Subspecies are often used in ways that require their evolutionary independence, for example as proxies for units of conservation. Mitochondrial DNA sequence data reveal that 97% of continentally ...distributed avian subspecies lack the population genetic structure indicative of a distinct evolutionary unit. Subspecies considered threatened or endangered, some of which have been targets of expensive restoration efforts, also generally lack genetic distinctiveness. Although sequence data show that species include 1.9 historically significant units on average, these units are not reflected by current subspecies nomenclature. Yet, it is these unnamed units and not named subspecies that should play a major role in guiding conservation efforts and in identifying biological diversity. Thus, a massive reorganization of classifications is required so that the lowest ranks, be they species or subspecies, reflect evolutionary diversity. Until such reorganization is accomplished, the subspecies rank will continue to hinder progress in taxonomy, evolutionary studies and especially conservation.
We present experimental evidence of a transverse thermopower, or planar Nernst effect, in ferromagnetic metal thin films driven by thermal gradients applied in the plane of the films. Samples of 20 ...nm thick Ni and Ni(80)Fe(20) were deposited on 500 nm thick suspended Si-N thermal isolation platforms with integrated platinum strips designed originally to allow measurement of thermally generated spin currents (the spin Seebeck effect). The low thermal conductivity of the thin supporting Si-N structure results in an essentially 2D geometry that approaches the zero substrate limit, dramatically reducing the contribution of thermal gradients perpendicular to the sample plane typically found in similar experiments on bulk substrates. The voltage on the platinum strips generated transverse to the applied thermal gradient (V(T)) is linear with increasing ΔT and exhibits a sign reversal on hot and cold sides of the sample. However, V(T) is always even in applied magnetic field and shows a sinθ cosθ angular dependence, both key indicators of the planar Nernst effect. Within the 5 nV estimated error of our experiment there is no evidence of a signal from the spin Seebeck effect, which would have cosθ angular dependence, suggesting a reduced spin Seebeck coefficient in a planar, entirely thin-film geometry.
Late Pleistocene glaciations have been ascribed a dominant role in sculpting present-day diversity and distributions of North American vertebrates. Molecular comparisons of recently diverged sister ...species now permit a test of this assertion. The Late Pleistocene Origins model predicts a mitochondrial DNA divergence value of less than 0.5 percent for avian sister species of Late Pleistocene origin. Instead, the average mitochondrial DNA sequence divergence for 35 such songbird species pairs is 5.1 percent, which exceeds the predicted value by a factor of 10. Molecular data suggest a relatively protracted history of speciation events among North American songbirds over the past 5 million years.
We present magnetic-field-dependent measurements of thermopower, alpha (H), and resistance, R(H), for Ni sub(80) Fe sub(20) and Ni thin films. We conducted these experiments in fields oriented ...parallel and perpendicular to the applied thermal gradient, HamiltonoperatorT, for alpha (H) and applied current for R(H). We deposited the 20-nm-thick films on 500-nm-thick suspended Si-N thermal isolation platforms that enable in-plane measurements of thermal and electrical properties of thin films. Both alpha (H) and R(H) in Ni-Fe and Ni films exhibit evidence of spin-dependent scattering through an even field dependence and a linear proportionality between and alpha (H) and 1/R(H). Finally, we use alpha (H sub(||)) and alpha (H sub(bottom)) to determine the planar Nernst coefficient in Ni-Fe and Ni thin films and use this coefficient to predict the size of the planar Nernst effect (PNE) in the micromachined platform. The measured field dependence of the PNE is well matched by the prediction obtained from our alpha (H) results.
ABSTRACT
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) were abundant in surface sediments of freshwater lakes and in catchment soils at altitudes from 10 to 2020 m in New Zealand. ...Significant differences in brGDGT compositions between lake sediments and soils indicate sources from separate microbial habitats. An expanded modern calibration dataset comprising 33 lakes has enabled a revised calibration function for determining past mean annual air temperature (MAT) from brGDGTs in New Zealand lake sediments: MAT (°C) = −31.664 × MBTm + 16.252 (n = 30). The function uses a modified methylation index of branched tetraethers (MBTm) that incorporates brGDGT III in the numerator to overcome the lower correlation found between our larger dataset and the unmodified MBT which had been used for previous calibrations. Calibrations combining the cyclization index of branched tetraethers (CBT) and the MBTm or using only certain brGDGTs are possible but have limitations. The revised function shows slightly higher correlation with MAT (R2 = 0.75) than previous calibrations, which were based on nine sites. The refined calibration function is applied to a ∼16 000‐year lake sediment sequence from northern South Island, New Zealand, and yields temperature reconstructions that are consistent with independently derived climate trends from the same sequence.
Helicoverpa armigera (Hübner) is one of the most important agricultural pests in the world. This historically Old World species was first reported in Brazil in 2013 and has since spread throughout ...much of South America and into the Caribbean. Throughout North America, H. armigera surveys are ongoing to detect any incursions. Each trap is capable of capturing hundreds of native Helicoverpa zea (Boddie). The two species cannot be separated without genitalic dissection or molecular methods. A ddPCR assay is currently used to screen large trap samples, but this equipment is relatively uncommon and expensive. Here, we optimized a newly designed assay for accurate and repeatable detection of H. armigera in bulk samples across both ddPCR and less costly, and more common, real-time PCR methods. Improvements over previously designed assays were sought through multiple means. Our results suggest bulk real-time PCR assays can be improved through changes in DNA extraction and purification, so that real-time PCR can be substituted for ddPCR in screening projects. While ddPCR remains a more sensitive method for detection of H. armigera in bulk samples, the improvements in assay design, DNA extraction, and purification presented here also enhance assay performance over previous protocols.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Determining the mechanisms responsible for the distribution of genetic diversity in natural populations has occupied a central role in molecular evolution. Our study was motivated by the ...unprecedented observation that a widespread Eurasian flycatcher, Ficedula albicilla, exhibited no variation at the mitochondrial DNA (mtDNA) ND2 gene in 75 individuals sampled over a 5000-km distance. In contrast, its sister species, F. parva, had low but considerably higher levels of mtDNA variation. We assessed whether natural selection or demographic factors could explain the absence of mtDNA variation in F. albicilla. Eighteen nuclear genes were sequenced to estimate the two species' phylogeographic histories, and for comparison to the mtDNA data. Multilocus coalescence analyses suggested that F. albicilla experienced a population expansion perhaps following a population bottleneck. Simulations based on this demographic history, however, did not replicate the extremely low level of mtDNA variation. Historical range changes based on ecological niche models also failed to explain the observed mtDNA patterns. Neutrality tests (DHEW and ML-HKA) suggested a non-neutral pattern in the mtDNA of F. albicilla. We found a transmembrane-skewed distribution of nonsynonymous substitutions between the two species, three of which caused functional change; the results implied that positive selection could have targeted mtDNA. Several lines of evidence support selection rather than demographic history as the main force influencing the patterns of mtDNA variation. Despite the influence of natural selection, many of the phylogeographic inferences derived from mtDNA were robust, including species limits and a high level of gene flow among populations within species.
Historical biogeography and comparative phylogeography have much in common. Both seek to discover common historical patterns in the elements of biotas, although typically at different tiers of ...evolutionary history. Comparative phylogeography is based on phylogeographic analyses of multiple taxa, usually widespread species. By comparing the phylogeographic structures of numerous widespread sympatric species, one can infer whether the current fauna has been historically stable, as evidenced by the relative frequency of geographically congruent reciprocally monophyletic groups. Alternatively, if species distributions are ephemeral over evolutionary time, a mixture of phylogeographic structures is expected. Coalescence analyses contribute information about history irrespective of whether haplotype phylogenies are structured or not. In the aridlands of North America, several isolating events are evident in the phylogeographic patterns of birds, mammals and herps. A mid-peninsular seaway in Baja California, dated at ca. one million years before present, had a pervasive effect, with 13 of 16 assayed species showing a concordant split. Hence, this community appears to have been a stable assemblage of species over the past one million years. In contrast, the avifauna of the Sonoran-Chihuahuan deserts consists of two species with a concordant split and three other species that are undifferentiated across both deserts. Hence, the species in this area have had different histories. The Baja biota appears to resemble its ancestral configuration to a greater degree than the Sonoran-Chihuahuan one. A deeper evolutionary event separated taxa in Baja California from the eastern deserts, showing that the aridlands fauna was affected by events at different times resulting in overlain tiers of history.
Pleistocene effects on North American songbird evolution Klicka, John; Zink, Robert M.
Proceedings - Royal Society. Biological sciences/Proceedings - Royal Society. Biological Sciences,
04/1999, Letnik:
266, Številka:
1420
Journal Article
Recenzirano
Odprti dostop
Recent studies have used comparisons of mitochondrial DNA (mtDNA) sequence divergence among populations and species to test existing hypotheses about avian evolution during the Pleistocene epoch. In ...1998, Avise and Walker concluded that the Pleistocene was an important time for avian evolution, including the initiation of phylogeographic separations and the completion of speciation events that began in the Pliocene. The study implied that these conclusions conflicted with the study, in 1997, by Klicka and Zink, which concluded that most species pairs previously thought to have originated in the past 250 000 years were much older. The two studies are complementary in the sense that Avise and Walker dealt primarily with phylogeographic (intraspecific) separations. Furthermore, Klicka and Zink concentrated on the inception of divergences whereas the Avise and Walker focused on the timing of the completion of speciation. To accomplish this, Avise and Walker analysed 'phylogroups', geographically coherent subsets of biological species in which mtDNA haplotypes exhibit reciprocal monophyly. The study used the average interphylogroup mtDNA distance (0.027), calibrated at 2% per million years, to conclude that speciation required on average one million years to complete. Hence, speciation events begun in the Late Pliocene would have been completed in the mid- to late Pleistocene. Although we appreciate the extended nature of the speciation process and Avise and Walker's insightful attempt to estimate its duration, we conclude that their value was an overestimate by a factor of two. In particular we question whether phylogroups can be used in the novel evolutionary role that Avise and Walker envisioned, because of the vagaries of taxonomic practices and lack of consensus regarding species concepts. To extend their analysis of intraspecific, phylogeographic separations, we compiled previously analysed and newly available data for divergence times for North American songbird (order Passeriformes) phylogroups. More than 80% were initiated at least one million years ago, which is inconsistent with the late Pleistocene origins model previously rejected by Klicka and Zink mentioned above. Although some divergence events can be traced to the late Pleistocene, the significance of the distribution must be judged with reference to a null model. Whether the Pleistocene was a profound time for avian phylogeographic differentiation is at present unknown.
The stomach bacterium Helicobacter pylori is one of the most prevalent human pathogens. It has dispersed globally with its human host, resulting in a distinct phylogeographic pattern that can be used ...to reconstruct both recent and ancient human migrations. The extant European population of H. pylori is known to be a hybrid between Asian and African bacteria, but there exist different hypotheses about when and where the hybridization took place, reflecting the complex demographic history of Europeans. Here, we present a 5300-year-old H. pylori genome from a European Copper Age glacier mummy. The "Iceman" H. pylori is a nearly pure representative of the bacterial population of Asian origin that existed in Europe before hybridization, suggesting that the African population arrived in Europe within the past few thousand years.
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