Drift Load Research Articles

Environmental variance in male mating success modulates the positive versus negative impacts of sexual selection on genetic load.

Sexual selection on males is predicted to increase population fitness, and delay population extinction, when mating success negatively covaries with genetic load across individuals. However, such benefits of sexual selection could be counteracted by simultaneous increases in genome-wide drift resulting from reduced effective population size caused by increased variance in fitness. Resulting fixation of deleterious mutations could be greatest in small populations, and when environmental variation in mating traits partially decouples sexual selection from underlying genetic variation. The net consequences of sexual selection for genetic load and population persistence are therefore likely to be context dependent, but such variation has not been examined. We use a genetically explicit individual-based model to show that weak sexual selection can increase population persistence time compared to random mating. However, for stronger sexual selection such positive effects can be overturned by the detrimental effects of increased genome-wide drift. Furthermore, the relative strengths of mutation-purging and drift critically depend on the environmental variance in the male mating trait. Specifically, increasing environmental variance caused stronger sexual selection to elevate deleterious mutation fixation rate and mean selection coefficient, driving rapid accumulation of drift load and decreasing population persistence times. These results highlight an intricate balance between conflicting positive and negative consequences of sexual selection on genetic load, even in the absence of sexually antagonistic selection. They imply that environmental variances in key mating traits, and intrinsic genetic drift, should be properly factored into future theoretical and empirical studies of the evolution of population fitness under sexual selection.

Whole-genome survey reveals extensive variation in genetic diversity and inbreeding levels among peregrine falcon subspecies.

In efforts to prevent extinction, resource managers are often tasked with increasing genetic diversity in a population of concern to prevent inbreeding depression or improve adaptive potential in a changing environment. The assumption that all small populations require measures to increase their genetic diversity may be unwarranted, and limited resources for conservation may be better utilized elsewhere. We test this assumption in a case study focused on the peregrine falcon (Falco peregrinus), a cosmopolitan circumpolar species with 19 named subspecies. We used whole-genome resequencing to generate over two million single nucleotide polymorphisms (SNPs) from multiple individuals of all peregrine falcon subspecies. Our analyses revealed extensive variation among subspecies, with many island-restricted and nonmigratory populations possessing lower overall genomic diversity, elevated inbreeding coefficients (F ROH)-among the highest reported, and extensive runs of homozygosity (ROH) compared to mainland and migratory populations. Similarly, the majority of subspecies that are either nonmigratory or restricted to islands show a much longer history of low effective population size (N e). While mutational load analyses indicated an increased proportion of homozygous-derived deleterious variants (i.e., drift load) among nonmigrant and island populations compared to those that are migrant or reside on the mainland, no significant differences in the proportion of heterozygous deleterious variants (i.e., inbreeding load) was observed. Our results provide evidence that high levels of inbreeding may not be an existential threat for some populations or taxa. Additional factors such as the timing and severity of population declines are important to consider in management decisions about extinction potential.

Dynamic analysis of the subsea production system with lazy-wave risers attached to FPSO.

The lazy-wave riser is an input and output riser for a flexible development system, which is widely used in all the riser and pipeline systems. Because of the influence of various factors, its configuration description, control and motion which have a strong nonlinear character are complex during the running process of the lazy-wave riser. Reference to the specific structure and environmental parameters of a certain lazy-wave risers system with a 300 thousand tons FPSO, with the basis of the specific process of the flexible riser system at work, the lazy-wave risers were discretized into lumped mass models, combined with AQWA, the simplified dynamic model of the whole system at the depth of 2100m has been established by the large hydrodynamic analysis software OrcaFlex. The dynamic response characteristics of the lazy-wave risers have been given by using time domain coupling method. With and without the consideration of the 2nd wave drift load in the simulation process, the effects of the 2nd wave drift load on the results are obtained. The simulation results reveal the difficulty of simulation convergence caused by a large number of risers and flexible components. The 2nd order wave drift loads have a significant effect on the riser system, resulting in the increasement of the effective tension at each end of each riser. To counteract the magnitude of the FPSO response caused by such loads, the number of mooring lines needed to be increased or combined with dynamic positioning techniques to optimize the design.

Development of near-fault loading protocols for first-story steel columns in dual systems with BRBs

ABSTRACT The AISC 341–16 (2016) has loading protocols for beam-to-column moment connections and buckling-restrained braces, but it does not have a loading protocol for columns. Because a first-story column base is expected to form a plastic hinge in a building, a loading protocol for first-story columns in a dual system that comprises moment-resisting frames and buckling-restrained braced frames under varied axial force and lateral drift is required. Moreover, the symmetric cyclic loading protocols of AISC 341 do not include the effect of near-fault earthquakes, which is common in Taiwan. Therefore, the effect of near-fault earthquakes on first-story columns must be considered during the development of a loading protocol for dual systems. In the present study, four dual systems (representing 3-story, 7-story, 15-story, and 25-story frames) are subjected to 11 near-fault earthquake motions, used for developing near-fault loading protocols of first-story columns for the lateral drift and axial load responses.

Numerical Study of Wave Drift Load and Turning Characteristics of KVLCC2 Ship in Regular Waves Based on TEBEM

Maritime traffic has increased considerably in recent years, making energy efficiency and navigation safety of ships more crucial than ever. Hence, a two-time scale model based on the Taylor expansion boundary element method (TEBEM) is proposed to predict ship turning trajectories in regular waves. The maneuvering motion is calculated using a three degrees of freedom MMG model that considers the wave drift loads. TEBEM overcomes the shortcomings of the constant panel method in solving tangential induced velocity at a non-smooth boundary and that of the high-order boundary element method in dealing with a high-order derivative of the velocity potential at the corner. This significantly improves the calculation accuracy of the induced velocity and high-order derivative of velocity potential. Firstly, based on the TEBEM, the surge and sway wave drift forces and yaw moment of the KVLCC2 model with drift angle under full wave headings are calculated and compared with computational fluid dynamics results, using which the calculation accuracy of TEBEM is verified. Subsequently, the two-time scale model is used to calculate the turning trajectories of the KVLCC2 model in regular waves with different wave headings, wave frequencies, and wave steepness. The numerical results show that the drift angle has a certain effect on the wave drift loads of the ship, and the proposed model can effectively predict the ship’s turning motion in regular waves.

Mean Wave Drift Forces on a Barge-Type Floating Wind Turbine Platform with Moonpools

Barge-type platforms with moonpools are a promising type of foundation for floating offshore wind turbines due to their good seakeeping performance. In this paper, the mean wave drift force on a barge-type vertical-axis floating wind turbine with multiple moonpools was investigated through physical model testing and numerical calculations using WAMIT. The focus was on the characteristics of mean drift load and its optimization potential. The present numerical results indicated that the application of moonpools was useful in reducing horizontal mean drift force at specific frequencies, and the reason was ascribed to the significant radiation effect of the resonant water oscillations in moonpools. The observed reduction effect on mean drift force was shown to be dependent on the viscous damping of moonpool resonance. The experimental results showed that the maximum response of the mean sway drift force was reduced by the gyroscopic effect of rotations of the vertical-axis wind turbine, and this reduction effect became stronger as the rotating speed of the wind turbine increased, but was weakened as wave amplitude increased. The comparisons between experimental data and potential flow predictions indicated that viscous effects should be taken into account to reasonably estimate the mean wave drift forces on barge-type floating wind turbines.

Addressing Darwin's dilemma: Can pseudo-overdominance explain persistent inbreeding depression and load?

Darwin spent years investigating the effects of self-fertilization, concluding that "nature abhors perpetual self-fertilization." Given that selection purges inbred populations of strongly deleterious mutations and drift fixes mild mutations, why does inbreeding depression (ID) persist in highly inbred taxa and why do no purely selfing taxa exist? Background selection, associations and interference among loci, and drift within small inbred populations all limit selection while often increasing fixation. These mechanisms help to explain why more inbred populations in most species consistently show more fixed load. This drift load is manifest in the considerable heterosis regularly observed in between-population crosses. Such heterosis results in subsequent high ID, suggesting a mechanism by which small populations could retain variation and inbreeding load. Multiple deleterious recessive mutations linked in repulsion generate pseudo-overdominance. Many tightly linked load loci could generate a balanced segregating load high enough to sustain ID over many generations. Such pseudo-overdominance blocks (or "PODs") are more likely to occur in regions of low recombination. They should also result in clear genetic signatures including genomic hotspots of heterozygosity; distinct haplotypes supporting alleles at intermediate frequency; and high linkage disequilibrium in and around POD regions. Simulation and empirical studies tend to support these predictions. Additional simulations and comparative genomic analyses should explore POD dynamics in greater detail to resolve whether PODs exist in sufficient strength and number to account for why ID and load persist within inbred lineages.

Parasite-mediated selection in a natural metapopulation of Daphnia magna.

Parasite-mediated selection varying across time and space in metapopulations is expected to result in host local adaptation and the maintenance of genetic diversity in disease-related traits. However, nonadaptive processes like migration and extinction-(re)colonization dynamics might interfere with adaptive evolution. Understanding how adaptive and nonadaptive processes interact to shape genetic variability in life-history and disease-related traits can provide important insights into their evolution in subdivided populations. Here we investigate signatures of spatially fluctuating, parasite-mediated selection in a natural metapopulation of Daphnia magna. Host genotypes from infected and uninfected populations were genotyped at microsatellite markers, and phenotyped for life-history and disease traits in common garden experiments. Combining phenotypic and genotypic data a QST -FST -like analysis was conducted to test for signatures of parasite mediated selection. We observed high variation within and among populations for phenotypic traits, but neither an indication of host local adaptation nor a cost of resistance. Infected populations have a higher gene diversity (Hs) than uninfected populations and Hs is strongly positively correlated with fitness. These results suggest a strong parasite effect on reducing population level inbreeding. We discuss how stochastic processes related to frequent extinction-(re)colonization dynamics as well as host and parasite migration impede the evolution of resistance in the infected populations. We suggest that the genetic and phenotypic patterns of variation are a product of dynamic changes in the host gene pool caused by the interaction of colonization bottlenecks, inbreeding, immigration, hybrid vigor, rare host genotype advantage and parasitism. Our study highlights the effect of the parasite in ameliorating the negative fitness consequences caused by the high drift load in this metapopulation.

Selfing ability and drift load evolve with range expansion

Colonization at expanding range edges often involves few founders, reducing effective population size. This process can promote the evolution of self‐fertilization, but implicating historical processes as drivers of trait evolution is often difficult and requires an explicit model of biogeographic history. In plants, contemporary limits to outcrossing are often invoked as evolutionary drivers of self‐fertilization, but historical expansions may shape mating system diversity, with leading‐edge populations evolving elevated selfing ability. In a widespread plant, Campanula americana, we identified a glacial refugium in the southern Appalachian Mountains from spatial patterns of genetic drift among 24 populations. Populations farther from this refugium have smaller effective sizes and fewer rare alleles. They also displayed elevated heterosis in among‐population crosses, reflecting the accumulation of deleterious mutations during range expansion. Although populations with elevated heterosis had reduced segregating mutation load, the magnitude of inbreeding depression lacked geographic pattern. The ability to self‐fertilize was strongly positively correlated with the distance from the refugium and mutation accumulation—a pattern that contrasts sharply with contemporary mate and pollinator limitation. In this and other species, diversity in sexual systems may reflect the legacy of evolution in small, colonizing populations, with little or no relation to the ecology of modern populations.

Biophysics and population size constrains speciation in an evolutionary model of developmental system drift.

Developmental system drift is a likely mechanism for the origin of hybrid incompatibilities between closely related species. We examine here the detailed mechanistic basis of hybrid incompatibilities between two allopatric lineages, for a genotype-phenotype map of developmental system drift under stabilising selection, where an organismal phenotype is conserved, but the underlying molecular phenotypes and genotype can drift. This leads to number of emergent phenomenon not obtainable by modelling genotype or phenotype alone. Our results show that: 1) speciation is more rapid at smaller population sizes with a characteristic, Orr-like, power law, but at large population sizes slow, characterised by a sub-diffusive growth law; 2) the molecular phenotypes under weakest selection contribute to the earliest incompatibilities; and 3) pair-wise incompatibilities dominate over higher order, contrary to previous predictions that the latter should dominate. The population size effect we find is consistent with previous results on allopatric divergence of transcription factor-DNA binding, where smaller populations have common ancestors with a larger drift load because genetic drift favours phenotypes which have a larger number of genotypes (higher sequence entropy) over more fit phenotypes which have far fewer genotypes; this means less substitutions are required in either lineage before incompatibilities arise. Overall, our results indicate that biophysics and population size provide a much stronger constraint to speciation than suggested by previous models, and point to a general mechanistic principle of how incompatibilities arise the under stabilising selection for an organismal phenotype.

Population structure promotes the evolution of costly sex in artificial gene networks.

We build on previous observations that Hill-Robertson interference generates an advantage of sex that, in structured populations, can be large enough to explain the evolutionary maintenance of costly sex. We employed a gene network model that explicitly incorporates interactions between genes. Mutations in the gene networks have variable effects that depend on the genetic background in which they appear. Consequently, our simulations include two costs of sex-recombination and migration loads-that were missing from previous studies of the evolution of costly sex. Our results suggest a critical role for population structure that lies in its ability to align the long- and short-term advantages of sex. We show that the addition of population structure favored the evolution of sex by disproportionately decreasing the equilibrium mean fitness of asexual populations, primarily by increasing the strength of Muller's Ratchet. Population structure also increased the ability of the short-term advantage of sex to counter the primary limit to the evolution of sex in the gene network model-recombination load. On the other hand, highly structured populations experienced migration load in the form of Dobzhansky-Muller incompatibilities, decreasing the effective rate of migration between demes and, consequently, accelerating the accumulation of drift load in the sexual populations.

Evaluation of reduced beam section (RBS) on castellated beam subjected to a seismic load

The ability of the beam to receive a seismic load can be explained in the experimental study of a castellated beam using RBS (reduced beam section) as a damping factor on earthquake resistant building structure. Evaluation of the structure of the exterior connections was done by subjecting cyclic loads gradually until collapsed to evaluate castellated beam test specimens with and without RBS. The experimental test conducted by cyclic test aimed to determine the ability of deformation and the force that worked due to the influence of RBS placement from the column face on the castellated beam. Investment of RBS positions on the columns in the column face shows that failures occurred due to the bending on the shaft. The results obtained by each load to the drift ratio were as follows. On the test specimens with RBS, the melting point was at 8 kN loading with a 1.05% drift ratio, 2.1% drift load at a 9 kN load, 3.20% drift at a peak load of 12 kN, and necking phase at 10.53 kN load with 4.17% drift.

Side Wall Effects on the Hydrodynamics of a Floating Body by Image Green Function Based on TEBEM

A novel numerical model based on the image Green function and first-order Taylor expansion boundary element method (TEBEM), which can improve the accuracy of the hydrodynamic simulation for the non-smooth body, was developed to calculate the side wall effects on first-order motion responses and second-order drift loads upon offshore structures in the wave tank. This model was confirmed by comparing it to the results from experiments on hydrodynamic coefficients, namely the first-order motion response and second-order drift load upon a hemisphere, prolate spheroid, and box-shaped barge in the wave tank. Then, the hydrodynamics of the KVLCC2 model were also calculated in two wave tanks with different widths. It was concluded that this model can predict the hydrodynamics for offshore structures effectively, and the side wall has a significant impact on the first-order quantities and second-order drift loads, which satisfied the resonant rule.

Strip Model Analysis for Steel Plate Shear Walls in Earthquake Resistant Structures

Unstiffened Steel Plate Shear Walls (SPSWs) are very effective structural systems designed to resist lateral forces. SPSW systems consist of thin web plates infilled within frames of steel horizontal and vertical boundary elements. The thin unstiffened web plates are expected to buckle in shear and to develop diagonal tension field after buckling under the action of horizontal loads. For unstiffened steel plates, buckling in shear occurs in the elastic range at low stress levels. This behaviour provides strength, stiffness and ductility and allows to have an appropriate level of energy dissipation through tension yielding of the web plates. This paper assesses the inelastic structural response and behaviour of Steel Plate Shear Wall systems using both a modified strip model approach and a new simplified strip model for only beam connected SPSWs. Both models are developed with plasticity concentrated elements and the performed analyses include the nonlinear behaviour of strips, also considering the compressive forces effects over the strip model elements. This research indicates fundamental aspects of the seismic performance of Steel Plate Shear Wall systems, such as energy dissipation capacity, panel ductility demand, seismic inter-story drift and design load demands in Vertical Boundary Elements (VBE) and Horizontal Boundary Elements (HBE) of the frame. The results obtained from the use of these models are compared with selected experimental and numerical results to enrich the research conclusions.

The power of evolutionary rescue is constrained by genetic load.

The risk of extinction faced by small isolated populations in changing environments can be reduced by rapid adaptation and subsequent growth to larger, less vulnerable sizes. Whether this process, called evolutionary rescue, is able to reduce extinction risk and sustain population growth over multiple generations is largely unknown. To understand the consequences of adaptive evolution as well as maladaptive processes in small isolated populations, we subjected experimental Tribolium castaneum populations founded with 10 or 40 individuals to novel environments, one more favorable, and one resource poor, and either allowed evolution, or constrained it by replacing individuals one‐for‐one each generation with those from a large population maintained in the natal environment. Replacement individuals spent one generation in the target novel environment before use to standardize effects due to the parental environment. After eight generations we mixed a subset of surviving populations to facilitate admixture, allowing us to estimate drift load by comparing performance of mixed to unmixed groups. Evolving populations had reduced extinction rates, and increased population sizes in the first four to five generations compared to populations where evolution was constrained. Performance of evolving populations subsequently declined. Admixture restored their performance, indicating high drift load that may have overwhelmed the beneficial effects of adaptation in evolving populations. Our results indicate that evolution may quickly reduce extinction risk and increase population sizes, but suggest that relying solely on adaptation from standing genetic variation may not provide long‐term benefits to small isolated populations of diploid sexual species, and that active management facilitating gene flow may be necessary for longer term persistence.

A central limit theorem for the spatial $\Lambda $-Fleming-Viot process with selection

We study the evolution of gene frequencies in a population living in $\mathbb{R}^d$, modelled by the spatial Lambda Fleming-Viot process with natural selection (Barton, Etheridge and Veber, 2010 and Etheridge, Veber and Yu, 2014). We suppose that the population is divided into two genetic types, $a$ and $A$, and consider the proportion of the population which is of type $a$ at each spatial location. If we let both the selection intensity and the fraction of individuals replaced during reproduction events tend to zero, the process can be rescaled so as to converge to the solution to a reaction-diffusion equation (typically the Fisher-KPP equation, as in Etheridge, Veber and Yu, 2014). We show that the rescaled fluctuations converge in distribution to the solution to a linear stochastic partial differential equation. Depending on whether offspring dispersal is only local or if large scale extinction-recolonization events are allowed to take place, the limiting equation is either the stochastic heat equation with a linear drift term driven by space-time white noise or the corresponding fractional heat equation driven by a coloured noise which is white in time. If individuals are diploid (i.e. either $AA$, $Aa$ or $aa$) and if natural selection favours heterozygous ($Aa$) individuals, a stable intermediate gene frequency is maintained in the population. We give estimates for the asymptotic effect of random fluctuations around the equilibrium frequency on the local average fitness in the population. In particular, we find that the size of this effect - known as the drift load - depends crucially on the dimension $d$ of the space in which the population evolves, and is reduced relative to the case without spatial structure.

Benefits of gene flow are mediated by individual variability in self-compatibility in small isolated populations of an endemic plant species.

Many rare and endemic species experience increased rates of self‐fertilization and mating among close relatives as a consequence of existing in small populations within isolated habitat patches. Variability in self‐compatibility among individuals within populations may reflect adaptation to local demography and genetic architecture, inbreeding, or drift. We use experimental hand‐pollinations under natural field conditions to assess the effects of gene flow in 21 populations of the central Appalachian endemic Trifolium virginicum that varied in population size and degree of isolation. We quantified the effects of distance from pollen source on pollination success and fruit set. Rates of self‐compatibility varied dramatically among maternal plants, ranging from 0% to 100%. This variation was unrelated to population size or degree of isolation. Nearly continuous variation in the success of selfing and near‐cross‐matings via hand pollination suggests that T. virginicum expresses pseudo‐self‐fertility, whereby plants carrying the same S‐allele mate successfully by altering the self‐incompatibility reaction. However, outcrossing among populations produced significantly higher fruit set than within populations, an indication of drift load. These results are consistent with strong selection acting to break down self‐incompatibility in these small populations and/or early‐acting inbreeding depression expressed upon selfing.

How does epistasis influence the response to selection?

Much of quantitative genetics is based on the 'infinitesimal model', under which selection has a negligible effect on the genetic variance. This is typically justified by assuming a very large number of loci with additive effects. However, it applies even when genes interact, provided that the number of loci is large enough that selection on each of them is weak relative to random drift. In the long term, directional selection will change allele frequencies, but even then, the effects of epistasis on the ultimate change in trait mean due to selection may be modest. Stabilising selection can maintain many traits close to their optima, even when the underlying alleles are weakly selected. However, the number of traits that can be optimised is apparently limited to ~4Ne by the 'drift load', and this is hard to reconcile with the apparent complexity of many organisms. Just as for the mutation load, this limit can be evaded by a particular form of negative epistasis. A more robust limit is set by the variance in reproductive success. This suggests that selection accumulates information most efficiently in the infinitesimal regime, when selection on individual alleles is weak, and comparable with random drift. A review of evidence on selection strength suggests that although most variance in fitness may be because of alleles with large Nes, substantial amounts of adaptation may be because of alleles in the infinitesimal regime, in which epistasis has modest effects.

Inbreeding depression and drift load in small populations at demographic disequilibrium

Inbreeding depression is a major driver of mating system evolution and has critical implications for population viability. Theoretical and empirical attention has been paid to predicting how inbreeding depression varies with population size. Lower inbreeding depression is predicted in small populations at equilibrium, primarily due to higher inbreeding rates facilitating purging and/or fixation of deleterious alleles (drift load), but predictions at demographic and genetic disequilibrium are less clear. In this study, we experimentally evaluate how lifetime inbreeding depression and drift load, estimated by heterosis, vary with census (Nc ) and effective (estimated as genetic diversity, He ) population size across six populations of the biennial Sabatia angularis as well as present novel models of inbreeding depression and heterosis under varying demographic scenarios at disequilibrium (fragmentation, bottlenecks, disturbances). Our experimental study reveals high average inbreeding depression and heterosis across populations. Across our small sample, heterosis declined with He , as predicted, whereas inbreeding depression did not vary with He and actually decreased with Nc . Our theoretical results demonstrate that inbreeding depression and heterosis levels can vary widely across populations at disequilibrium despite similar He and highlight that joint demographic and genetic dynamics are key to predicting patterns of genetic load in nonequilibrium systems.

Impact of season-long water abstraction on invertebrate drift composition and concentration

Surface water abstraction from rivers for irrigated agriculture is one of the largest uses of freshwater resources in the world. Water abstraction has important impacts on the structure of riverine assemblages. However, little work has examined the chronic, season-long impacts on ecosystem functions. Invertebrate drift is an important ecosystem function of river systems influencing nutrient cycling, food webs, and invertebrate population dynamics. We examined the season-long impact of reduced discharge resulting from multiple points of abstraction on drift assemblage composition, concentration, and total drift load. Early in the season, water abstraction had little impact on drift assemblage composition. However, later in the irrigation season, the drift assemblage at sites impacted by water abstraction diverged from upstream, control sites. The degree of change in assemblage composition at impacted sites was related to the amount of water abstracted such that sites with the lowest discharge also had assemblages that differed most strongly from control sites. Drift assemblages at impacted sites became dominated by tolerant microcrustaceans. In addition, water abstraction resulted in an increase in drift concentration (ind./m3). However, despite this increase in concentration at impacted sites, total drift load (# of invertebrates drifting in the river) decreased with decreasing discharge.