Rapid trait evolution drives increased speed and variance in experimental range expansions

The great tit HapMap project: A continental-scale analysis of genomic variation in a songbird

A major aim of evolutionary biology is to understand why patterns of genomic diversity vary within taxa and space. Large-scale genomic studies of widespread species are useful for studying how environment and demography shape patterns of genomic divergence. Here, we describe one of the most geographically comprehensive surveys of genomic variation in a wild vertebrate to date; the great tit (Parus major) HapMap project. We screened ca 500,000 SNP markers across 647 individuals from 29 populations, spanning ~30 degrees of latitude and 40 degrees of longitude - almost the entire geographical range of the European subspecies. Genome-wide variation was consistent with a recent colonisation across Europe from a South-East European refugium, with bottlenecks and reduced genetic diversity in island populations. Differentiation across the genome was highly heterogeneous, with clear 'islands of differentiation', even among populations with very low levels of genome-wide differentiation. Low local recombination rates were a strong predictor of high local genomic differentiation (FST), especially in island and peripheral mainland populations, suggesting that the interplay between genetic drift and recombination causes highly heterogeneous differentiation landscapes. We also detected genomic outlier regions that were confined to one or more peripheral great tit populations, probably as a result of recent directional selection at the species' range edges. Haplotype-based measures of selection were related to recombination rate, albeit less strongly, and highlighted population-specific sweeps that likely resulted from positive selection. Our study highlights how comprehensive screens of genomic variation in wild organisms can provide unique insights into spatio-temporal evolutionary dynamics.

Environmental gradients mediate dispersal evolution during biological invasions

Rapid evolution of increased dispersal at the edge of a range expansion can accelerate invasions. However, populations expanding across environmental gradients often face challenging environments that reduce fitness of dispersing individuals. We used an eco-evolutionary model to explore how environmental gradients influence dispersal evolution and, in turn, modulate the speed and predictability of invasion. Environmental gradients opposed evolution of increased dispersal during invasion, even leading to evolution of reduced dispersal along steeper gradients. Counterintuitively, reduced dispersal could allow for faster expansion by minimizing maladaptive gene flow and facilitating adaptation. While dispersal evolution across homogenous landscapes increased both the mean and variance of expansion speed, these increases were greatly dampened by environmental gradients. We illustrate our model's potential application to prediction and management of invasions by parameterizing it with data from a recent invertebrate range expansion. Overall, we find that environmental gradients strongly modulate the effect of dispersal evolution on invasion trajectories.

Climate change reduces long-term population benefits from no-take marine protected areas through selective pressures on species movement

Marine protected areas (MPAs) are important conservation tools that confer ecosystem benefits by removing fishing within their borders to allow stocks to rebuild. Fishing mortality outside a traditionally fixed MPA can exert selective pressure for low movement alleles, resulting in enhanced protection. While evolving to move less may be useful for conservation presently, it could be detrimental in the face of climate change for species that need to move to track their thermal optimum. Here, we build a spatially explicit simulation model to assess the impact of movement evolution in and around static MPAs resulting from both fishing mortality and temperature-dependent natural mortality on conservation benefits across five climate scenarios: (i) linear mean temperature shift, (ii) El Niño/La Niña conditions, (iii) heat waves, (iv) heatwaves with a mean temperature shift, and (v) no climate change. While movement evolution allows populations within MPAs to survive longer, we find that over time, climate change degrades the benefits by selecting for higher movement genotypes. Resulting population declines within MPAs are faster than expected based on climate mortality alone, even within the largest MPAs. Our findings suggest that while static MPAs may conserve species for a time, other strategies, such as dynamic MPA networks or assisted migration, may also be required to effectively incorporate climate change into conservation planning.

Spatial sorting promotes rapid (mal)adaptation in the red-shouldered soapberry bug after hurricane-driven local extinctions

Predicting future evolutionary change is a critical challenge in the Anthropocene as geographic range shifts and local extinction emerge as hallmarks of planetary change. Hence, spatial sorting-a driver of rapid evolution in which dispersal-associated traits accumulate along expanding range edges and within recolonized habitats-might be of growing importance in ecology and conservation. We report on the results of a natural experiment that monitored recolonization of host plants by the seed-feeding, red-shouldered soapberry bug, Jadera haematoloma, after local extinctions from catastrophic flooding in an extreme hurricane. We tested the contribution of spatial sorting to generate rapid and persistent evolution in dispersal traits, as well as in feeding traits unrelated to dispersal. Long-winged dispersal forms accumulated in recolonized habitats and due to genetic correlation, mouthparts also became longer and this shift persisted across generations. Those longer mouthparts were probably adaptive on one host plant species but maladaptive on two others based on matching the optimum depth of seeds within their host fruits. Moreover, spatial sorting eroded recently evolved adaptive divergence in mouthpart length among all host-associated biotypes, an outcome pointing to profound practical consequences of the extreme weather event for local adaptation, population resilience and evolutionary futures.

Travelling with a parasite: the evolution of resistance and dispersal syndromes during experimental range expansion

Rapid evolutionary change during range expansions can lead to diverging range core and front populations, with the emergence of dispersal syndromes (coupled responses in dispersal and life-history traits). Besides intraspecific effects, range expansions may be impacted by interspecific interactions such as parasitism. Yet, despite the potentially large impact of parasites imposing additional selective pressures on the host, their role on range expansions remains largely unexplored. Using microcosm populations of the ciliate Paramecium caudatum and its bacterial parasite Holospora undulata, we studied experimental range expansions under parasite presence or absence. We found that the interaction of range expansion and parasite treatments affected the evolution of host dispersal syndromes. Namely, front populations showed different associations of population growth parameters and swimming behaviours than core populations, indicating divergent evolution. Parasitism reshaped trait associations, with hosts evolved in the presence of the parasite exhibiting overall increased resistance and reduced dispersal. Nonetheless, when comparing infected range core and front populations, we found a positive association, suggesting joint evolution of resistance and dispersal at the front. We conclude that host-parasite interactions during range expansions can change evolutionary trajectories; this in turn may feedback on the ecological dynamics of the range expansion and parasite epidemics.

Understanding the drivers of dispersal evolution in range expansions and their ecological consequences

Research has conclusively demonstrated the potential for dispersal evolution in range expansions and shifts, however the degree of dispersal evolution observed has varied substantially among organisms. Further, it is unknown how the factors influencing dispersal evolution might impact other ecological processes at play. We use an individual-based model to investigate the effects of the underlying genetics of dispersal and mode of reproduction in range expansions and shifts. Consistent with predictions from stationary populations, dispersal evolution increases with sexual reproduction and loci number. Contrary to our predictions, however, increased dispersal does not always improve a population’s ability to track changing conditions. The mate finding Allee effect inherent to sexual reproduction increases extinction risk during range shifts, counteracting the beneficial effect of increased dispersal evolution. Our results demonstrate the importance of considering both ecological and evolutionary processes for understanding range expansions and shifts.

Tribolium castaneum: A Model Insect for Fundamental and Applied Research

Tribolium castaneum has a long history as a model species in many distinct subject areas, but improved connections among the genetics, genomics, behavioral, ecological, and pest management fields are needed to fully realize this species' potential as a model. Tribolium castaneum was the first beetle whose genome was sequenced, and a new genome assembly and enhanced annotation, combined with readily available genomic research tools, have facilitated its increased use in a wide range of functional genomics research. Research into T. castaneum's sensory systems, response to pheromones and kairomones, and patterns of movement and landscape utilization has improved our understanding of behavioral and ecological processes. Tribolium castaneum has also been a model in the development of pest monitoring and management tactics, including evaluation of insecticide resistance mechanisms. Application of functional genomics approaches to behavioral, ecological, and pest management research is in its infancy but offers a powerful tool that can link mechanism with function and facilitate exploitation of these relationships to better manage this important food pest.

Increased rates of dispersal of free-ranging cane toads (Rhinella marina) during their global invasion

Invasions often accelerate through time, as dispersal-enhancing traits accumulate at the expanding range edge. How does the dispersal behaviour of individual organisms shift to increase rates of population spread? We collate data from 44 radio-tracking studies (in total, of 650 animals) of cane toads (Rhinella marina) to quantify distances moved per day, and the frequency of displacement in their native range (French Guiana) and two invaded areas (Hawai’i and Australia). We show that toads in their native-range, Hawai’i and eastern Australia are relatively sedentary, while toads dispersing across tropical Australia increased their daily distances travelled from 20 to 200 m per day. That increase reflects an increasing propensity to change diurnal retreat sites every day, as well as to move further during each nocturnal displacement. Daily changes in retreat site evolved earlier than did changes in distances moved per night, indicating a breakdown in philopatry before other movement behaviours were optimised to maximise dispersal.

Predicting responses to marine heatwaves using functional traits

Marine heatwaves (MHWs), discrete but prolonged periods of anomalously warm seawater, can fundamentally restructure marine communities and ecosystems. Although our understanding of these events has improved in recent years, key knowledge gaps hinder our ability to predict how MHWs will affect patterns of biodiversity. Here, we outline a functional trait approach that enables a better understanding of which species and communities will be most vulnerable to MHWs, and how the distribution of species and composition of communities are likely to shift through time. Our perspective allows progress toward unifying extreme events and longer term environmental trends as co-drivers of ecological change, with the incorporation of species traits into our predictions allowing for a greater capacity to make management decisions.

Dispersal evolution in temporally variable environments: implications for plant range dynamics

Dispersal-the movement of an individual from the site of birth to a different site for reproduction-is an ecological and evolutionary driver of species ranges that shapes patterns of colonization, connectivity, gene flow, and adaptation. In plants, the traits that influence dispersal often vary within and among species, are heritable, and evolve in response to the fitness consequences of moving through heterogeneous landscapes. Spatial and temporal variation in the quality and quantity of habitat are important sources of selection on dispersal strategies across species ranges. While recent reviews have evaluated the interactions between spatial variation in habitat and dispersal dynamics, the extent to which geographic variation in temporal variability can also shape range-wide patterns in dispersal traits has not been synthesized. In this paper, we summarize key predictions from metapopulation models that evaluate how dispersal evolves in response to spatial and temporal habitat variability. Next, we compile empirical data that quantify temporal variability in plant demography and patterns of dispersal trait variation across species ranges to evaluate the hypothesis that higher temporal variability favors increased dispersal at plant range limits. We found some suggestive evidence supporting this hypothesis while more generally identifying a major gap in empirical work evaluating plant metapopulation dynamics across species ranges and geographic variation in dispersal traits. To address this gap, we propose several future research directions that would advance our understanding of the interplay between spatiotemporal variability and dispersal trait variation in shaping the dynamics of current and future species ranges.

Morphological evolution reduces downstream displacement in juvenile landlocked salmon

Severe flooding often leads to downstream displacement of aquatic animals. Despite this, many salmonid populations persist in habitats located upstream of tall barriers, such as artificial check dams and/or natural waterfalls, that completely block fishes from returning to the upstream areas after flooding. The evolution of such populations may be affected by spatial sorting due to differential rates of downstream displacement. This study examined whether a morphological trait (increased body depth) that allows individuals to better maintain their position during flooding has evolved in juvenile amago salmon Oncorhynchus masou ishikawae inhabiting above-barrier habitats in two rivers. In both rivers, juveniles collected at the stations with multiple downstream barriers had deeper bodies than those collected at other stations. Similar differences were found in juveniles reared in a common-garden experiment. Field experiments with natural flooding also indicated that deep bodies help juveniles resist downstream displacement. These results consistently suggest that juveniles in some above-barrier habitats have evolved deep bodies to resist downstream displacement due to flooding. Our study is the first to show the evolutionary outcomes of passive spatial sorting during severe climate events.

The evolutionary dynamics of biological invasions: A multi-approach perspective

Biological invasions, the establishment and spread of non-native species in new regions, can have extensive economic and environmental consequences. Increased global connectivity accelerates introduction rates, while climate and land-cover changes may decrease the barriers to invasive populations spread. A detailed knowledge of the invasion history, including assessing source populations, routes of spread, number of independent introductions, and the effects of genetic bottlenecks and admixture on the establishment success, adaptive potential, and further spread, is crucial from an applied perspective to mitigate socioeconomic impacts of invasive species, as well as for addressing fundamental questions on the evolutionary dynamics of the invasion process. Recent advances in genomics together with the development of geographic information systems provide unprecedented large genetic and environmental datasets at global and local scales to link population genomics, landscape ecology, and species distribution modeling into a common framework to study the invasion process. Although the factors underlying population invasiveness have been extensively reviewed, analytical methods currently available to optimally combine molecular and environmental data for inferring invasive population demographic parameters and predicting further spreading are still under development. In this review, we focus on the few recent insect invasion studies that combine different datasets and approaches to show how integrating genetic, observational, ecological, and environmental data pave the way to a more integrative biological invasion science. We provide guidelines to study the evolutionary dynamics of invasions at each step of the invasion process, and conclude on the benefits of including all types of information and up-to-date analytical tools from different research areas into a single framework.

How Dispersal Evolution and Local Adaptation Affect the Range Dynamics of Species Lagging Behind Climate Change

AbstractAs climate changes, species' ability to spatially track suitable climate depends on their spread velocity, a function of their population growth and dispersal capacity. When climate changes faster than species can spread, the climate experienced at species' expanding range edges may ameliorate as conditions become increasingly similar to those of the range core. When this boosts species' growth rates, their spread accelerates. Here, we use simulations of a spreading population with an annual life history to explore how climatic amelioration interacts with dispersal evolution and local adaptation to determine the dynamics of spread. We found that depending on the timing of dispersal evolution, spread velocity can show contrasting trajectories, sometimes transiently exceeding the climate velocity before decelerating. Climatic amelioration can also accelerate the spread of populations composed of genotypes best adapted to local climatic conditions, but the exact dynamics depends on the pattern of climatic adaptation. We conclude that failing to account for demographic variation across climatic gradients can lead to erroneous conclusions about species' capacity to spatially track suitable climate.

Back to the future: Reintegrating biology to understand how past eco-evolutionary change can predict future outcomes

During the last few decades, biologists have made remarkable progress in understanding the fundamental processes that shape life. But despite the unprecedented level of knowledge now available, large gaps still remain in our understanding of the complex interplay of eco-evolutionary mechanisms across scales of life. Rapidly changing environments on Earth provide a pressing need to understand the potential implications of eco-evolutionary dynamics, which can be achieved by improving existing eco-evolutionary models and fostering convergence among the sub-fields of biology. We propose a new, data-driven approach that harnesses our knowledge of the functioning of biological systems to expand current conceptual frameworks and develop corresponding models that can more accurately represent and predict future eco-evolutionary outcomes. We suggest a roadmap toward achieving this goal. This long-term vision will move biology in a direction that can wield these predictive models for scientific applications that benefit humanity and increase the resilience of natural biological systems. We identify short, medium, and long-term key objectives to connect our current state of knowledge to this long-term vision, iteratively progressing across three stages: 1) utilizing knowledge of biological systems to better inform eco-evolutionary models, 2) generating models with more accurate predictions, and 3) applying predictive models to benefit the biosphere. Within each stage, we outline avenues of investigation and scientific applications related to the timescales over which evolution occurs, the parameter space of eco-evolutionary processes, and the dynamic interactions between these mechanisms. The ability to accurately model, monitor, and anticipate eco-evolutionary changes would be transformational to humanity's interaction with the global environment, providing novel tools to benefit human health, protect the natural world, and manage our planet's biosphere.

Initial abundance and stochasticity influence competitive outcome in communities

Predicting competitive outcomes in communities frequently involves inferences based on deterministic population models since these provide clear criteria for exclusion (e.g. R* rule) or long-term coexistence (e.g. mutual invasibility). However, incorporating stochasticity into population- or community-level processes into models is necessary if the goal is to explain variation in natural systems, which are inherently stochastic. Similarly, in systems with demographic or environmental stochasticity, weaker competitors have the potential to exclude superior competitors, contributing to what is known as 'competitive indeterminacy'. The importance of such effects for natural communities is unknown, in part because it is difficult to demonstrate that multiple forms of stochasticity are present in these communities. Moreover, the effects of multiple forms of stochasticity on competitive outcomes are largely untested, even in theory. Here, we address these issues by examining the role of stochasticity in replicated communities of flour beetles (Tribolium sp.). To do so, we developed a set of two-species stochastic Ricker models incorporating four distinct forms of stochasticity: environmental stochasticity, demographic stochasticity, demographic heterogeneity and stochastic sex determination. By fitting models to experimental data, and simulating fit models to examine long- term behaviour, we found that both the duration of transient coexistence and the degree of competitive indeterminacy were sensitive to the forms of stochasticity included in our models. These findings suggest the current estimates of extinction risk, coexistence and time until competitive exclusion in communities may not be accurate when based on models that exclude relevant forms of stochasticity.

Two centuries of monarch butterfly collections reveal contrasting effects of range expansion and migration loss on wing traits

Migratory animals exhibit traits that allow them to exploit seasonally variable habitats. In environments where migration is no longer beneficial, such as oceanic islands, migration-association traits may be selected against or be under relaxed selection. Monarch butterflies are best known for their continent-scale migration in North America but have repeatedly become established as nonmigrants in the tropical Americas and on Atlantic and Pacific Islands. These replicated nonmigratory populations provide natural laboratories for understanding the rate of evolution of migration-associated traits. We measured >6,000 museum specimens of monarch butterflies collected from 1856 to the present as well as contemporary wild-caught monarchs from around the world. We determined 1) how wing morphology varies across the monarch's global range, 2) whether initial long-distance founders were particularly suited for migration, and 3) whether recently established nonmigrants show evidence for contemporary phenotypic evolution. We further reared >1,000 monarchs from six populations around the world under controlled conditions and measured migration-associated traits. Historical specimens show that 1) initial founders are well suited for long-distance movement and 2) loss of seasonal migration is associated with reductions in forewing size and elongation. Monarch butterflies raised in a common garden from four derived nonmigratory populations exhibit genetically based reductions in forewing size, consistent with a previous study. Our findings provide a compelling example of how migration-associated traits may be favored during the early stages of range expansion, and also the rate of reductions in those same traits upon loss of migration.

Westward range expansion from middle latitudes explains the Mississippi River discontinuity in a forest herb of eastern North America

It is often expected that temperate plants have expanded their geographical ranges northward from primarily southern refugia. Evidence for this hypothesis is mixed in eastern North American species, and there is increasing support for colonization from middle latitudes. We studied genome-wide patterns of variation in RADseq loci to test hypotheses concerning range expansion in a North American forest herb (Campanula americana). First, spatial patterns of genetic differentiation were determined. Then phylogenetic relationships and divergence times were estimated. Spatial signatures of genetic drift were also studied to identify the directionality of recent range expansion and its geographical origins. Finally, spatially explicit scenarios for the spread of plants across the landscape were compared, using variation in the population mutation parameter and Tajima's D. We found strong longitudinal subdivision, with populations clustering into groups west and east of the Mississippi River. While the southeastern region was probably part of a diverse Pleistocene refugium, there is little evidence that range expansion involved founders from these southern locales. Instead, declines in genetic diversity and the loss of rare alleles support a westward colonization wave from a middle latitude refugium near the southern Appalachian Mountains, with subsequent expansion from a Pleistocene staging ground in the Mississippi River Valley (0.51-1.27 million years ago). These analyses implicate stepping stone colonization from middle latitudes as an important mechanism of species range expansion in eastern North America. This study further demonstrates the utility of population genetics as a tool to infer the routes travelled by organisms during geographical range expansion.

Eco-evolutionary dynamics of range expansion

Understanding the movement of species' ranges is a classic ecological problem that takes on urgency in this era of global change. Historically treated as a purely ecological process, range expansion is now understood to involve eco-evolutionary feedbacks due to spatial genetic structure that emerges as populations spread. We synthesize empirical and theoretical work on the eco-evolutionary dynamics of range expansion, with emphasis on bridging directional, deterministic processes that favor evolved increases in dispersal and demographic traits with stochastic processes that lead to the random fixation of alleles and traits. We develop a framework for understanding the joint influence of these processes in changing the mean and variance of expansion speed and its underlying traits. Our synthesis of recent laboratory experiments supports the consistent role of evolution in accelerating expansion speed on average, and highlights unexpected diversity in how evolution can influence variability in speed: results not well predicted by current theory. We discuss and evaluate support for three classes of modifiers of eco-evolutionary range dynamics (landscape context, trait genetics, and biotic interactions), identify emerging themes, and suggest new directions for future work in a field that stands to increase in relevance as populations move in response to global change.

Dispersal evolution diminishes the negative density dependence in dispersal

In many organisms, dispersal varies with the local population density. Such patterns of density-dependent dispersal (DDD) are expected to shape the dynamics, spatial spread, and invasiveness of populations. Despite their ecological importance, empirical evidence for the evolution of DDD patterns remains extremely scarce. This is especially relevant because rapid evolution of dispersal traits has now been empirically confirmed in several taxa. Changes in DDD of dispersing populations could help clarify not only the role of DDD in dispersal evolution, but also the possible pattern of subsequent range expansion. Here, we investigate the relationship between dispersal evolution and DDD using a long-term experimental evolution study on Drosophila melanogaster. We compared the DDD patterns of four dispersal-selected populations and their non-selected controls. The control populations showed negative DDD, which was stronger in females than in males. In contrast, the dispersal-selected populations showed DDD, where neither males nor females exhibited DDD. We compare our results with previous evolutionary predictions that focused largely on positive DDD, and highlight how the direction of evolutionary change depends on the initial DDD pattern of a population. Finally, we discuss the implications of DDD evolution for spatial ecology and evolution.

Gene swamping alters evolution during range expansions in the protist Tetrahymena thermophila

At species' range edges, individuals often face novel environmental conditions that may limit range expansion until populations adapt. The potential to adapt depends on genetic variation upon which selection can act. However, populations at species' range edges are often genetically depauperate. One mechanism increasing genetic variation is reshuffling existing variation through sex. Sex, however, can potentially limit adaptation by breaking up existing beneficial allele combinations (recombination load). The gene swamping hypothesis predicts this is specifically the case when populations expand along an abiotic gradient and asymmetric dispersal leads to numerous maladapted dispersers from the range core swamping the range edge. We used the ciliate Tetrahymena thermophila as a model for testing the gene swamping hypothesis. We performed replicated range expansions in landscapes with or without a pH-gradient, while simultaneously manipulating the occurrence of gene flow and sexual versus asexual reproduction. We show that sex accelerated evolution of local adaptation in the absence of gene flow, but hindered it in the presence of gene flow. However, sex affected adaptation independently of the pH-gradient, indicating that both abiotic gradients and the biotic gradient in population density lead to gene swamping. Overall, our results show that gene swamping alters adaptation in life-history strategies.

A Review of the Phenotypic Traits Associated with Insect Dispersal Polymorphism, and Experimental Designs for Sorting out Resident and Disperser Phenotypes

Dispersal represents a key life-history trait with several implications for the fitness of organisms, population dynamics and resilience, local adaptation, meta-population dynamics, range shifting, and biological invasions. Plastic and evolutionary changes of dispersal traits have been intensively studied over the past decades in entomology, in particular in wing-dimorphic insects for which literature reviews are available. Importantly, dispersal polymorphism also exists in wing-monomorphic and wingless insects, and except for butterflies, fewer syntheses are available. In this perspective, by integrating the very latest research in the fast moving field of insect dispersal ecology, this review article provides an overview of our current knowledge of dispersal polymorphism in insects. In a first part, some of the most often used experimental methodologies for the separation of dispersers and residents in wing-monomorphic and wingless insects are presented. Then, the existing knowledge on the morphological and life-history trait differences between resident and disperser phenotypes is synthetized. In a last part, the effects of range expansion on dispersal traits and performance is examined, in particular for insects from range edges and invasion fronts. Finally, some research perspectives are proposed in the last part of the review.

Stochastic processes drive rapid genomic divergence during experimental range expansions

Range expansions are crucibles for rapid evolution, acting via both selective and neutral mechanisms. While selection on traits such as dispersal and fecundity may increase expansion speed, neutral mechanisms arising from repeated bottlenecks and genetic drift in edge populations (i.e. gene surfing) could slow spread or make it less predictable. Thus, it is necessary to disentangle the effects of selection from neutral mechanisms to robustly predict expansion dynamics. This is difficult to do with expansions in nature, as replicated expansions are required to distinguish selective and neutral processes in the genome. Using replicated microcosms of the red flour beetle ( Tribolium castaneum), we identify a robust signature of stochastic, neutral mechanisms in genomic changes arising over only eight generations of expansion and assess the role of standing variation and de novo mutations in driving these changes. Average genetic diversity was reduced within edge populations, but with substantial among-replicate variability in the changes at specific genomic windows. Such variability in genomic changes is consistent with a large role for stochastic, neutral processes. This increased genomic divergence among populations was mirrored by heightened variation in population size and expansion speed, suggesting that stochastic variation in the genome could increase unpredictability of range expansions.

Dispersal and life-history traits in a spider with rapid range expansion

BACKGROUND

Dispersal and reproduction are key life-history traits that jointly determine species' potential to expand their distribution, for instance in light of ongoing climate change. These life-history traits are known to be under selection by changing local environmental conditions, but they may also evolve by spatial sorting. While local natural selection and spatial sorting are mainly studied in model organisms, we do not know the degree to which these processes are relevant in the wild, despite their importance to a comprehensive understanding of species' resistance and tolerance to climate change.

METHODS

The wasp spider Argiope bruennichi has undergone a natural range expansion - from the Mediterranean to Northern Europe during the recent decades. Using reciprocal common garden experiments in the laboratory, we studied differences in crucial traits between replicated core (Southern France) and edge (Baltic States) populations. We tested theoretical predictions of enhanced dispersal (ballooning behaviour) and reproductive performance (fecundity and winter survival) at the expansion front due to spatial sorting and local environmental conditions.

RESULTS

Dispersal rates were not consistently higher at the northern expansion front, but were impacted by the overwintering climatic conditions experienced, such that dispersal was higher when spiderlings had experienced winter conditions as occur in their region. Hatching success and winter survival were lower at the range border. In agreement with theoretical predictions, spiders from the northern leading edge invested more in reproduction for their given body size.

CONCLUSIONS

We found no evidence for spatial sorting leading to higher dispersal in northern range edge populations of A. bruennichi. However, reproductive investment and overwintering survival between core and edge populations differed. These life-history traits that directly affect species' expansion rates seem to have diverged during the recent range expansion of A. bruennichi. We discuss the observed changes with respect to the species' natural history and the ecological drivers associated with range expansion to northern latitudes.

Flight capacity increases then declines from the core to the margins of an invasive species' range

Individuals that disperse farther than other individuals are more likely to be on the frontlines of spreading populations and may be more likely to mate with one another as a consequence of their spatial proximity. Over generations, this process-known as spatial sorting-can produce patterns of increasing dispersal ability from a population's core towards the spreading front. By contrast, when the spread of a population is limited by the availability of suitable habitat, theory predicts that range boundaries can select against more dispersive phenotypes and produce patterns of decreasing dispersal capacity towards population margins. In a common garden study of invasive kudzu bugs (Megacopta cribraria)-which are limited by the availability of hostplants in their southern and western margins-I show that midrange individuals fly 49% farther than individuals in the core and 37% farther than individuals at margins. This result highlights that other processes, such as maternal effects or selection at range boundaries, may create more complicated patterns of dispersal ability across landscapes than predicted by models of spatial sorting alone.

Habitat availability explains variation in climate-driven range shifts across multiple taxonomic groups

Range shifting is vital for species persistence, but there is little consensus on why individual species vary so greatly in the rates at which their ranges have shifted in response to recent climate warming. Here, using 40 years of distribution data for 291 species from 13 invertebrate taxa in Britain, we show that interactions between habitat availability and exposure to climate change at the range margins explain up to half of the variation in rates of range shift. Habitat generalists expanded faster than more specialised species, but this intrinsic trait explains less of the variation in range shifts than habitat availability, which additionally depends on extrinsic factors that may be rare or widespread at the range margin. Similarly, while climate change likely underlies polewards expansions, we find that more of the between-species variation is explained by differences in habitat availability than by changes in climatic suitability. A model that includes both habitat and climate, and their statistical interaction, explains the most variation in range shifts. We conclude that climate-change vulnerability assessments should focus as much on future habitat availability as on climate sensitivity and exposure, with the expectation that habitat restoration and protection will substantially improve species' abilities to respond to uncertain future climates.

Genetic admixture accelerates invasion via provisioning rapid adaptive evolution

Genetic admixture, the intraspecific hybridization among divergent introduced sources, can immediately facilitate colonization via hybrid vigor and profoundly enhance invasion via contributing novel genetic variation to adaption. As hybrid vigor is short-lived, provisioning adaptation is anticipated to be the dominant and long-term profit of genetic admixture, but the evidence for this is rare. We employed the 30 years' geographic-scale invasion of the salt marsh grass, Spartina alterniflora, as an evolutionary experiment and evaluated the consequences of genetic admixture by combining the reciprocal transplant experiment with quantitative and population genetic surveys. Consistent with the documentation, we found that the invasive populations in China had multiple origins from the southern Atlantic coast and the Gulf of Mexico in the US. Interbreeding among these multiple sources generated a "hybrid swarm" that spread throughout the coast of China. In the northern and mid-latitude China, natural selection greatly enhanced fecundity, plant height and shoot regeneration compared to the native populations. Furthermore, genetic admixture appeared to have broken the negative correlation between plant height and shoot regeneration, which was genetically-based in the native range, and have facilitated the evolution of super competitive genotypes in the invasive range. In contrast to the evolved northern and mid-latitude populations, the southern invasive populations showed slight increase of plant height and shoot regeneration compared to the native populations, possibly reflecting the heterotic effect of the intraspecific hybridization. Therefore, our study suggests a critical role of genetic admixture in accelerating the geographic invasion via provisioning rapid adaptive evolution.

Evolution at the Edge of Expanding Populations

Predicting the evolution of expanding populations is critical to controlling biological threats such as invasive species and cancer metastasis. Expansion is primarily driven by reproduction and dispersal, but nature abounds with examples of evolution where organisms pay a reproductive cost to disperse faster. When does selection favor this "survival of the fastest"? We searched for a simple rule, motivated by evolution experiments where swarming bacteria evolved into a hyperswarmer mutant that disperses ∼100% faster but pays a growth cost of ∼10% to make many copies of its flagellum. We analyzed a two-species model based on the Fisher equation to explain this observation: the population expansion rate (v) results from an interplay of growth (r) and dispersal (D) and is independent of the carrying capacity: v = 2 ( rD ) 1 / 2 . A mutant can take over the edge only if its expansion rate (v2) exceeds the expansion rate of the established species (v1); this simple condition ( v 2 > v 1 ) determines the maximum cost in slower growth that a faster mutant can pay and still be able to take over. Numerical simulations and time-course experiments where we tracked evolution by imaging bacteria suggest that our findings are general: less favorable conditions delay but do not entirely prevent the success of the fastest. Thus, the expansion rate defines a traveling wave fitness, which could be combined with trade-offs to predict evolution of expanding populations.

Range expansion is associated with increased survival and fecundity in a long-lived bat species

The speed and dynamics of range expansions shape species distributions and community composition. Despite the critical impact of population growth rates for range expansion, they are neglected in existing empirical studies, which focus on the investigation of selected life-history traits. Here, we present an approach based on non-invasive genetic capture-mark-recapture data for the estimation of adult survival, fecundity and juvenile survival, which determine population growth. We demonstrate the reliability of our method with simulated data, and use it to investigate life-history changes associated with range expansion in 35 colonies of the bat species Rhinolophus hipposideros. Comparing the demographic parameters inferred for 19 of those colonies which belong to an expanding population with those inferred for the remaining 16 colonies from a non-expanding population reveals that range expansion is associated with higher net reproduction. Juvenile survival was the main driver of the observed reproduction increase in this long-lived bat species with low per capita annual reproductive output. The higher average growth rate in the expanding population was not associated with a trade-off between increased reproduction and survival, suggesting that the observed increase in reproduction stems from a higher resource acquisition in the expanding population. Environmental conditions in the novel habitat hence seem to have an important influence on range expansion dynamics, and warrant further investigation for the management of range expansion in both native and invasive species.

Local adaptation, dispersal evolution, and the spatial eco-evolutionary dynamics of invasion

Local adaptation and dispersal evolution are key evolutionary processes shaping the invasion dynamics of populations colonizing new environments. Yet their interaction is largely unresolved. Using a single-species population model along a one-dimensional environmental gradient, we show how local competition and dispersal jointly shape the eco-evolutionary dynamics and speed of invasion. From a focal introduction site, the generic pattern predicted by our model features a temporal transition from wave-like to pulsed invasion. Each regime is driven primarily by local adaptation, while the transition is caused by eco-evolutionary feedbacks mediated by dispersal. The interaction range and cost of dispersal arise as key factors of the duration and speed of each phase. Our results demonstrate that spatial eco-evolutionary feedbacks along environmental gradients can drive strong temporal variation in the rate and structure of population spread, and must be considered to better understand and forecast invasion rates and range dynamics.