Evolutionary rescue: an emerging focus at the intersection between ecology and evolution

Evolutionary adaptation under climate change: Aedes sp. demonstrates potential to adapt to warming

Climate warming is expected to shift the distributions of mosquitoes and mosquito-borne diseases, facilitating expansions at cool range edges and contractions at warm range edges. However, whether mosquito populations could maintain their warm edges through evolutionary adaptation remains unknown. Here, we investigate the potential for thermal adaptation in Aedes sierrensis, a congener of the major disease vector species that experiences large thermal gradients in its native range, by assaying tolerance to prolonged and acute heat exposure, and its genetic basis in a diverse, field-derived population. We found pervasive evidence of heritable genetic variation in acute heat tolerance, which phenotypically trades off with tolerance to prolonged heat exposure. A simple evolutionary model based on our data shows that the estimated maximum rate of evolutionary adaptation in mosquito heat tolerance typically exceeds that of projected climate warming under idealized conditions. Our findings indicate that natural mosquito populations may have the potential to track projected warming via genetic adaptation. Prior climate-based projections may thus underestimate the range of mosquito and mosquito-borne disease distributions under future climate conditions.

Reprint: Acclimatization and Adaptive Capacity of Marine Species in a Changing Ocean

To persist in an ocean changing in temperature, pH and other stressors related to climate change, many marine species will likely need to acclimatize or adapt to avoid extinction. If marine populations possess adequate genetic variation in tolerance to climate change stressors, species might be able to adapt to environmental change. Marine climate change research is moving away from single life stage studies where individuals are directly placed into projected scenarios ('future shock' approach), to focus on the adaptive potential of populations in an ocean that will gradually change over coming decades. This review summarizes studies that consider the adaptive potential of marine invertebrates to climate change stressors and the methods that have been applied to this research, including quantitative genetics, laboratory selection studies and trans- and multigenerational experiments. Phenotypic plasticity is likely to contribute to population persistence providing time for genetic adaptation to occur. Transgenerational and epigenetic effects indicate that the environmental and physiological history of the parents can affect offspring performance. There is a need for long-term, multigenerational experiments to determine the influence of phenotypic plasticity, genetic variation and transgenerational effects on species' capacity to persist in a changing ocean. However, multigenerational studies are only practicable for short generation species. Consideration of multiple morphological and physiological traits, including changes in molecular processes (eg, DNA methylation) and long-term studies that facilitate acclimatization will be essential in making informed predictions of how the seascape and marine communities will be altered by climate change.

Effect of the Seed Bank on Evolutionary Rescue in Small Populations: Univariate and Multivariate Demogenetic Dynamics

AbstractUnder global change, the impact of seed banks on evolutionary rescue is uncertain. They buffer plant populations from demographic and genetic stochasticity but extend generation time and can become a reservoir of maladapted alleles. We built analytical and individual-based models to predict the effect of seed banks on the persistence of small annual plant populations facing an abrupt or sustained directional change in uni- or multivariate trait optima. Demogenetic dynamics predict that under most scenarios seed banks increase the lag yet enhance persistence to 200-250 years by absorbing demographic losses. Simulations indicate that the seed bank has a minimal impact on the genetic skew, although we suggest that this result could depend on the fitness component under selection. Our multivariate model reveals that by enlarging and reshaping the G matrix, seed banks can diminish the impact of mutational correlation and even accelerate adaptation under antagonistic pleiotropy relative to populations without a bank. We illustrate how the magnitude of optimum fluctuations, type and degree of optimum change, selection strength, and vital rates are weights that tip the scales determining persistence. Finally, our work highlights that migration from the past is not maladaptative when optimum fluctuations are large enough to create stepping stones to the new optimum.

Conserving Evolutionary Potential: Combining Landscape Genomics with Established Methods to Inform Plant Conservation

Biodiversity conservation requires conserving evolutionary potential-the capacity for wild populations to adapt. Understanding genetic diversity and evolutionary dynamics is critical for informing conservation decisions that enhance adaptability and persistence under environmental change. We review how emerging landscape genomic methods provide plant conservation programs with insights into evolutionary dynamics, including local adaptation and its environmental drivers. Landscape genomic approaches that explore relationships between genomic variation and environments complement rather than replace established population genomic and common garden approaches for assessing adaptive phenotypic variation, population structure, gene flow, and demography. Collectively, these approaches inform conservation actions, including genetic rescue, maladaptation prediction, and assisted gene flow. The greatest on-the-ground impacts from such studies will be realized when conservation practitioners are actively engaged in research and monitoring. Understanding the evolutionary dynamics shaping the genetic diversity of wild plant populations will inform plant conservation decisions that enhance the adaptability and persistence of species in an uncertain future.

Immigration allows population persistence and maintains genetic diversity despite an attempted experimental extinction

Widespread fragmentation and degradation of habitats make organisms increasingly vulnerable to declines in population size. Immigration is a key process potentially affecting the rescue and persistence of populations in the face of such pressures. Field research addressing severe demographic declines in the context of immigration among interconnected local populations is limited owing to difficulties in detecting such demographic events and the need for long-term monitoring of populations. In a 17-subpopulation metapopulation of the butterfly, Parnassius smintheus, all adults observed in two adjacent patches were removed over eight consecutive generations. Despite this severe and long-term reduction in survival and reproduction, the targeted populations did not go extinct. Here, we use genetic data to assess the role of immigration versus in situ reproduction in allowing the persistence of these populations. We genotyped 471 samples collected from the targeted populations throughout the removal experiment at 152 single nucleotide polymorphisms. We found no reduction in the genetic diversity of the targeted populations over time, but a decrease in the number of loci in Hardy-Weinberg equilibrium, consistent with a high level of immigration from multiple surrounding populations. Our results highlight the role of connectivity and movement in making metapopulations resilient to even severe and protracted localized population reductions.

Adaptive responses to habitat change: Theory and tests with field experiments

Habitat loss and change are often implicated as the primary causes of species extinction. Although any population can be instantly imperiled by catastrophe, most habitat loss occurs gradually, thus enabling affected individuals an adaptive advantage to occupy the best of their dwindling opportunities. I demonstrate how to infer the advantage between two habitats for any density and frequency-dependent strategy of habitat selection. I explore the concept of an Adaptive Dispersal Strategy Landscape to reveal the Evolutionarily Stable Strategy separately for ideal-free and ideal preemptive habitat selectors. Both solutions reveal an initially counterintuitive expectation that individuals living at high density gain insufficient adaptive advantage to disperse from a deteriorating habitat. Adaptive dispersal is constrained at high density because habitats of better quality are fully occupied. I test the theory with measures of movement and foraging in crossover experiments on a seminatural population of meadow voles. The experiment allowed the voles to choose among patches and between enclosures in which I differentially manipulated food and shelter. Although photographs from an infrared camera documented voles venturing from one habitat to the other, none became resident. Voles preferentially foraged in the richer of the two enclosures, even when I reversed treatments, and they foraged more in patches protected by mulched straw. The adaptive advantage of dispersal using a surrogate fitness proxy based on the voles' giving-up densities mirrored that generated by theory. The convergence between theory and experiment yields much-needed insight into our ability to test, predict, and hopefully resolve, the ecological, evolutionary, and conservation consequences of habitat loss.

Evolutionary rescue in resistance to pesticides

Evolutionary rescue occurs when the genetic evolution of adaptation saves a population from decline or extinction after environmental change. The evolution of resistance to pesticides is a special scenario of abrupt environmental change, where rescue occurs under (very) strong selection for one or a few de novo resistance mutations of large effect. Here, a population genetic model of evolutionary rescue with density-dependent population change is developed, with a focus on deriving results that are important to resistance management. Massive stochastic simulations are used to generate observations, which are accurately predicted using analytical approximations. Key results include the probability density function for the time to resistance and the probability of population extinction. The distribution of resistance times shows a lag period, a narrow peak and a long tail. Surprisingly, the mean time to resistance can increase with the strength of selection because, if a mutation does not occur early on, then its emergence is delayed by the pesticide reducing the population size. The probability of population extinction shows a sharp transition, in that when extinction is possible, it is also highly likely. Consequently, population suppression and (local) eradication can be theoretically achievable goals, as novel strategies to delay resistance evolution.

Warming-induced "plastic floors" improve hypoxia vulnerability, not aerobic scope, in red drum (Sciaenops ocellatus)

Ocean warming is a prevailing threat to marine ectotherms. Recently the "plastic floors, concrete ceilings" hypothesis was proposed, which suggests that a warmed fish will acclimate to higher temperatures by reducing standard metabolic rate (SMR) while keeping maximum metabolic rate (MMR) stable, therefore improving aerobic scope (AS). Here we evaluated this hypothesis on red drum (Sciaenops ocellatus) while incorporating measures of hypoxia vulnerability (critical oxygen threshold; Pcrit) and mitochondrial performance. Fish were subjected to a 12-week acclimation to 20 °C or 28 °C. Respirometry was performed every 4 weeks to obtain metabolic rate and Pcrit; mitochondrial respirometry was performed on liver and heart samples at the end of the acclimation. 28 °C fish had a significantly higher SMR, MMR, and Pcrit than 20 °C controls at time 0, but SMR declined by 36.2 % over the 12-week acclimation. No change in SMR was observed in the control treatment. Contrary to expectations, SMR suppression did not improve AS relative to time 0 owing to a progressive decline in MMR over acclimation time. Pcrit decreased by 27.2 % in the warm-acclimated fishes, which resulted in temperature treatments having statistically similar values by 12-weeks. No differences in mitochondrial traits were observed in the heart - despite a Δ8 °C assay temperature - while liver respiratory and coupling control ratios were significantly improved, suggesting that mitochondrial plasticity may contribute to the reduced SMR with warming. Overall, this work suggests that warming induced metabolic suppression offsets the deleterious consequences of high oxygen demand on hypoxia vulnerability, and in so doing greatly expands the theoretical range of metabolically available habitats for red drum.

Gene expression plasticity facilitates acclimatization of a long-lived Caribbean coral across divergent reef environments

Local adaptation can increase fitness under stable environmental conditions. However, in rapidly changing environments, compensatory mechanisms enabled through plasticity may better promote fitness. Climate change is causing devastating impacts on coral reefs globally and understanding the potential for adaptive and plastic responses is critical for reef management. We conducted a four-year, three-way reciprocal transplant of the Caribbean coral Siderastrea siderea across forereef, backreef, and nearshore populations in Belize to investigate the potential for environmental specialization versus plasticity in this species. Corals maintained high survival within forereef and backreef environments, but transplantation to nearshore environments resulted in high mortality, suggesting that nearshore environments present strong environmental selection. Only forereef-sourced corals demonstrated evidence of environmental specialization, exhibiting the highest growth in the forereef. Gene expression profiling 3.5 years post-transplantation revealed that transplanted coral hosts exhibited profiles more similar to other corals in the same reef environment, regardless of their source location, suggesting that transcriptome plasticity facilitates acclimatization to environmental change in S. siderea. In contrast, algal symbiont (Cladocopium goreaui) gene expression showcased functional variation between source locations that was maintained post-transplantation. Our findings suggest limited acclimatory capacity of some S. siderea populations under strong environmental selection and highlight the potential limits of coral physiological plasticity in reef restoration.

Incorporating eco-evolutionary information into species distribution models provides comprehensive predictions of species range shifts under climate change

As climate changes increasingly influence species distributions, ecosystem functions, and biodiversity, the urgency to understand how species' ranges shift under those changes is great. Species distribution models (SDMs) are vital approaches that can predict species distributions under changing climates. However, SDMs based on the species' current occurrences may underestimate the species' climatic tolerances. Integrating species' realized niches at different periods, also known as multi-temporal calibration, can provide an estimation closer to its fundamental niche. Based on this, we further proposed an integrated framework that combines eco-evolutionary data and SDMs (phylogenetically-informed SDMs) to provide comprehensive predictions of species range shifts under climate change. To evaluate our approach's performance, we applied it to a group of related species, the Chrysanthemum zawadskii species complex (Anthemidae, Asteracee). First, we investigated the niche differentiation between species and intraspecific lineages of the complex and estimated their rates of niche evolution. Next, using both standard SDMs and our phylogenetically-informed SDMs, we generated predictions of suitability areas for all species and lineages and compared the results. Finally, we reconstructed the historical range dynamics for the species of this complex. Our results showed that the species and intraspecific lineages of the complex had varying degrees of niche differentiation and different rates of niche evolution. Lineage-level SDMs can provide more realistic predictions for species with intraspecific differentiation than species-level models can. The phylogenetically-informed SDMs provided more complete environmental envelopes and predicted broader potential distributions for all species than the standard SDMs did. Range dynamics varied among the species that have different rates of niche evolution. Our framework integrating eco-evolutionary data and SDMs contributes to a better understanding of the species' responses to climate change and can help to make more targeted conservation efforts for the target species under climate change, particularly for rare species.

Demographic feedbacks during evolutionary rescue can slow or speed adaptive evolution

Populations declining toward extinction can persist via genetic adaptation in a process called evolutionary rescue. Predicting evolutionary rescue has applications ranging from conservation biology to medicine, but requires understanding and integrating the multiple effects of a stressful environmental change on population processes. Here we derive a simple expression for how generation time, a key determinant of the rate of evolution, varies with population size during evolutionary rescue. Change in generation time is quantitatively predicted by comparing how intraspecific competition and the source of maladaptation each affect the rates of births and deaths in the population. Depending on the difference between two parameters quantifying these effects, the model predicts that populations may experience substantial changes in their rate of adaptation in both positive and negative directions, or adapt consistently despite severe stress. These predictions were then tested by comparison to the results of individual-based simulations of evolutionary rescue, which validated that the tolerable rate of environmental change varied considerably as described by analytical results. We discuss how these results inform efforts to understand wildlife disease and adaptation to climate change, evolution in managed populations and treatment resistance in pathogens.

Multi-scale effects of habitat loss and the role of trait evolution

Habitat loss (HL) is a major cause of species extinctions. Although the effects of HL beyond the directly impacted area have been previously observed, they have not been modelled explicitly, especially in an eco-evolutionary context. To start filling this gap, we study a two-patch deterministic consumer-resource model, with one of the patches experiencing loss of resources as a special case of HL. Our model allows foraging and mating within a patch as well as between patches. We then introduce heritable variation in consumer traits related to resource utilization and patch use to investigate eco-evolutionary dynamics and compare results with constant and no trait variation scenarios. Our results show that HL in one patch can indeed reduce consumer densities in the neighbouring patch but can also increase consumer densities in the neighbouring patch when the resources are overexploited. Yet at the landscape scale, the effect of HL on consumer densities is consistently negative. Patch isolation increases consumer density in the patch experiencing HL but has generally negative effects on the neighbouring patch, with context-dependent results at the landscape scale. With high cross-patch dependence and coupled foraging and mating preferences, local HL can sometimes even lead to landscape-level consumer extinction. Eco-evolutionary dynamics can rescue consumers from such extinction in some cases if their death rates are sufficiently small. More generally, trait evolution had positive or negative effects on equilibrium consumer densities after HL, depending on the evolving trait and the spatial scale considered. In summary, our findings show that HL at a local scale can affect the neighbouring patch and the landscape as a whole, where heritable trait variation can, in some cases, alleviate the impact of HL. We thus suggest joint consideration of multiple spatial scales and trait variation when assessing and predicting the impacts of HL.

High importance of indirect evolutionary rescue in a small food web

Evolutionary rescue may allow species to survive environmental change, but how this mechanism operates in food webs is poorly understood. Here, the evolutionary rescue was investigated in a small model food web, systematically allowing the evolution of each single species in order to reveal how its adaptation affects the persistence of itself and others. The impact of evolution was highly species-specific and not necessarily positive: only one species, the specialist predator, consistently had a positive impact on overall persistence. Most strikingly, evolution overwhelmingly affected other species: rescue of others (indirect rescue) was far more frequent than self-rescue, and negative effects were nearly always indirect. This demonstrates that evolutionary rescue in food webs is inextricably bound up with species interactions, as the effects of evolution in one species ripple through the entire community. It is therefore critically important to consider the food web context in efforts to understand how species may survive global change.

Low interspecific variation and no phylogenetic signal in additive genetic variance in wild bird and mammal populations

Evolutionary adaptation through genetic change requires genetic variation and is a key mechanism enabling species to persist in changing environments. Although a substantial body of work has focused on understanding how and why additive genetic variance (V A) differs among traits within species, we still know little about how they vary among species. Here we make a first attempt at testing for interspecific variation in two complementary measures of V A and the role of phylogeny in shaping this variation. To this end, we performed a phylogenetic comparative analysis using 1822 narrow-sense heritability (h 2) for 68 species of birds and mammals and 378 coefficients of additive genetic variance (CV A) estimates for 23 species. Controlling for within-species variation attributable to estimation method and trait type, we found some interspecific variation in h 2 (~15%) but not CV A. Although suggestive of interspecific variation in the importance of non-(additive) genetic sources of variance, sample sizes were insufficient to test this hypothesis directly. Additionally, although power was low, no phylogenetic signal was detected for either measure. Hence, while this suggests interspecific variation in V A is probably small, our understanding of interspecific variation in the adaptive potential of wild vertebrate populations is currently hampered by data limitations, a scarcity of CV A estimates and a measure of their uncertainty in particular.

Genetic mixing in conservation translocations increases diversity of a keystone threatened species, Bettongia lesueur

Translocation programmes are increasingly being informed by genetic data to monitor and enhance conservation outcomes for both natural and established populations. These data provide a window into contemporary patterns of genetic diversity, structure and relatedness that can guide managers in how to best source animals for their translocation programmes. The inclusion of historical samples, where possible, strengthens monitoring by allowing assessment of changes in genetic diversity over time and by providing a benchmark for future improvements in diversity via management practices. Here, we used reduced representation sequencing (ddRADseq) data to report on the current genetic health of three remnant and seven translocated boodie (Bettongia lesueur) populations, now extinct on the Australian mainland. In addition, we used exon capture data from seven historical mainland specimens and a subset of contemporary samples to compare pre-decline and current diversity. Both data sets showed the significant impact of population founder source (whether multiple or single) on the genetic diversity of translocated populations. Populations founded by animals from multiple sources showed significantly higher genetic diversity than the natural remnant and single-source translocation populations, and we show that by mixing the most divergent populations, exon capture heterozygosity was restored to levels close to that observed in pre-decline mainland samples. Relatedness estimates were surprisingly low across all contemporary populations and there was limited evidence of inbreeding. Our results show that a strategy of genetic mixing has led to successful conservation outcomes for the species in terms of increasing genetic diversity and provides strong rationale for mixing as a management strategy.

Coevolution of species colonisation rates controls food-chain length in spatially structured food webs

How the complexity of food webs depends on environmental variables is a long-standing ecological question. It is unclear though how food-chain length should vary with adaptive evolution of the constitutive species. Here we model the evolution of species colonisation rates and its consequences on occupancies and food-chain length in metacommunities. When colonisation rates can evolve, longer food-chains can persist. Extinction, perturbation and habitat loss all affect evolutionarily stable colonisation rates, but the strength of the competition-colonisation trade-off has a major role: weaker trade-offs yield longer chains. Although such eco-evo dynamics partly alleviates the spatial constraint on food-chain length, it is no magic bullet: the highest, most vulnerable, trophic levels are also those that least benefit from evolution. We provide qualitative predictions regarding how trait evolution affects the response of communities to disturbance and habitat loss. This highlights the importance of eco-evolutionary dynamics at metacommunity level in determining food-chain length.

Extending eco-evolutionary theory with oligomorphic dynamics

Understanding the interplay between ecological processes and the evolutionary dynamics of quantitative traits in natural systems remains a major challenge. Two main theoretical frameworks are used to address this question, adaptive dynamics and quantitative genetics, both of which have strengths and limitations and are often used by distinct research communities to address different questions. In order to make progress, new theoretical developments are needed that integrate these approaches and strengthen the link to empirical data. Here, we discuss a novel theoretical framework that bridges the gap between quantitative genetics and adaptive dynamics approaches. 'Oligomorphic dynamics' can be used to analyse eco-evolutionary dynamics across different time scales and extends quantitative genetics theory to account for multimodal trait distributions, the dynamical nature of genetic variance, the potential for disruptive selection due to ecological feedbacks, and the non-normal or skewed trait distributions encountered in nature. Oligomorphic dynamics explicitly takes into account the effect of environmental feedback, such as frequency- and density-dependent selection, on the dynamics of multi-modal trait distributions and we argue it has the potential to facilitate a much tighter integration between eco-evolutionary theory and empirical data.

The General Trends of Genetic Diversity Change in Alien Plants' Invasion

Genetic diversity is associated with invasion dynamics during establishment and expansion stages by affecting the viability and adaptive potential of exotics. There have been many reports on the comparison between the genetic diversity of invasive alien species (IAS) in and out of their native habitats, but the conclusions were usually inconsistent. In this work, a standard meta-analysis of the genetic diversity of 19 invasive plants based on 26 previous studies was carried out to investigate the general trend for the change of IASs' genetic diversity during their invasion process and its real correlation with the invasion fate. Those 26 studies were screened from a total of 3557 peer-reviewed publications from the ISI Web of Science database during the period of January 2000 to May 2022. Based on the selected studies in this work, a general reduction of IASs' genetic diversity was found in non-native populations compared to that in native ones, while the difference was not significant. This finding suggested that regardless of the change in genetic diversity, it had no substantial effect on the outcome of the invasion process. Therefore, genetic diversity might not serve as a reliable indicator for risk assessment and prediction of invasion dynamic prediction in the case of IASs.

Hidden genetic variation in plasticity provides the potential for rapid adaptation to novel environments

Rapid environmental change is forcing populations into environments where plasticity will no longer maintain fitness. When populations are exposed to novel environments, evolutionary theory predicts that genetic variation in fitness will increase and should be associated with genetic differences in plasticity. If true, then genetic variation in plasticity can increase adaptive potential in novel environments, and population persistence via evolutionary rescue is more likely. To test whether genetic variation in fitness increases in novel environments and is associated with plasticity, we transplanted 8149 clones of 314 genotypes of a Sicilian daisy (Senecio chrysanthemifolius) within and outside its native range, and quantified genetic variation in fitness, and plasticity in leaf traits and gene expression. Although mean fitness declined by 87% in the novel environment, genetic variance in fitness increased threefold and was correlated with plasticity in leaf traits. High fitness genotypes showed greater plasticity in gene expression, but lower plasticity in most leaf traits. Interestingly, genotypes with the highest fitness in the novel environment had the lowest fitness at the native site. These results suggest that standing genetic variation in plasticity could help populations to persist and adapt to novel environments, despite remaining hidden in native environments.

Evolutionary rescue in a fluctuating environment: periodic versus quasi-periodic environmental changes

No environment is constant over time, and environmental fluctuations impact the outcome of evolutionary dynamics. Survival of a population not adapted to some environmental conditions is threatened unless, for example, a mutation rescues it, an eco-evolutionary process termed evolutionary rescue. We here investigate evolutionary rescue in an environment that fluctuates between a favourable state, in which the population grows, and a harsh state, in which the population declines. We develop a stochastic model that includes both population dynamics and genetics. We derive analytical predictions for the mean extinction time of a non-adapted population given that it is not rescued, the probability of rescue by a mutation, and the mean appearance time of a rescue mutant, which we validate using numerical simulations. We find that stochastic environmental fluctuations, resulting in quasi-periodic environmental changes, accelerate extinction and hinder evolutionary rescue compared with deterministic environmental fluctuations, resulting in periodic environmental changes. We demonstrate that high equilibrium population sizes and per capita growth rates maximize the chances of evolutionary rescue. We show that an imperfectly harsh environment, which does not fully prevent births but makes the death rate to birth rate ratio much greater than unity, has almost the same rescue probability as a perfectly harsh environment, which fully prevents births. Finally, we put our results in the context of antimicrobial resistance and conservation biology.

Frank Beach Award Winner: The centrality of the hypothalamic-pituitary-adrenal axis in dealing with environmental change across temporal scales

Understanding if and how individuals and populations cope with environmental change is an enduring question in evolutionary ecology that has renewed importance given the pace of change in the Anthropocene. Two evolutionary strategies of coping with environmental change may be particularly important in rapidly changing environments: adaptive phenotypic plasticity and/or bet hedging. Adaptive plasticity could enable individuals to match their phenotypes to the expected environment if there is an accurate cue predicting the selective environment. Diversifying bet hedging involves the production of seemingly random phenotypes in an unpredictable environment, some of which may be adaptive. Here, I review the central role of the hypothalamic-pituitary-adrenal (HPA) axis and glucocorticoids (GCs) in enabling vertebrates to cope with environmental change through adaptive plasticity and bet hedging. I first describe how the HPA axis mediates three types of adaptive plasticity to cope with environmental change (evasion, tolerance, recovery) over short timescales (e.g., 1-3 generations) before discussing how the implications of GCs on phenotype integration may depend upon the timescale under consideration. GCs can promote adaptive phenotypic integration, but their effects on phenotypic co-variation could also limit the dimensions of phenotypic space explored by animals over longer timescales. Finally, I discuss how organismal responses to environmental stressors can act as a bet hedging mechanism and therefore enhance evolvability by increasing genetic or phenotypic variability or reducing patterns of genetic and phenotypic co-variance. Together, this emphasizes the crucial role of the HPA axis in understanding fundamental questions in evolutionary ecology.

Evolutionary rescue and geographic range shifts under climate change for global amphibians

By the end of this century, human-induced climate change and habitat loss may drastically reduce biodiversity, with expected effects on many amphibian lineages. One of these effects is the shift in the geographic distributions of species when tracking suitable climates. Here, we employ a macroecological approach to dynamically model geographic range shifts by coupling ecological niche models and eco-evolutionary mechanisms, aiming to assess the probability of evolutionary rescue (i.e., rapid adaptation) and dispersal under climate change. Evolutionary models estimated the probability of population persistence by adapting to changes in the temperature influenced by precipitation in the following decades, while compensating the fitness reduction and maintaining viable populations in the new climates. In addition, we evaluated emerging patterns of species richness and turnover at the assemblage level. Our approach was able to identify which amphibian populations among 7,193 species at the global scale could adapt to temperature changes or disperse into suitable regions in the future. Without evolutionary adaptation and dispersal, 47.7% of the species could go extinct until the year 2,100, whereas adding both processes will slightly decrease this extinction rate to 36.5%. Although adaptation to climate is possible for populations in about 25.7% of species, evolutionary rescue is the only possibility to avoid extinction in 4.2% of them. Dispersal will allow geographic range shifts for 49.7% of species, but only 6.5% may avoid extinction by reaching climatically suitable environments. This reconfiguration of species distributions and their persistence creates new assemblage-level patterns at the local scale. Temporal beta-diversity across the globe showed relatively low levels of species turnover, mainly due to the loss of species. Despite limitations with obtaining data, our approach provides more realistic assessments of species responses to ongoing climate changes. It shows that, although dispersal and evolutionary rescue may attenuate species losses, they are not enough to avoid a significant reduction of species’ geographic ranges in the future. Actions that guarantee a higher potential of adaptation (e.g., genetic diversity through larger population sizes) and increased connectivity for species dispersion to track suitable climates become essential, increasing the resilience of biodiversity to climate change.

Evolution of thermal performance curves: A meta-analysis of selection experiments

Temperatures are increasing due to global changes, putting biodiversity at risk. Organisms are faced with a limited set of options to cope with this situation: adapt, disperse or die. We here focus on the first possibility, more specifically, on evolutionary adaptations to temperature. Ectotherms are usually characterized by a hump-shaped relationship between fitness and temperature, a non-linear reaction norm that is referred to as thermal performance curve (TPC). To understand and predict impacts of global change, we need to know whether and how such TPCs evolve. Therefore, we performed a systematic literature search and a statistical meta-analysis focusing on experimental evolution and artificial selection studies. This focus allows us to directly quantify relative fitness responses to temperature selection by calculating fitness differences between TPCs from ancestral and derived populations after thermal selection. Out of 7561 publications screened, we found 47 studies corresponding to our search criteria representing taxa across the tree of life, from bacteria, to plants and vertebrates. We show that, independently of species identity, the studies we found report a positive response to temperature selection. Considering entire TPC shapes, adaptation to higher temperatures traded off with fitness at lower temperatures, leading to niche shifts. Effects were generally stronger in unicellular organisms. By contrast, we do not find statistical support for the often discussed "Hotter is better" hypothesis. While our meta-analysis provides evidence for adaptive potential of TPCs across organisms, it also highlights that more experimental work is needed, especially for under-represented taxa, such as plants and non-model systems.

Eco-evolutionary extinction and recolonization dynamics reduce genetic load and increase time to extinction in highly inbred populations

Understanding how genetic and ecological effects can interact to shape genetic loads within and across local populations is key to understanding ongoing persistence of systems that should otherwise be susceptible to extinction through mutational meltdown. Classic theory predicts short persistence times for metapopulations comprising small local populations with low connectivity, due to accumulation of deleterious mutations. Yet, some such systems have persisted over evolutionary time, implying the existence of mechanisms that allow metapopulations to avoid mutational meltdown. We first hypothesize a mechanism by which the combination of stochasticity in the numbers and types of mutations arising locally (genetic stochasticity), resulting local extinction, and recolonization through evolving dispersal facilitates metapopulation persistence. We then test this mechanism using a spatially and genetically explicit individual-based model. We show that genetic stochasticity in highly structured metapopulations can result in local extinctions, which can favor increased dispersal, thus allowing recolonization of empty habitat patches. This causes fluctuations in metapopulation size and transient gene flow, which reduces genetic load and increases metapopulation persistence over evolutionary time. Our suggested mechanism and simulation results provide an explanation for the conundrum presented by the continued persistence of highly structured populations with inbreeding mating systems that occur in diverse taxa.

Susceptibility to a sexually transmitted disease in a wild koala population shows heritable genetic variance but no inbreeding depression

The koala, one of the most iconic Australian wildlife species, is facing several concomitant threats that are driving population declines. Some threats are well known and have clear methods of prevention (e.g., habitat loss can be reduced with stronger land-clearing control), whereas others are less easily addressed. One of the major current threats to koalas is chlamydial disease, which can have major impacts on individual survival and reproduction rates and can translate into population declines. Effective management strategies for the disease in the wild are currently lacking, and, to date, we know little about the determinants of individual susceptibility to disease. Here, we investigated the genetic basis of variation in susceptibility to chlamydia using one of the most intensively studied wild koala populations. We combined data from veterinary examinations, chlamydia testing, genetic sampling and movement monitoring. Out of our sample of 342 wild koalas, 60 were found to have chlamydia. Using genotype information on 5007 SNPs to investigate the role of genetic variation in determining disease status, we found no evidence of inbreeding depression, but a heritability of 0.11 (95% CI: 0.06-0.23) for the probability that koalas had chlamydia. Heritability of susceptibility to chlamydia could be relevant for future disease management, as it suggests adaptive potential for the population.

Symbiont genotype influences holobiont response to increased temperature

As coral reefs face warming oceans and increased coral bleaching, a whitening of the coral due to loss of microalgal endosymbionts, the possibility of evolutionary rescue offers some hope for reef persistence. In tightly linked mutualisms, evolutionary rescue may occur through evolution of the host and/or endosymbionts. Many obligate mutualisms are composed of relatively small, fast-growing symbionts with greater potential to evolve on ecologically relevant time scales than their relatively large, slower growing hosts. Numerous jellyfish species harbor closely related endosymbiont taxa to other cnidarian species such as coral, and are commonly used as a model system for investigating cnidarian mutualisms. We examined the potential for adaptation of the upside-down jellyfish Cassiopea xamachana to increased temperature via evolution of its microalgal endosymbiont, Symbiodinium microadriaticum. We quantified trait variation among five algal genotypes in response to three temperatures (26 °C, 30 °C, and 32 °C) and fitness of hosts infected with each genotype. All genotypes showed positive growth rates at each temperature, but rates of respiration and photosynthesis decreased with increased temperature. Responses varied among genotypes but were unrelated to genetic similarity. The effect of temperature on asexual reproduction and the timing of development in the host also depended on the genotype of the symbiont. Natural selection could favor different algal genotypes at different temperatures, affecting host fitness. This eco-evolutionary interaction may be a critical component of understanding species resilience in increasingly stressful environments.

Developing an empirical model for spillover and emergence: Orsay virus host range in Caenorhabditis

A lack of tractable experimental systems in which to test hypotheses about the ecological and evolutionary drivers of disease spillover and emergence has limited our understanding of these processes. Here we introduce a promising system: Caenorhabditis hosts and Orsay virus, a positive-sense single-stranded RNA virus that naturally infects C. elegans. We assayed species across the Caenorhabditis tree and found Orsay virus susceptibility in 21 of 84 wild strains belonging to 14 of 44 species. Confirming patterns documented in other systems, we detected effects of host phylogeny on susceptibility. We then tested whether susceptible strains were capable of transmitting Orsay virus by transplanting exposed hosts and determining whether they transmitted infection to conspecifics during serial passage. We found no evidence of transmission in 10 strains (virus undetectable after passaging in all replicates), evidence of low-level transmission in 5 strains (virus lost between passage 1 and 5 in at least one replicate) and evidence of sustained transmission in 6 strains (including all three experimental C. elegans strains) in at least one replicate. Transmission was strongly associated with viral amplification in exposed populations. Variation in Orsay virus susceptibility and transmission among Caenorhabditis strains suggests that the system could be powerful for studying spillover and emergence.

The immediate costs and long-term benefits of assisted gene flow in large populations

With the genetic health of many plant and animal populations deteriorating due to climate change outpacing adaptation, interventions, such as assisted gene flow (AGF), may provide genetic variation necessary for populations to adapt to climate change. We ran genetic simulations to mimic different AGF scenarios in large populations and measured their outcomes on population-level fitness to determine circumstances in which it is worthwhile to perform AGF. In the absence of inbreeding depression, AGF was beneficial within a few generations only when introduced genotypes had much higher fitness than local individuals and traits affecting fitness were controlled by a few genes of large effect. AGF was harmful over short periods (e.g., first ∼10-20 generations) if there was strong outbreeding depression or introduced deleterious genetic variation. When the adaptive trait was controlled by many loci of small effect, the benefits of AGF took over 10 generations to realize-potentially too long for most climate-related management scenarios. The genomic integrity of the recipient population typically remained intact following AGF; the amount of genetic material from the donor population usually constituted no more of the recipient population's genome than the fraction of the population introduced. Significant genomic turnover (e.g., >50% replacement) only occurred when the selective advantage of the adaptive trait and translocation fraction were extremely high. Our results will be useful when adaptive management is used to maintain the genetic health and productivity of large populations under climate change.

Hybridization may aid evolutionary rescue of an endangered East African passerine

Introgressive hybridization is a process that enables gene flow across species barriers through the backcrossing of hybrids into a parent population. This may make genetic material, potentially including relevant environmental adaptations, rapidly available in a gene pool. Consequently, it has been postulated to be an important mechanism for enabling evolutionary rescue, that is the recovery of threatened populations through rapid evolutionary adaptation to novel environments. However, predicting the likelihood of such evolutionary rescue for individual species remains challenging. Here, we use the example of Zosterops silvanus, an endangered East African highland bird species suffering from severe habitat loss and fragmentation, to investigate whether hybridization with its congener Zosterops flavilateralis might enable evolutionary rescue of its Taita Hills population. To do so, we employ an empirically parameterized individual-based model to simulate the species' behaviour, physiology and genetics. We test the population's response to different assumptions of mating behaviour and multiple scenarios of habitat change. We show that as long as hybridization does take place, evolutionary rescue of Z. silvanus is likely. Intermediate hybridization rates enable the greatest long-term population growth, due to trade-offs between adaptive and maladaptive introgressed alleles. Habitat change did not have a strong effect on population growth rates, as Z. silvanus is a strong disperser and landscape configuration is therefore not the limiting factor for hybridization. Our results show that targeted gene flow may be a promising avenue to help accelerate the adaptation of endangered species to novel environments, and demonstrate how to combine empirical research and mechanistic modelling to deliver species-specific predictions for conservation planning.

Rate-induced collapse in evolutionary systems

Recent work has highlighted the possibility of 'rate-induced tipping', in which a system undergoes an abrupt transition when a perturbation exceeds a critical rate of change. Here, we argue that this is widely applicable to evolutionary systems: collapse, or extinction, may occur when external changes occur too fast for evolutionary adaptation to keep up. To bridge existing theoretical frameworks, we develop a minimal evolutionary-ecological model showing that rate-induced extinction and the established notion of 'evolutionary rescue' are fundamentally two sides of the same coin: the failure of one implies the other, and vice versa. We compare the minimal model's behaviour with that of a more complex model in which the large-scale dynamics emerge from the interactions of many individual agents; in both cases, there is a well-defined threshold rate to induce extinction, and a consistent scaling law for that rate as a function of timescale. Due to the fundamental nature of the underlying mechanism, we suggest that a vast range of evolutionary systems should in principle be susceptible to rate-induced collapse. This would include ecosystems on all scales as well as human societies; further research is warranted.

Change in prey genotype frequency rescues predator from extinction

Indirect evolutionary rescue (IER) is a mechanism where a non-evolving species is saved from extinction in an otherwise lethal environment by evolution in an interacting species. This process has been described in a predator-prey model, where extinction of the predator is prevented by a shift in the frequency of defended towards undefended prey when reduced predator densities lower selection for defended prey. We test here how increased mortality and the initial frequencies of the prey types affect IER. Combining the analysis of model simulations and experiments with rotifers feeding on algae we show IER in the presence of increased predator mortality. We found that IER was dependent on the ability of the prey to evolve as well as on the frequency of the defended prey. High initial frequencies of defended prey resulted in predator extinction despite the possibility for prey evolution, as the increase in undefended prey was delayed too much to allow predator rescue. This frequency dependency for IER was more pronounced for higher predator mortalities. Our findings can help informing the development of conservation and management strategies that consider evolutionary responses in communities to environmental changes.

Plant-microbe eco-evolutionary dynamics in a changing world

Both plants and their associated microbiomes can respond strongly to anthropogenic environmental changes. These responses can be both ecological (e.g. a global change affecting plant demography or microbial community composition) and evolutionary (e.g. a global change altering natural selection on plant or microbial populations). As a result, global changes can catalyse eco-evolutionary feedbacks. Here, we take a plant-focused perspective to discuss how microbes mediate plant ecological responses to global change and how these ecological effects can influence plant evolutionary response to global change. We argue that the strong and functionally important relationships between plants and their associated microbes are particularly likely to result in eco-evolutionary feedbacks when perturbed by global changes and discuss how improved understanding of plant-microbe eco-evolutionary dynamics could inform conservation or even agriculture.

Genetic variance in fitness indicates rapid contemporary adaptive evolution in wild animals

The rate of adaptive evolution, the contribution of selection to genetic changes that increase mean fitness, is determined by the additive genetic variance in individual relative fitness. To date, there are few robust estimates of this parameter for natural populations, and it is therefore unclear whether adaptive evolution can play a meaningful role in short-term population dynamics. We developed and applied quantitative genetic methods to long-term datasets from 19 wild bird and mammal populations and found that, while estimates vary between populations, additive genetic variance in relative fitness is often substantial and, on average, twice that of previous estimates. We show that these rates of contemporary adaptive evolution can affect population dynamics and hence that natural selection has the potential to partly mitigate effects of current environmental change.

Introduction to the theme issue 'Species' ranges in the face of changing environments'

Understanding where, when and how species' ranges will be modified is both a fundamental problem and essential to predicting how spatio-temporal environmental changes in abiotic and biotic factors impact biodiversity. Notably, different species may respond disparately to similar environmental changes: some species may overcome an environmental change only with difficulty or not at all, while other species may readily overcome the same change. Ranges may contract, expand or move. The drivers and consequences of this variability in species' responses remain puzzling. Importantly, changes in a species' range creates feedbacks to the environmental conditions, populations and communities in its previous and current range, rendering population genetic, population dynamic and community processes inextricably linked. Understanding these links is critical in guiding biodiversity management and conservation efforts. This theme issue presents current thinking about the factors and mechanisms that limit and/or modify species' ranges. It also outlines different approaches to detect changes in species' distributions, and illustrates cases of range modifications in several taxa. Overall, this theme issue highlights the urgency of understanding species' ranges but shows that we are only just beginning to disentangle the processes involved. One way forward is to unite ecology with evolutionary biology and empirical with modelling approaches. This article is part of the theme issue 'Species' ranges in the face of changing environments (Part II)'.

Lichens and animal camouflage: some observations from central Asian ecoregions

Camouflage is a fitness-relevant trait that supports survival and fosters evolutionary adaptation by which animals match their body pattern to a background setting. Lichens are among the most common of these backgrounds that several animal species use for camouflage. Lichens are omnipresent and grow in wide arrays of colorations and compositions. Their composition and phenotypic diversity might facilitate cryptic coloration and habitat matching by various animal species. Here, we describe the role of lichens in providing camouflage to various animal species in central Asian and Caucasus mountain ecoregions, which are categorized as global biodiversity hotspots. Despite multiple ecological studies, no information is available on the role of this regions‘ lichen diversity in providing animal camouflage. Casual field observations of lichen camouflage are reported for four (one mammal and three reptile) species: the Persian Leopard’s Panthera pardus saxicolor body coat seems to closely match the colors and patterns of saxicolous lichens (Acarospora sp. and Circinaria sp.) in their habitat.  A similar background matching pattern was observed in both morphs of the Caucasian Rock Agama Paralaudakia caucasia upon crustose lichens: Caloplaca spp., Circinaria spp., and the Radde’s Rock Lizard Darevskia raddei to the crustose lichens Acarospora sp. and Caloplaca sp. Likewise, the Horny-scaled Agama’s Trapelus ruderatus grey matches with the color of multiple lichens (Lecanora spp., Circinaria spp., Protoparmeliopsis spp., Rinodina spp., and Anaptychia spp.). Our observations preliminarily suggest that lichens play an important role for species of different trophic levels, ensuring adaptation and survival through camouflage. We call for more field-based empirical and experimental studies in various terrestrial ecosystems in other parts of the world to test the role of lichens in local adaption and evolutionary plasticity of regional species.

The recent evolutionary rescue of a staple crop depended on over half a century of global germplasm exchange

Rapid environmental change can lead to population extinction or evolutionary rescue. The global staple crop sorghum (Sorghum bicolor) has recently been threatened by a global outbreak of an aggressive new biotype of sugarcane aphid (SCA; Melanaphis sacchari). We characterized genomic signatures of adaptation in a Haitian breeding population that had rapidly adapted to SCA infestation, conducting evolutionary population genomics analyses on 296 Haitian lines versus 767 global accessions. Genome scans and geographic analyses suggest that SCA adaptation has been conferred by a globally rare East African allele of RMES1, which spread to breeding programs in Africa, Asia, and the Americas. De novo genome sequencing revealed potential causative variants at RMES1. Markers developed from the RMES1 sweep predicted resistance in eight independent commercial and public breeding programs. These findings demonstrate the value of evolutionary genomics to develop adaptive trait technology and highlight the benefits of global germplasm exchange to facilitate evolutionary rescue.

Genotype diversity promotes the persistence of Daphnia populations exposed to severe copper stress

When environmental stressors of high intensity are sustained for long periods of time, populations face high probabilities of being extirpated. However, depending on the intensity of the stressor, large populations with sufficient genetic diversity may persist. We report the results of an experiment that tracked the persistence of Daphnia populations exposed to copper contamination. We assessed whether genotypic diversity reduced the risk of extinction. We created monoclonal and multiclonal populations and monitored their population sizes during a 32-week experiment. Cu was applied at a sub-lethal concentration and then increased every week until the population sizes dropped to about 10% of the carrying capacity (Cu at 180 μg/L). The concentration was then increased up to 186 μg/L and held stable until the end of the experiment. A survival analysis showed that clonal diversity extended the persistence of Daphnia populations, but copper contamination caused a substantial genetic erosion followed by population extirpation. However, some Cu-treated populations, mostly multiclonal, showed U-shaped patterns of growth consistent with evolutionary rescue but these did not lead to lasting population recovery. These results highlight the importance of genetic variation for population persistence, but they also show how quickly it can be lost in contaminated environments.

Evolutionary rescue can prevent rate-induced tipping

The transformation of ecosystems proceeds at unprecedented rates. Recent studies suggest that high rates of environmental change can cause rate-induced tipping. In ecological models, the associated rate-induced critical transition manifests during transient dynamics in which populations drop to dangerously low densities. In this work, we study how indirect evolutionary rescue—due to the rapid evolution of a predator’s trait—can save a prey population from the rate-induced collapse. Therefore, we explicitly include the time-dependent dynamics of environmental change and evolutionary adaptation in an eco-evolutionary system. We then examine how fast the evolutionary adaptation needs to be to counteract the response to environmental degradation and express this relationship by means of a critical rate. Based on this critical rate, we conclude that indirect evolutionary rescue is more probable if the predator population possesses a high genetic variation and, simultaneously, the environmental change is slow. Hence, our results strongly emphasize that the maintenance of biodiversity requires a deceleration of the anthropogenic degradation of natural habitats.

Impacts of salinization on aquatic communities: Abrupt vs. gradual exposures

Increasing chloride concentrations from road salt applications are an emerging threat to freshwater diversity in cold weather regions. Few studies have focused on how road salt affects freshwater biota and even fewer have focused on how the rate of exposure alters organism responses. We hypothesized that road salt concentrations delivered gradually would result in slower population declines and more rapid rebounds due to evolved tolerance. To test this hypothesis, we examined the responses of freshwater lake organisms to four environmentally relevant salt concentrations (100, 230, 860, and 1600 mg Cl^-/L) that differed in application rate (abrupt vs. gradual). We used outdoor aquatic mesocosms containing zooplankton, filamentous algae, phytoplankton, periphyton, and macroinvertebrates. We found negative effects of road salt on zooplankton and macroinvertebrate abundance, but positive effects on phytoplankton and periphyton, likely resulting from reduced grazing. Only rarely did we detect a difference between abrupt vs gradual salt applications and the directions of those differences were not consistent. This affirms the need for additional research on how road salt pollution entering ecosystems at different frequencies and magnitudes will alter freshwater communities.

Drug-induced resistance evolution necessitates less aggressive treatment

Increasing body of experimental evidence suggests that anticancer and antimicrobial therapies may themselves promote the acquisition of drug resistance by increasing mutability. The successful control of evolving populations requires that such biological costs of control are identified, quantified and included to the evolutionarily informed treatment protocol. Here we identify, characterise and exploit a trade-off between decreasing the target population size and generating a surplus of treatment-induced rescue mutations. We show that the probability of cure is maximized at an intermediate dosage, below the drug concentration yielding maximal population decay, suggesting that treatment outcomes may in some cases be substantially improved by less aggressive treatment strategies. We also provide a general analytical relationship that implicitly links growth rate, pharmacodynamics and dose-dependent mutation rate to an optimal control law. Our results highlight the important, but often neglected, role of fundamental eco-evolutionary costs of control. These costs can often lead to situations, where decreasing the cumulative drug dosage may be preferable even when the objective of the treatment is elimination, and not containment. Taken together, our results thus add to the ongoing criticism of the standard practice of administering aggressive, high-dose therapies and motivate further experimental and clinical investigation of the mutagenicity and other hidden collateral costs of therapies.

How will mosquitoes adapt to climate warming?

The potential for adaptive evolution to enable species persistence under a changing climate is one of the most important questions for understanding impacts of future climate change. Climate adaptation may be particularly likely for short-lived ectotherms, including many pest, pathogen, and vector species. For these taxa, estimating climate adaptive potential is critical for accurate predictive modeling and public health preparedness. Here, we demonstrate how a simple theoretical framework used in conservation biology-evolutionary rescue models-can be used to investigate the potential for climate adaptation in these taxa, using mosquito thermal adaptation as a focal case. Synthesizing current evidence, we find that short mosquito generation times, high population growth rates, and strong temperature-imposed selection favor thermal adaptation. However, knowledge gaps about the extent of phenotypic and genotypic variation in thermal tolerance within mosquito populations, the environmental sensitivity of selection, and the role of phenotypic plasticity constrain our ability to make more precise estimates. We describe how common garden and selection experiments can be used to fill these data gaps. Lastly, we investigate the consequences of mosquito climate adaptation on disease transmission using Aedes aegypti-transmitted dengue virus in Northern Brazil as a case study. The approach outlined here can be applied to any disease vector or pest species and type of environmental change.

Why are viral genomes so fragile? The bottleneck hypothesis

If they undergo new mutations at each replication cycle, why are RNA viral genomes so fragile, with most mutations being either strongly deleterious or lethal? Here we provide theoretical and numerical evidence for the hypothesis that genetic fragility is partly an evolutionary response to the multiple population bottlenecks experienced by viral populations at various stages of their life cycles. Modelling within-host viral populations as multi-type branching processes, we show that mutational fragility lowers the rate at which Muller’s ratchet clicks and increases the survival probability through multiple bottlenecks. In the context of a susceptible-exposed-infectious-recovered epidemiological model, we find that the attack rate of fragile viral strains can exceed that of more robust strains, particularly at low infectivities and high mutation rates. Our findings highlight the importance of demographic events such as transmission bottlenecks in shaping the genetic architecture of viral pathogens.

Given that most mutations are deleterious, high mutation rates carry a significant evolutionary cost. To reduce this burden, an obvious evolutionary solution would be to reduce the fitness cost of mutations by becoming more robust; this solution is indeed selected in populations of constantly large size. Here, we show that when populations regularly experience bottlenecks, as viruses do upon transmission to a new host, a less obvious solution becomes more viable: namely, to increase the fitness cost of mutations so that unfit mutants are less likely to fix at each passage. This could explain why viruses—especially RNA viruses—do in fact have very fragile genomes.

Quercus Conservation Genetics and Genomics: Past, Present, and Future

Quercus species (oaks) have been an integral part of the landscape in the northern hemisphere for millions of years. Their ability to adapt and spread across different environments and their contributions to many ecosystem services is well documented. Human activity has placed many oak species in peril by eliminating or adversely modifying habitats through exploitative land usage and by practices that have exacerbated climate change. The goal of this review is to compile a list of oak species of conservation concern, evaluate the genetic data that is available for these species, and to highlight the gaps that exist. We compiled a list of 124 Oaks of Concern based on the Red List of Oaks 2020 and the Conservation Gap Analysis for Native U.S. Oaks and their evaluations of each species. Of these, 57% have been the subject of some genetic analysis, but for most threatened species (72%), the only genetic analysis was done as part of a phylogenetic study. While nearly half (49%) of published genetic studies involved population genetic analysis, only 16 species of concern (13%) have been the subject of these studies. This is a critical gap considering that analysis of intraspecific genetic variability and genetic structure are essential for designing conservation management strategies. We review the published population genetic studies to highlight their application to conservation. Finally, we discuss future directions in Quercus conservation genetics and genomics.