Urbanization drives genetic differentiation in physiology and structures the evolution of pace-of-life syndromes in the water flea Daphnia magna

Rapid evolution of consumptive and non-consumptive predator effects on prey population densities, bioenergetics and stoichiometry

Predators can strongly influence prey populations not only through consumptive effects (CE) but also through non-consumptive effects (NCE) imposed by predation risk. Yet, the impact of NCE on bioenergetic and stoichiometric body contents of prey, traits that are shaping life histories, population and food web dynamics, is largely unknown. Moreover, the degree to which NCE can evolve and can drive evolution in prey populations is rarely studied. A 6-week outdoor mesocosm experiment with Caged-Fish (NCE) and Free-Ranging-Fish (CE and NCE) treatments was conducted to quantify and compare the effects of CE and NCE on population densities, bioenergetic and stoichiometric body contents of Daphnia magna, a keystone species in freshwater ecosystems. We tested for evolution of CE and NCE by using experimental populations consisting of D. magna clones from two periods of a resurrected natural pond population: a pre-fish period without fish and a high-fish period with high predation pressure. Both Caged-Fish and Free-Ranging-Fish treatments decreased the body size and population densities, especially in Daphnia from the high-fish period. Only the Free-Ranging-Fish treatment affected bioenergetic variables, while both the Caged-Fish and Free-Ranging-Fish treatments shaped body stoichiometry. The effects of CE and NCE were different between both periods indicating their rapid evolution in the natural resurrected population. Both the Caged-Fish and Free-Ranging-Fish treatments changed the clonal frequencies of the experimental Daphnia populations of the pre-fish as well as the high-fish period, indicating that not only CE but also NCE induced clonal sorting, hence rapid evolution during the mesocosm experiment in both periods. Our results demonstrate that CE as well as NCE have the potential to change not only the body size and population density but also the bioenergetic and stoichiometric characteristics of prey populations. Moreover, we show that these responses not only evolved in the studied resurrected population, but that CE and NCE also caused differential rapid evolution in a time frame of 6 weeks (ca. four to six generations). As NCE can evolve as well as can drive evolution, they may play an important role in shaping eco-evolutionary dynamics in predator-prey interactions.

Cities of the Anthropocene: urban sustainability in an eco-evolutionary perspective

Cities across the globe are driving systemic change in social and ecological systems by accelerating the rates of interactions and intensifying the links between human activities and Earth's ecosystems, thereby expanding the scale and influence of human activities on fundamental processes that sustain life. Increasing evidence shows that cities not only alter biodiversity, they change the genetic makeup of many populations, including animals, plants, fungi and microorganisms. Urban-driven rapid evolution in species traits might have significant effects on socially relevant ecosystem functions such as nutrient cycling, pollination, water and air purification and food production. Despite increasing evidence that cities are causing rapid evolutionary change, current urban sustainability strategies often overlook these dynamics. The dominant perspectives that guide these strategies are essentially static, focusing on preserving biodiversity in its present state or restoring it to pre-urban conditions. This paper provides a systemic overview of the socio-eco-evolutionary transition associated with global urbanization. Using examples of observed changes in species traits that play a significant role in maintaining ecosystem function and resilience, I propose that these evolutionary changes significantly impact urban sustainability. Incorporating an eco-evolutionary perspective into urban sustainability science and planning is crucial for effectively reimagining the cities of the Anthropocene. This article is part of the theme issue 'Evolution and sustainability: gathering the strands for an Anthropocene synthesis'.

Daphnia as a model organism to probe biological responses to nanomaterials-from individual to population effects via adverse outcome pathways

The importance of the cladoceran Daphnia as a model organism for ecotoxicity testing has been well-established since the 1980s. Daphnia have been increasingly used in standardised testing of chemicals as they are well characterised and show sensitivity to pollutants, making them an essential indicator species for environmental stress. The mapping of the genomes of D. pulex in 2012 and D. magna in 2017 further consolidated their utility for ecotoxicity testing, including demonstrating the responsiveness of the Daphnia genome to environmental stressors. The short lifecycle and parthenogenetic reproduction make Daphnia useful for assessment of developmental toxicity and adaption to stress. The emergence of nanomaterials (NMs) and their safety assessment has introduced some challenges to the use of standard toxicity tests which were developed for soluble chemicals. NMs have enormous reactive surface areas resulting in dynamic interactions with dissolved organic carbon, proteins and other biomolecules in their surroundings leading to a myriad of physical, chemical, biological, and macromolecular transformations of the NMs and thus changes in their bioavailability to, and impacts on, daphnids. However, NM safety assessments are also driving innovations in our approaches to toxicity testing, for both chemicals and other emerging contaminants such as microplastics (MPs). These advances include establishing more realistic environmental exposures via medium composition tuning including pre-conditioning by the organisms to provide relevant biomolecules as background, development of microfluidics approaches to mimic environmental flow conditions typical in streams, utilisation of field daphnids cultured in the lab to assess adaption and impacts of pre-exposure to pollution gradients, and of course development of mechanistic insights to connect the first encounter with NMs or MPs to an adverse outcome, via the key events in an adverse outcome pathway. Insights into these developments are presented below to inspire further advances and utilisation of these important organisms as part of an overall environmental risk assessment of NMs and MPs impacts, including in mixture exposure scenarios.

A fast pace-of-life is traded off against a high thermal performance

The integration of life-history, behavioural and physiological traits into a ‘pace-of-life syndrome’ is a powerful concept in understanding trait variation in nature. Yet, mechanisms maintaining variation in ‘pace-of-life’ are not well understood. We tested whether decreased thermal performance is an energetic cost of a faster pace-of-life. We characterized the pace-of-life of larvae of the damselfly Ischnura elegans from high-latitude and low-latitude regions when reared at 20°C or 24°C in a common-garden experiment, and estimated thermal performance curves for a set of behavioural, physiological and performance traits. Our results confirm a faster pace-of-life (i.e. faster growth and metabolic rate, more active and bold behaviour) in the low-latitude and in warm-reared larvae, and reveal increased maximum performance, R_max, but not thermal optimum T_opt, in low-latitude larvae. Besides a clear pace-of-life syndrome integration at the individual level, larvae also aligned along a ‘cold–hot’ axis. Importantly, a faster pace-of-life correlated negatively with a high thermal performance (i.e. higher T_opt for swimming speed, metabolic rate, activity and boldness), which was consistent across latitudes and rearing temperatures. This trade-off, potentially driven by the energetically costly maintenance of a fast pace-of-life, may be an alternative mechanism contributing to the maintenance of variation in pace-of-life within populations.

Evolution in Cities

Although research performed in cities will not uncover new evolutionary mechanisms, it could provide unprecedented opportunities to examine the interplay of evolutionary forces in new ways and new avenues to address classic questions. However, while the variation within and among cities affords many opportunities to advance evolutionary biology research, careful alignment between how cities are used and the research questions being asked is necessary to maximize the insights that can be gained. In this review, we develop a framework to help guide alignment between urban evolution research approaches and questions. Using this framework, we highlight what has been accomplished to date in the field of urban evolution and identify several up-and-coming research directions for further expansion. We conclude that urban environments can be used as evolutionary test beds to tackle both new and long-standing questions in evolutionary biology.

Cryptic eco-evolutionary feedback in the city: Urban evolution of prey dampens the effect of urban evolution of the predator

Most research on eco-evolutionary feedbacks focuses on ecological consequences of evolution in a single species. This ignores the fact that evolution in response to a shared environmental factor in multiple species involved in interactions could alter the net cumulative effect of evolution on ecology. We empirically tested whether urbanization-driven evolution in a predator (nymphs of the damselfly Ischnura elegans) and its prey (the water flea Daphnia magna) jointly shape the outcome of predation under simulated heatwaves. Both interactors show genetic trait adaptation to urbanization, particularly to higher temperatures. We cross-exposed common-garden reared damselflies and Daphnia from replicated urban and rural populations, and quantified predation rates and functional response traits. Urban damselfly nymphs showed higher encounter and predation rates than rural damselflies when exposed to rural prey, but this difference disappeared when they preyed on urban Daphnia. This represents a case of a cryptic evo-to-eco feedback, where the evolution of one species dampens the effects of the evolution of another species on their interaction strength. The effects of evolution of each single species were strong: the scenario in which only the predator or prey was adapted to urbanization resulted in a c. 250% increase in encounter rate and a c. 25% increase in predation rate, compared to the rural predator-rural prey combination. Our results provide unique evidence for eco-evolutionary feedbacks in cities, and underscore the importance of a multi-species approach in eco-evolutionary dynamics research.

Genetic differentiation in pesticide resistance between urban and rural populations of a nontarget freshwater keystone interactor, Daphnia magna

There is growing evidence that urbanization drives adaptive evolution in response to thermal gradients. One such example is documented in the water flea Daphnia magna. However, organisms residing in urban lentic ecosystems are increasingly exposed to chemical pollutants such as pesticides through run-off and aerial transportation. The extent to which urbanization drives the evolution of pesticide resistance in aquatic organisms and whether this is impacted by warming and thermal adaptation remains limitedly studied. We performed a common garden rearing experiment using multiple clonal lineages originating from five replicated urban and rural D. magna populations, in which we implemented an acute toxicity test exposing neonates (<24h) to either a solvent control or the organophosphate pesticide chlorpyrifos. Pesticide exposures were performed at two temperatures (20°C vs. 24°C) to test for temperature-associated differences in urbanization-driven evolved pesticide resistance. We identified a strong overall effect of pesticide exposure on Daphnia survival probability (-72.8 percentage points). However, urban Daphnia genotypes showed higher survival probabilities compared to rural ones in the presence of chlorpyrifos (+29.7 percentage points). Our experiment did not reveal strong temperature x pesticide or temperature x pesticide x urbanization background effects on survival probability. The here observed evolution of resistance to an organophosphate pesticide is a first indication Daphnia likely also adapts to pesticide pollution in urban areas. Increased pesticide resistance could facilitate their population persistence in urban ponds, and feed back to ecosystem functions, such as top-down control of algae. In addition, adaptive evolution of nontarget organisms to pest control strategies and occupational pesticide use may modulate how pesticide applications affect genetic and species diversity in urban areas.

Seasonality and social factors, but not noise pollution, influence the song characteristics of two leaf warbler species

Changes in the acoustic signalling of animals occupying urban ecosystems is often associated with the masking effects of noise pollution, but the way in which they respond to noise pollution is not straightforward. An increasing number of studies indicate that responses can be case specific, and some species have been found to respond differently to high levels of natural versus anthropogenic noise, as well as different levels of the latter. While the perception of noise between species may vary with its source, amplitude and temporal features, some species may possess broader environmental tolerance to noise pollution, as they use higher frequency vocalizations that are less masked by low-frequency urban noise. In this study, we explored the song variation of two closely related leaf warblers, the Common Chiffchaff Phylloscopus collybita and the Willow Warbler Phylloscopus trochilus, inhabiting urban green spaces and nonurban forests. The main goal of our study was to evaluate the impact of moderate levels of noise pollution on the songs of species which use higher frequency vocalizations and large frequency bandwidth. Previous studies found that the Common Chiffchaff modified their song in response to intense noise pollution, while no such data is available for the Willow Warbler. However, the majority of urban green spaces, which serve as wildlife hot spots in urban environments are usually polluted with moderate noise levels, which may not mask the acoustic signals of species that communicate with higher frequency. We analysed the spectral and temporal song parameters of both warblers and described the ambient noise present in males’ territories. Additionally, we looked at the social and seasonal aspects of bird song, since there is more than just noise in urban ecosystems which may affect acoustic communication. We found no evidence for noise-related bird song divergence in either species, however, we showed that social factors, time of day and season influence certain Common Chiffchaff and Willow Warbler song characteristics. Lack of noise-related bird song divergence may be due to the relatively low variation in its amplitude or other noise features present within the song frequency range of the studied species. Similar results have previously been shown for a few songbird species inhabiting urban ecosystems. Although in many cases such results remain in the shadow of the positive ones, they all contribute to a better understanding of animal communication in urban ecosystems.

Natural and anthropogenic sources of habitat variation influence exploration behaviour, stress response, and brain morphology in a coastal fish

Evolutionary ecology aims to better understand how ecologically important traits respond to environmental heterogeneity. Environments vary both naturally and as a result of human activities, and investigations that simultaneously consider how natural and human-induced environmental variation affect diverse trait types grow increasingly important as human activities drive species endangerment. Here, we examined how habitat fragmentation and structural habitat complexity affect disparate trait types in Bahamas mosquitofish Gambusia hubbsi inhabiting tidal creeks. We tested a priori predictions for how these factors might influence exploratory behaviour, stress reactivity and brain anatomy. We examined approximately 350 adult Bahamas mosquitofish from seven tidal-creek populations across Andros Island, The Bahamas that varied in both human-caused fragmentation (three fragmented and four unfragmented) and natural habitat complexity (e.g. fivefold variation in rock habitat). Populations that had experienced severe human-induced fragmentation, and thus restriction of tidal exchange from the ocean, exhibited greater exploration of a novel environment, stronger physiological stress responses to a mildly stressful event and smaller telencephala (relative to body size). These changes matched adaptive predictions based mostly on (a) reduced chronic predation risk and (b) decreased demands for navigating tidally dynamic habitats. Populations from sites with greater structural habitat complexity showed a higher propensity for exploration and a relatively larger optic tectum and cerebellum. These patterns matched adaptive predictions related to increased demands for navigating complex environments. Our findings demonstrate environmental variation, including recent anthropogenic impacts (<50 years), can significantly affect complex, ecologically important traits. Yet trait-specific patterns may not be easily predicted, as we found strong support for only six of 12 predictions. Our results further highlight the utility of simultaneously quantifying multiple environmental factors-for example had we failed to account for habitat complexity, we would not have detected the effects of fragmentation on exploratory behaviours. These responses, and their ecological consequences, may be complex: rapid and adaptive phenotypic responses to anthropogenic impacts can facilitate persistence in human-altered environments, but may come at a cost of population vulnerability if ecological restoration was to occur without consideration of the altered traits.

Long-term changes of plumage between urban and rural populations of white-crowned sparrows (Zonotrichia leucophrys)

Urbanization is one of the most extreme forms of land transformation and results in changes to ecosystems and species compositions. As a result, there are strong directional selection pressures compared to nearby rural areas. Despite a surge in research on the different selection pressures on acoustic communication in urban and rural areas, there has been comparatively little investigation into traits involved with visual communication. We measured the plumage of museum specimens of white-crowned sparrows (Zonotrichia leucophrys) from urban and adjacent rural habitats in San Francisco, CA, to assess the effects of divergent habitats on plumage. We found significant differences in dorsal plumage, but not crown plumage, between urban and rural populations that have been diverging over the past 100 years. Urban birds have increasingly darker and duller dorsal plumage, whereas rural birds in adjacent areas have plumage with richer hues and more color complexity. Our findings suggest a newly observed adaptation to urban environments by native species and suggest that many traits, in addition to acoustic signals, may be changing in response to urban selection pressures. Additional collections in urban areas are needed to explore likely divergences in plumage coloration between urban and rural environments.

Physiological adaptation to cities as a proxy to forecast global-scale responses to climate change

Cities are emerging as a new venue to overcome the challenges of obtaining data on compensatory responses to climatic warming through phenotypic plasticity and evolutionary change. In this Review, we highlight how cities can be used to explore physiological trait responses to experimental warming, and also how cities can be used as human-made space-for-time substitutions. We assessed the current literature and found evidence for significant plasticity and evolution in thermal tolerance trait responses to urban heat islands. For those studies that reported both plastic and evolved components of thermal tolerance, we found evidence that both mechanisms contributed to phenotypic shifts in thermal tolerance, rather than plastic responses precluding or limiting evolved responses. Interestingly though, for a broader range of studies, we found that the magnitude of evolved shifts in thermal tolerance was not significantly different from the magnitude of shift in those studies that only reported phenotypic results, which could be a product of evolution, plasticity, or both. Regardless, the magnitude of shifts in urban thermal tolerance phenotypes was comparable to more traditional space-for-time substitutions across latitudinal and altitudinal clines in environmental temperature. We conclude by considering how urban-derived estimates of plasticity and evolution of thermal tolerance traits can be used to improve forecasting methods, including macrophysiological models and species distribution modelling approaches. Finally, we consider areas for further exploration including sub-lethal performance traits and thermal performance curves, assessing the adaptive nature of trait shifts, and taking full advantage of the environmental thermal variation that cities generate.

Evidence for the evolution of thermal tolerance, but not desiccation tolerance, in response to hotter, drier city conditions in a cosmopolitan, terrestrial isopod

Cities are often hotter and drier compared with nearby undeveloped areas, but how organisms respond to these multifarious stressors associated with urban heat islands is largely unknown. Terrestrial isopods are especially susceptible to temperature and aridity stress as they have retained highly permeable gills from their aquatic ancestors. We performed a two temperature common garden experiment with urban and rural populations of the terrestrial isopod, Oniscus asellus, to uncover evidence for plastic and evolutionary responses to urban heat islands. We focused on physiological tolerance traits including tolerance of heat, cold, and desiccation. We also examined body size responses to urban heat islands, as size can modulate physiological tolerances. We found that different mechanisms underlie responses to urban heat islands. While evidence suggests urban isopods may have evolved higher heat tolerance, urban and rural isopods had statistically indistinguishable cold and desiccation tolerances. In both populations, plasticity to warmer rearing temperature diminished cold tolerance. Although field-collected urban and rural isopods were the same size, rearing temperature positively affected body size. Finally, larger size improved desiccation tolerance, which itself was influenced by rearing temperature. Our study demonstrates how multifarious changes associated with urban heat islands will not necessarily contribute to contemporary evolution in each of the corresponding physiological traits.

Effects of Urbanization and Landscape on Gut Microbiomes in White-Crowned Sparrows

Habitats are changing rapidly around the globe and urbanization is one of the primary drivers. Urbanization changes food availability, environmental stressors, and the prevalence of disease for many species. These changes can lead to divergence in phenotypic traits, including behavioral, physiological, and morphological features between urban and rural populations. Recent research highlights that urbanization is also changing the gut microbial communities found in a diverse group of host species. These changes have not been uniform, leaving uncertainty as to how urban habitats are shaping gut microbial communities. To better understand these effects, we investigated the gut bacterial communities of White-Crowned Sparrow (Zonotrichia leucophrys) populations along an urbanization gradient in the San Francisco Bay area. We examined how gut bacterial communities vary with the local environment and host morphological characteristics. We found direct effects of environmental factors, including urban noise levels and territory land cover, as well as indirect effects through body size and condition, on alpha and beta diversity of gut microbial communities. We also found that urban and rural birds' microbiomes differed in which variables predicted their diversity, with urban communities driven by host morphology, and rural communities driven by environmental factors. Elucidating these effects provides a better understanding of how urbanization affects wild avian physiology.

Lower bioenergetic costs but similar immune responsiveness under a heat wave in urban compared to rural damselflies

There is mounting evidence that the widespread phenotypic changes in response to urbanization may reflect adaptations caused by rapid evolutionary processes driven by urban-related stressors. Compared to increased habitat fragmentation and pollution, adaptations towards another typical urban-related stressor, that is higher and longer lasting very high temperatures (heat waves), are much less studied. Notably, the sensitivities to heat waves of life-history traits and important fitness-related physiological traits such as immune responsiveness and bioenergetic variables (energy availability, energy consumption and their balance) have never been contrasted between urban and rural populations. By conducting a laboratory common-garden experiment, we compared effects of a simulated heat wave on life history (survival and growth rate), immune responsiveness and bioenergetic variables between three urban and three rural populations of the damselfly Coenagrion puella. Because energy-mediated trade-off patterns may only be detected under energetically costly manipulations, all larvae were immune-challenged by simulating ectoparasitism by water mites. As expected, the simulated heat wave caused negative effects on nearly all response variables. The immune responsiveness, on the other hand, increased under the heat wave, consistent with a trade-off pattern between immune function and growth, and this similarly between urban and rural populations. A key finding was that urban larvae suffered less from the simulated heat wave compared to the rural larvae in terms of a lower heat wave-induced depletion in energy availability. This suggests an adaptation of urban populations to better cope with the stronger and more frequent heat waves in cities. Notably, this urbanization-driven evolution in the bioenergetic variables was not apparent in the absence of a heat wave. Given that changes in energy budgets have strong fitness consequences, our findings suggest that the evolved higher ability to cope with heat waves is fundamental for the survival of urban damselfly populations.

Urban evolution comes into its own: Emerging themes and future directions of a burgeoning field

Urbanization has recently emerged as an exciting new direction for evolutionary research founded on our growing understanding of rapid evolution paired with the expansion of novel urban habitats. Urbanization can influence adaptive and nonadaptive evolution in urban-dwelling species, but generalized patterns and the predictability of urban evolutionary responses within populations remain unclear. This editorial introduces the special feature "Evolution in Urban Environments" and addresses four major emerging themes, which include: (a) adaptive evolution and phenotypic plasticity via physiological responses to urban climate, (b) adaptive evolution via phenotype-environment relationships in urban habitats, (c) population connectivity and genetic drift in urban landscapes, and (d) human-wildlife interactions in urban spaces. Here, we present the 16 articles (12 empirical, 3 review, 1 capstone) within this issue and how they represent each of these four emerging themes in urban evolutionary biology. Finally, we discuss how these articles address previous questions and have now raised new ones, highlighting important new directions for the field.

Pedal to the metal: Cities power evolutionary divergence by accelerating metabolic rate and locomotor performance

Metabolic rates of ectotherms are expected to increase with global trends of climatic warming. But the potential for rapid, compensatory evolution of lower metabolic rate in response to rising temperatures is only starting to be explored. Here, we explored rapid evolution of metabolic rate and locomotor performance in acorn-dwelling ants (Temnothorax curvispinosus) in response to urban heat island effects. We reared ant colonies within a laboratory common garden (25°C) to generate a laboratory-born cohort of workers and tested their acute plastic responses to temperature. Contrary to expectations, urban ants exhibited a higher metabolic rate compared with rural ants when tested at 25°C, suggesting a potentially maladaptive evolutionary response to urbanization. Urban and rural ants had similar metabolic rates when tested at 38°C, as a consequence of a diminished plastic response of the urban ants. Locomotor performance also evolved such that the running speed of urban ants was faster than rural ants under warmer test temperatures (32°C and 42°C) but slower under a cooler test temperature (22°C). The resulting specialist-generalist trade-off and higher thermal optimum for locomotor performance might compensate for evolved increases in metabolic rate by allowing workers to more quickly scout and retrieve resources.

Adaptive Evolution in Cities: Progress and Misconceptions

Current narratives suggest that urban adaptation - the adaptive evolution of organisms to cities - is pervasive across taxa and cities. However, in reviewing hundreds of studies, we find only six comprehensive examples of species adaptively evolving to urbanization. We discuss the utility and shortcomings of methods for studying urban adaptation. We then review diverse systems offering preliminary evidence for urban adaptation and outline a research program for advancing its study. Urban environments constitute diverse, interacting selective agents that test the limits of adaptation. Understanding urban adaptation therefore offers unique opportunities for addressing fundamental questions in evolutionary biology and for better conserving biodiversity in cities. However, capitalizing on these opportunities requires appropriate research methods and dissemination of accurate narratives.

Genetic Adaptation in New York City Rats

Brown rats (Rattus norvegicus) thrive in urban environments by navigating the anthropocentric environment and taking advantage of human resources and by-products. From the human perspective, rats are a chronic problem that causes billions of dollars in damage to agriculture, health, and infrastructure. Did genetic adaptation play a role in the spread of rats in cities? To approach this question, we collected whole-genome sequences from 29 brown rats from New York City (NYC) and scanned for genetic signatures of adaptation. We tested for 1) high-frequency, extended haplotypes that could indicate selective sweeps and 2) loci of extreme genetic differentiation between the NYC sample and a sample from the presumed ancestral range of brown rats in northeast China. We found candidate selective sweeps near or inside genes associated with metabolism, diet, the nervous system, and locomotory behavior. Patterns of differentiation between NYC and Chinese rats at putative sweep loci suggest that many sweeps began after the split from the ancestral population. Together, our results suggest several hypotheses on adaptation in rats living in proximity to humans.

Urban biodiversity management using evolutionary tools

Cities are fully functioning ecosystems and are home to no-analogue communities of species that interact with each other and which are subject to novel urban stressors. As such, biodiversity can evolve in response to these new urban conditions, making urban species a moving target for conservation and management efforts. An evolving urban biodiversity necessitates integrating evolutionary insights into management for these efforts to be successful in a dynamic urban milieu. Here we present a framework for categorizing urban biodiversity from a management perspective. We then discuss a suite of example management tools and their potential evolutionary implications-both their opportunities for and potential consequence to management. Urban ecosystems are proliferating but, far from being ecological lost causes, they may provide unique insights and opportunities for biodiversity conservation. Determining how to achieve urban biodiversity priorities while managing pest species requires evolutionary thinking.

Remarkable insensitivity of acorn ant morphology to temperature decouples the evolution of physiological tolerance from body size under urban heat islands

Environmental temperature can alter body size and thermal tolerance, yet the effects of temperature rise on the size-tolerance relationship remain unclear. Terrestrial ectotherms with larger body sizes typically exhibit greater tolerance of high (and low) temperatures. However, while warming tends to increase tolerance of high temperatures through phenotypic plasticity and evolutionary change, warming tends to decrease body size through these mechanisms and thus might indirectly contribute to worse tolerance of high temperatures. These contrasting effects of warming on body size, thermal tolerance, and their relationship are increasingly important in light of global climate change. Here, we used replicated urban heat islands to explore the size-tolerance relationship in response to warming. We performed a common garden experiment with a small acorn-dwelling ant species collected from urban and rural populations across three different cities and reared under five laboratory rearing temperatures from 21 to 29 °C. We found that acorn ant body size was remarkably insensitive to laboratory rearing temperature (ant workers exhibited no phenotypic plasticity in body size across rearing temperature) and among populations experiencing cooler rural versus warmer urban environmental temperatures (no evolved differences in body size between urban and rural populations). Further, this insensitivity of body size to temperature was highly consistent across each of the three cities we examined. Because body size was robust to temperature variation, previously described plastic and evolved shifts in heat (and cold) tolerance in acorn ant responses to urbanization were shown to be independent of shifts in body size. Indeed, genetic (colony-level) correlations between heat and cold tolerance traits and body size revealed no significant association between size and tolerance. Our results show how typical trait correlations, such as between size and thermal tolerance, might be decoupled as populations respond to contemporary environmental change.

A roadmap for urban evolutionary ecology

Urban ecosystems are rapidly expanding throughout the world, but how urban growth affects the evolutionary ecology of species living in urban areas remains largely unknown. Urban ecology has advanced our understanding of how the development of cities and towns change environmental conditions and alter ecological processes and patterns. However, despite decades of research in urban ecology, the extent to which urbanization influences evolutionary and eco-evolutionary change has received little attention. The nascent field of urban evolutionary ecology seeks to understand how urbanization affects the evolution of populations, and how those evolutionary changes in turn influence the ecological dynamics of populations, communities, and ecosystems. Following a brief history of this emerging field, this Perspective article provides a research agenda and roadmap for future research aimed at advancing our understanding of the interplay between ecology and evolution of urban-dwelling organisms. We identify six key questions that, if addressed, would significantly increase our understanding of how urbanization influences evolutionary processes. These questions consider how urbanization affects nonadaptive evolution, natural selection, and convergent evolution, in addition to the role of urban environmental heterogeneity on species evolution, and the roles of phenotypic plasticity versus adaptation on species' abundance in cities. Our final question examines the impact of urbanization on evolutionary diversification. For each of these six questions, we suggest avenues for future research that will help advance the field of urban evolutionary ecology. Lastly, we highlight the importance of integrating urban evolutionary ecology into urban planning, conservation practice, pest management, and public engagement.

Toads phenotypically adjust their chemical defences to anthropogenic habitat change

Despite the well-documented effects of human-induced environmental changes on the morphology, physiology, behaviour and life history of wild animals, next to nothing is known about how anthropogenic habitats influence anti-predatory chemical defence, a crucial fitness component of many species. We investigated the amount and composition of defensive toxins in adult common toads (Bufo bufo) captured in natural, agricultural and urban habitats, and in their offspring raised in a common-garden experiment. We found that, compared to toads captured from natural habitats, adults from both types of anthropogenic habitats had larger toxin glands (parotoids) and their toxin secretion contained higher concentrations of bufagenins, the more potent class of bufadienolide toxins. Furthermore, urban toads had lower concentrations of bufotoxins, the compounds with lower toxicity. None of these differences were present in the captive-raised juveniles; instead, toadlets originating from agricultural habitats had smaller parotoids and lower bufotoxin concentrations. These results suggest that toads' chemical defences respond to the challenges of anthropogenic environments via phenotypic plasticity. These responses may constitute non-adaptive consequences of pollution by endocrine-disrupting chemicals as well as adaptive adjustments to the altered predator assemblages of urban and agricultural habitats.

Urban colonization through multiple genetic lenses: The city-fox phenomenon revisited

Urbanization is driving environmental change on a global scale, creating novel environments for wildlife to colonize. Through a combination of stochastic and selective processes, urbanization is also driving evolutionary change. For instance, difficulty in traversing human-modified landscapes may isolate newly established populations from rural sources, while novel selective pressures, such as altered disease risk, toxicant exposure, and light pollution, may further diverge populations through local adaptation. Assessing the evolutionary consequences of urban colonization and the processes underlying them is a principle aim of urban evolutionary ecology. In the present study, we revisited the genetic effects of urbanization on red foxes (Vulpes vulpes) that colonized Zurich, Switzerland. Through use of genome-wide single nucleotide polymorphisms and microsatellite markers linked to the major histocompatibility complex (MHC), we expanded upon a previous neutral microsatellite study to assess population structure, characterize patterns of genetic diversity, and detect outliers associated with urbanization. Our results indicated the presence of one large evolutionary cluster, with substructure evident between geographic sampling areas. In urban foxes, we observed patterns of neutral and functional diversity consistent with founder events and reported increased differentiation between populations separated by natural and anthropogenic barriers. We additionally reported evidence of selection acting on MHC-linked markers and identified outlier loci with putative gene functions related to energy metabolism, behavior, and immunity. We concluded that demographic processes primarily drove patterns of diversity, with outlier tests providing preliminary evidence of possible urban adaptation. This study contributes to our overall understanding of urban colonization ecology and emphasizes the value of combining datasets when examining evolutionary change in an increasingly urban world.

Urban environment and cancer in wildlife: available evidence and future research avenues

While it is generally known that the risk of several cancers in humans is higher in urban areas compared with rural areas, cancer is often deemed a problem of human societies with modern lifestyles. At the same time, more and more wild animals are affected by urbanization processes and are faced with the need to adapt or acclimate to urban conditions. These include, among other things, increased exposure to an assortment of pollutants (e.g. chemicals, light and noise), novel types of food and new infections. According to the abundant literature available for humans, all of these factors are associated with an increased probability of developing cancerous neoplasias; however, the link between the urban environment and cancer in wildlife has not been discussed in the scientific literature. Here, we describe the available evidence linking environmental changes resulting from urbanization to cancer-related physiological changes in wild animals. We identify the knowledge gaps in this field and suggest future research avenues, with the ultimate aim of understanding how our modern lifestyle affects cancer prevalence in urbanizing wild populations. In addition, we consider the possibilities of using urban wild animal populations as models to study the association between environmental factors and cancer epidemics in humans, as well as to understand the evolution of cancer and defence mechanisms against it.

The evolution of city life

Urbanization represents a dominant and growing form of disturbance to Earth's natural ecosystems, affecting biodiversity and ecosystem services on a global scale. While decades of research have illuminated the effects of urban environmental change on the structure and function of ecological communities in cities, only recently have researchers begun exploring the effects of urbanization on the evolution of urban populations. The 15 articles in this special feature represent the leading edge of urban evolutionary biology and address existing gaps in our knowledge. These gaps include: (i) the absence of theoretical models examining how multiple evolutionary mechanisms interact to affect evolution in urban environments; (ii) a lack of data on how urbanization affects natural selection and local adaptation; (iii) poor understanding of whether urban areas consistently affect non-adaptive and adaptive evolution in similar ways across multiple cities; (iv) insufficient data on the genetic and especially genomic signatures of urban evolutionary change; and (v) limited understanding of the evolutionary processes underlying the origin of new human commensals. Using theory, observations from natural populations, common gardens, genomic data and cutting-edge population genomic and landscape genetic tools, the papers in this special feature address these gaps and highlight the power of urban evolutionary biology as a globally replicated 'experiment' that provides a powerful approach for understanding how human altered environments affect evolution.

Urbanization drives genetic differentiation in physiology and structures the evolution of pace-of-life syndromes in the water flea Daphnia magna

Natural and human-induced stressors elicit changes in energy metabolism and stress physiology in populations of a wide array of species. Cities are stressful environments that may lead to differential selection on stress-coping mechanisms. Given that city ponds are exposed to the urban heat island effect and receive polluted run-off, organisms inhabiting these ecosystems might show genetic differentiation for physiological traits enabling them to better cope with higher overall stress levels. A common garden study with 62 Daphnia magna genotypes from replicated urban and rural populations revealed that urban Daphnia have significantly higher concentrations of total body fat, proteins and sugars. Baseline activity levels of the antioxidant defence enzymes superoxide dismutase (SOD) and glutathione-S-transferase (GST) were higher in rural compared with city populations, yet urban animals were equally well protected against lipid peroxidation. Our results add to the recent evidence of urbanization-driven changes in stress physiology and energy metabolism in terrestrial organisms. Combining our results with data on urban life history evolution in Daphnia revealed that urban genotypes show a structured pace-of-life syndrome involving both life-history and physiological traits, whereas this is absent in rural populations.