting old records

   https://doi.org/10.1111/mec.15524  

   Lado, Paula ; Smith, Megan L. ; Carstens, Bryan C. ; Klompen, Hans ( July 2020 , Molecular Ecology)

   Range expansions are a potential outcome of changes in habitat suitability, which commonly result as a consequence of climate change. Hypotheses on such changes in the geographic distribution of a certain species can be evaluated using population genetic structure and demography. In this study we explore the population genetic structure, genetic variability, demographic history of, and habitat suitability forAmblyomma americanum, a North American tick species that is a known vector of several pathogenic microorganisms. We used a double digestion restriction site‐associated DNA sequencing technique (dd‐RAD seq) and discovered 8,181 independent single nucleotide polymorphisms (SNPs) in 189 ticks from across the geographic range of the species. Genetic diversity was low, particularly when considering the broad geographic range of this species. The edge populations were less diverse than populations belonging to the historic range, possibly indicative of a range expansion, but this hypothesis was not statistically supported by a test based on genetic data. Nonetheless, moderate levels of population structure and substructure were detected between geographic regions. For New England, demographic and species distribution models support a scenario whereA. americanumwas present in more northern locations in the past, underwent a bottleneck, and subsequently recovered. These results are consistent with a hypothesis that this species is re‐establishing in this area, rather than one focused on range expansion from the south. This hypothesis is consistent with old records describing the presence ofA. americanumin the northeastern US in the early colonial period.

    

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