The heat is on: Genetic adaptation to urbanization mediated by thermal tolerance and body size

Pollution offsets the rapid evolution of increased heat tolerance in a natural population

Despite the increasing evidence for rapid thermal evolution in natural populations, evolutionary rescue under global warming may be constrained by the presence of other stressors. Highly relevant in our polluted planet, is the largely ignored evolutionary trade-off between heat tolerance and tolerance to pollutants. By using two subpopulations (separated 40 years in time) from a resurrected natural population of the water flea Daphnia magna that experienced a threefold increase in heat wave frequency during this period, we tested whether rapid evolution of heat tolerance resulted in reduced tolerance to the widespread metal zinc and whether this would affect heat tolerance upon exposure to the pollutant. Our results revealed rapid evolution of increased heat tolerance in the recent subpopulation. Notably, the sensitivity to the metal tended to be stronger (reduction in net energy budget) or was only present (reductions in heat tolerance and in sugar content) in the recent subpopulation. As a result, the rapidly evolved higher heat tolerance of the recent subpopulation was fully offset when exposed to zinc. Our results highlight that the many reports of evolutionary rescue to global change stressors may give a too optimistic view as our warming planet is polluted by metals and other pollutants.

Urbanization reduces diversity, simplifies community and filter bird species based on their functional traits in a tropical city

Understanding how organisms are coping with major changes imposed by urban intensification is a complex task. In fact, our understanding of the impacts of urbanization on biodiversity is scarce in the global south compared to the north. In this study, we evaluated how bird communities are affected by impact of urban intensification in a tropical city. Thus, we assessed whether increased urban intensification 1) jeopardizes bird diversity (taking into account taxonomic-TD, phylogenetic-PD, and functional-FD dimensions), 2) drives changes in bird community composition and enables the detection of indicator species of such impact, and 3) leads to changes in bird functional traits linked to reproduction, resource acquisition, and survival. We found that urban intensification has a direct impact on the bird community, reducing all three types of diversity. Communities in areas of greater urban intensity are represented by fewer species, and these species are PD and FD less distinct. In addition, we detected at least ten species of areas of lower urban intensity that proved to be more sensitive to urban intensification. With regard to bird traits, we found no significant responses from reproductive, habitat use and feeding variables. Body weight and tail length were the only variables with significant results, with higher urbanization intensity areas selecting for species with lower weights and longer tails. Given the global biodiversity loss we are observing, this information can guide urban managers and planners in designing urban landscapes to maintain biodiversity in cities.

Local thermal adaption mediates the sensitivity of Daphnia magna to nanoplastics under global warming scenarios

The toxicity of nanoplastics at environmentally relevant concentrations has received widespread attention in the context of global warming. Despite numerous studies on the impact of mean temperature (MT), the effects of daily temperature fluctuations (DTFs) on the ecotoxicity of nanoplastics remains largely unexplored. Moreover, the role of evolutionary adaptation in assessing long-term ecological risks is unclear. Here, we investigated the effects of polystyrene nanoplastics (5 μg L-1) on Daphnia magna under varying MT (20 °C and 24 °C) and DTFs (0 °C, 5 °C, and 10 °C). Capitalizing on a space-for-time substitution approach, we further assessed how local thermal adaptation affect the sensitivity of Daphnia to nanoplastics under global warming. Our results indicated that nanoplastics exposure in general reduced heartbeat rate, thoracic limb activity and feeding rate, and increased CytP450, ETS activity and Hgb concentrations. Higher MT and DTFs enhanced these effects. Notably, clones originating from their respective sites performed better under their native temperature conditions, indicating local thermal adaptation. Warm-adapted low-latitude D. magna showed stronger nanoplastics-induced increases in CytP450, ETS activity and Hgb concentrations under local MT 24 °C, while cold-adapted high-latitude D. magna showed stronger nanoplastics-induced decreases in heartbeat rate, thoracic limb activity and feeding rate under high MT than under low MT.

Evidence of plasticity, but not evolutionary divergence, in the thermal limits of a highly successful urban butterfly

Despite the generally negative impact of urbanization on insect biodiversity, some insect species persist in urban habitats. Understanding the mechanisms underpinning the ability of insects to tolerate urban habitats is critical given the contribution of land-use change to the global insect decline. Compensatory mechanisms such as phenotypic plasticity and evolutionary change in thermal physiological traits could allow urban populations to persist under the altered thermal regimes of urban habitats. It is important to understand the contributions of plasticity and evolution to trait change along urbanization gradients as the two mechanisms operate under different constraints and timescales. Here, we examine the plastic and evolutionary responses of heat and cold tolerance (critical thermal maximum [CTmax] and critical thermal minimum [CTmin]) to warming among populations of the cabbage white butterfly, Pieris rapae, from urban and non-urban (rural) habitats using a two-temperature common garden experiment. Although we expected populations experiencing urban warming to exhibit greater CTmax and diminished CTmin through plastic and evolutionary mechanisms, our study revealed evidence only for plasticity in the expected direction of both thermal tolerance traits. We found no evidence of evolutionary divergence in either heat or cold tolerance, despite each trait showing evolutionary potential. Our results suggest that thermal tolerance plasticity contributes to urban persistence in this system. However, as the magnitude of the plastic response was low and comparable to other insect species, other compensatory mechanisms likely further underpin this species' success in urban habitats.

Differential interference effects of thermal pollution on the induced defense of different body-sized cladocerans

Climate warming influences the biological activities of aquatic organisms, including feeding, growth, and reproduction, thereby affecting predator-prey interactions. This study explored the variation in thermal sensitivity of anti-predator responses in two cladoceran species with varying body sizes, Daphnia pulex and Ceriodaphnia cornuta. These species were cultured with or without the fish (Rhodeus ocellatus) kairomone at temperatures of 15, 20, 25, and 30 °C for 15 days. Results revealed that cladocerans of different body sizes exhibited varying responses to fish kairomones in aspects such as individual size, first-brood neonate size, total offspring number, average brood size, growth rate, and reproductive effort. Notably, low temperature differently affected defense responses in cladocerans of different body sizes. Both high and low temperatures moderated the intensity of the kairomone-induced response on body size at maturity. Additionally, low temperature reversed the reducing effect of fish kairomone on the total offspring number, average brood size, and reproductive effort in D. pulex. Conversely, it enhanced the increasing effect of fish kairomone on these parameters in C. cornuta. These results suggest that inducible anti-predator responses in cladocerans are modifiable by temperature. The differential effects of fish kairomones on various cladocerans under temperature influence offer crucial insights for predicting changes in predator-prey interactions within freshwater ecosystems under future climate conditions.

Urban socioeconomic variation influences the ecology and evolution of trophic interactions

As urbanization expands, it is becoming increasingly important to understand how anthropogenic activity is affecting ecological and evolutionary processes. Few studies have examined how human social patterns within cities can modify eco-evolutionary dynamics. We tested how socioeconomic variation corresponds with changes in trophic interactions and natural selection on prey phenotypes using the classic interaction between goldenrod gall flies (Eurosta solidaginis) and their natural enemies: birds, beetles, and parasitoid wasps. We sampled galls from 84 sites across neighbourhoods with varying socioeconomic levels, and quantified the frequency of predation/parasitism on flies and natural selection by each enemy. We found that bird predation was higher in the highest income neighbourhoods, increasing the strength of selection for smaller galls. Wasp and beetle attack, but not their strength of selection, increased in lower income neighbourhoods. We show that socioeconomic variation in cities can have strong unintended consequences for the ecology and evolution of trophic interactions.

Continent-wide parallel urban evolution of increased heat tolerance in a common moth

Urbanization and its urban-heat-island effect (UHI) have expanding footprints worldwide. The UHI means that urban habitats experience a higher mean and more frequent extreme high temperatures than rural habitats, impacting the ontogeny and resilience of urban biodiversity. However, many organisms occupy different microhabitats during different life stages and thus may experience the UHI differently across their development. While evolutionary changes in heat tolerance in line with the UHI have been demonstrated, it is unknown whether such evolutionary responses can vary across development. Here, using common-garden-reared Chiasmia clathrata moths from urban and rural populations from three European countries, we tested for urban evolution of heat shock tolerance in two life stages: larvae and adults. Our results indicate widespread urban evolution of increased heat tolerance in the adult stage only, suggesting that the UHI may be a stronger selective agent in adults. We also found that the difference in heat tolerance between urban and rural populations was similar to the difference between Mid- and North-European regions, suggesting similarity between adaptation to the UHI and natural, latitudinal temperature variation. Our observations incentivize further research to quantify the impact of these UHI adaptations on fitness during urbanization and climate change, and to check whether life-stage-specific adaptations in heat tolerance are typical of other ectothermic species that manage to survive in urbanized settings.

Urbanization and a green corridor do not impact genetic divergence in common milkweed (Asclepias syriaca L.)

Urbanization is altering landscapes globally at an unprecedented rate. While ecological differences between urban and rural environments often promote phenotypic divergence among populations, it is unclear to what degree these trait differences arise from genetic divergence as opposed to phenotypic plasticity. Furthermore, little is known about how specific landscape elements, such as green corridors, impact genetic divergence in urban environments. We tested the hypotheses that: (1) urbanization, and (2) proximity to an urban green corridor influence genetic divergence in common milkweed (Asclepias syriaca) populations for phenotypic traits. Using seeds from 52 populations along three urban-to-rural subtransects in the Greater Toronto Area, Canada, one of which followed a green corridor, we grew ~ 1000 plants in a common garden setup and measured > 20 ecologically-important traits associated with plant defense/damage, reproduction, and growth over four years. We found significant heritable variation for nine traits within common milkweed populations and weak phenotypic divergence among populations. However, neither urbanization nor an urban green corridor influenced genetic divergence in individual traits or multivariate phenotype. These findings contrast with the expanding literature demonstrating that urbanization promotes rapid evolutionary change and offer preliminary insights into the eco-evolutionary role of green corridors in urban environments.

Rural selection drives the evolution of an urban-rural cline in coat color in gray squirrels

Phenotypic differences between urban and rural populations are well-documented, but the evolutionary processes driving trait variation along urbanization gradients are often unclear. We combined spatial data on abundance, trait variation, and measurements of fitness to understand cline structure and test for natural selection on heritable coat color morphs (melanic, gray) of eastern gray squirrels (Sciurus carolinensis) along an urbanization gradient. Population surveys using remote cameras and visual counts at 76 sites along the urbanization gradient revealed a significant cline in melanism, decreasing from 48% in the city center to <5% in rural woodlands. Among 76 squirrels translocated to test for phenotypic selection, survival was lower for the melanic than gray morph in rural woodlands, whereas there was no difference in survival between color morphs in the city. These results suggest the urban-rural cline in melanism is explained by natural selection favoring the gray morph in rural woodlands combined with relaxed selection in the city. Our study illustrates how trait variation between urban and rural populations can emerge from selection primarily in rural populations rather than adaptation to novel features of the urban environment.

Preferred temperature in the warmth of cities: Body size, sex and development stage matter more than urban climate in a ground-dwelling spider

Most ectotherms rely on behavioural thermoregulation to maintain body temperatures close to their physiological optimum. Hence, ectotherms can drastically limit their exposure to thermal extremes by selecting a narrower range of temperatures, which includes their preferred temperature (Tpref). Despite evidence that behavioural thermoregulation can be adjusted by phenotypic plasticity or constrained by natural selection, intraspecific Tpref variations across environmental gradients remain overlooked as compared to other thermal traits like thermal tolerance. Here, we analyzed Tpref variation of spider populations found along a gradient of urban heat island (UHI) which displays large thermal variations over small distances. We measured two components of the thermal preference, namely the mean Tpref and the Tpref range (i.e., standard deviation) in 557 field-collected individuals of a common ground-dwelling spider (Pardosa saltans, Lycosidae) using a laboratory thermal gradient. We determined if Tpref values differed among ten populations from contrasting thermal zones. We showed that endogenous factors such as body size or sex primarily determine both mean Tpref and Tpref range. The Tpref range was also linked to the UHI intensity to a lesser extent, yet only in juveniles. The absence of relationship between Tpref metrics and UHI in adult spiders suggests a Bogert effect according to which the ability of individuals to detect and exploit optimal microclimates weakens the selection pressure of temperatures (here driven by UHI) on their thermal physiology. Alternatively, this lack of relationship could also indicate that temperature patterns occurring at the scale of the spiders' micro-habitat differ from measured ones. This study shows the importance of considering both inter-individual and inter-population variations of the Tpref range when conducting Tpref experiments, and supports Tpref range as being a relevant measure to inform on the strength of behavioural thermoregulation in a given population.

Shifts in survival and reproduction after chronic warming enhance the potential of a marine copepod to persist under extreme heat events

The study of a species' thermal tolerance and vital rates responses provides useful metrics to characterize its vulnerability to ocean warming. Under prolonged thermal stress, plastic and adaptive processes can adjust the physiology of organisms. Yet it is uncertain whether the species can expand their upper thermal limits to cope with rapid and extreme changes in environmental temperature. In this study, we reared the marine copepod Paracartia grani at control (19°C) and warmer conditions (25°C) for >18 generations and assessed their survival and fecundity under short-term exposure to a range of temperatures (11-34°C). After multigenerational warming, the upper tolerance to acute exposure (24 h) increased by 1-1.3°C, although this enhancement decreased to 0.3-0.8°C after longer thermal stress (7 days). Warm-reared copepods were smaller and produced significantly fewer offspring at the optimum temperature. No shift in the thermal breadth of the reproductive response was observed. Yet the fecundity rates of the warm-reared copepods in the upper thermal range were up to 21-fold higher than the control. Our results show that chronic warming improved tolerance to stress temperatures and fecundity of P. grani, therefore, enhancing its chances to persist under extreme heat events.

Life cycle complexity and body mass drive erratic changes in climate vulnerability across ontogeny in a seasonally migrating butterfly

Physiological traits are often used for vulnerability assessments of organismal responses to climate change. Trait values can change dramatically over the life cycle of organisms but are typically assessed at a single developmental stage. Reconciling ontogenetic changes in physiological traits with vulnerability assessments often reveals early life-stage vulnerabilities. The degree to which ontogenetic changes in physiological traits are due to changes in body mass over development versus stage-specific responses determines the degree to which mass can be used as a proxy for vulnerability. Here, we use the painted lady butterfly, Vanessa cardui, to test ontogenetic changes in two physiological traits, the acute thermal sensitivity of routine metabolic rate (RMR Q10) and the critical thermal maximum (CTmax). RMR Q10 generally followed ontogenetic changes in body mass, with stages characterized by smaller body mass exhibiting lower acute thermal sensitivity. However, CTmax was largely decoupled from ontogenetic changes in body mass. In contrast with trends from other studies showing increasing vulnerability among progressively earlier developmental stages, our study revealed highly erratic patterns of vulnerability across ontogeny. Specifically, we found the lowest joint-trait vulnerability (both RMR Q10 and CTmax) in the earliest developmental stage we tested (3rd instar larvae), the highest vulnerabilities in the next two developmental stages (4th and 5th instar larvae), and reduced vulnerability into the pupal and adult stages. Our study supports growing evidence of mechanistic decoupling of physiology across developmental stages and suggests that body mass is not a universal proxy for all physiological trait indicators of climate vulnerability.

Latitude-specific urbanization effects on life history traits in the damselfly Ischnura elegans

Many species are currently adapting to cities at different latitudes. Adaptation to urbanization may require eco-evolutionary changes in response to temperature and invasive species that may differ between latitudes. Here, we studied single and combined effects of increased temperatures and an invasive alien predator on the phenotypic response of replicated urban and rural populations of the damselfly Ischnura elegans and contrasted these between central and high latitudes. Adult females were collected in rural and urban ponds at central and high latitudes. Their larvae were exposed to temperature treatments (current [20°C], mild warming [24°C], and heat wave [28°C; for high latitude only]) crossed with the presence or absence of chemical cues released by the spiny-cheek crayfish (Faxonius limosus), only present at the central latitude. We measured treatment effects on larval development time, mass, and growth rate. Urbanization type affected all life history traits, yet these responses were often dependent on latitude, temperature, and sex. Mild warming decreased mass in rural and increased growth rate in urban populations. The effects of urbanization type on mass were latitude-dependent, with central-latitude populations having a greater phenotypic difference. Urbanization type effects were sex-specific with urban males being lighter and having a lower growth rate than rural males. At the current temperature and mild warming, the predator cue reduced the growth rate, and this independently of urbanization type and latitude of origin. This pattern was reversed during a heat wave in high-latitude damselflies. Our results highlight the context-dependency of evolutionary and plastic responses to urbanization, and caution for generalizing how populations respond to cities based on populations at a single latitude.

Species- and sex-dependent changes in body size between 1892 and 2017, and recent biochemical signatures in rural and urban populations of two ground beetle species

Increasing urbanisation and intensified agriculture lead to rapid transitions of ecosystems. Species that persist throughout rapid transitions may respond to environmental changes across space and/or time, for instance by altering morphological and/or biochemical traits. We used natural history museum specimens, covering the Anthropocene epoch, to obtain long-term data combined with recent samples. We tested whether rural and urban populations of two ground beetle species, Harpalus affinis and H. rufipes, exhibit spatio-temporal intraspecific differences in body size. On a spatial scale, we tested signatures of nitrogen and carbon stable isotopes enrichments in different tissues and body components in recent populations of both species from urban and agricultural habitats. For body size examinations, we used beetles, collected from the early 20th century until 2017 in the Berlin-Brandenburg region, Germany, where urbanisation and agriculture have intensified throughout the last century. For stable isotope examinations, we used recent beetles from urban and agricultural habitats. Our results revealed no spatio-temporal changes in body size in both species' females. Body size of H. rufipes males decreased in the city but remained constant in rural areas over time. We discuss our findings with respect to habitat quality, urban heat and interspecific differences in activity pattern. Although nitrogen isotope ratios were mostly higher in specimens from agricultural habitats, some urban beetles reached equal enrichments. Carbon signatures of both species did not differ between habitats, detecting no differences in energy sources. Our results indicate that increasing urbanisation and intensified agriculture are influencing species' morphology and/or biochemistry. However, changes may be species- and sex-specific.

Urban evolutionary ecology brings exaptation back into focus

The contribution of pre-existing phenotypic variation to evolution in novel environments has long been appreciated. Nevertheless, evolutionary ecologists have struggled with communicating these aspects of the adaptive process. In 1982, Gould and Vrba proposed terminology to distinguish character states shaped via natural selection for the roles they currently serve ('adaptations') from those shaped under preceding selective regimes ('exaptations'), with the intention of replacing the inaccurate 'preadaptation'. Forty years later, we revisit Gould and Vrba's ideas which, while often controversial, continue to be widely debated and highly cited. We use the recent emergence of urban evolutionary ecology as a timely opportunity to reintroduce the ideas of Gould and Vrba as an integrated framework to understand contemporary evolution in novel environments.

Can behaviour and physiology mitigate effects of warming on ectotherms? A test in urban ants

Global climate change is expected to have pervasive effects on the diversity and distribution of species, particularly ectotherms whose body temperatures depend on environmental temperatures. However, these impacts remain difficult to predict, in part because ectotherms may adapt or acclimate to novel conditions or may use behavioural thermoregulation to reduce their exposure to stressful microclimates. Here we examine the potential for physiological and behavioural changes to mitigate effects of environmental warming on five species of ants in a temperate forest habitat subject to urban warming. We worked in eight urban and eight non-urban forest sites in North Carolina, USA; sites experienced a 1.1°C range of mean summer air temperatures. At each site, we documented species-specific microclimates (ant operative temperatures, T_e ) and ant activity on a transect of 14 bait stations at three times of day. In the laboratory, we measured upper thermal tolerance (CT_max ) and thermal preference (T_pref ) for each focal species. We then asked whether thermal traits shifted at hotter sites, and whether ants avoided non-preferred microclimates in the field. CT_max and T_pref did not increase at warmer sites, indicating that these populations did not adapt or acclimate to urban warming. Consistent with behavioural thermoregulation, four of the five species were less likely to occupy baits where T_e departed from T_pref . Apparent thermoregulation resulted from fixed diel activity patterns that helped ants avoid the most inappropriate temperatures but did not compensate for daily or spatial temperature variation: Hotter sites had hotter ants. This study uses a novel approach to detect behavioural thermoregulation and sublethal warming in foraging insects. The results suggest that adaptation and behaviour may not protect common temperate forest ants from a warming climate, and highlight the need to evaluate effects of chronic, sublethal warming on small ectotherms.

Microplastics increases the heat tolerance of Daphnia magna under global warming via hormetic effects

The ecological risk assessment of microplastics under global warming receives increasing attention. Yet, such studies mostly focused on increased mean temperatures (MT), ignoring another key component of global warming, namely daily temperature fluctuations (DTF). Moreover, we know next to nothing about the combined effects of multigenerational exposure to microplastics and warming. In this study, Daphnia magna was exposed to an environmentally relevant concentration of polystyrene microplastics (5 μg L-1) under six thermal conditions (MT: 20 ℃, 24 ℃; DTF: 0 ℃, 5 ℃, 10 ℃) over two generations to investigate the interactive effects of microplastics and global warming. Results showed that microplastics had no effects on Daphnia at standard thermal conditions (constant 20 °C). Yet, microplastics increased the fecundity, heat tolerance, amount of energy storage, net energy budget and cytochrome P450 activity, and decreased the energy consumption when tested under an increased MT or DTF, indicating a hormesis effect induced by microplastics under warming. The unexpected increase in heat tolerance upon exposure to microplastics could be partly explained by the reduced energy consumption and/or increased energy availability. Overall, the present study highlighted the importance of including DTF and multigenerational exposure to improve the ecological risk assessment of microplastics under global warming.

Urban evolution of thermal physiology in a range-expanding, mycophagous fruit fly, Drosophila tripunctata

In Drosophila spp., their often high number of annual generations, large population sizes and large amounts of standing genetic variation should predispose them to undergo contemporary adaptation to climatic warming. However, a number of laboratory experimental evolution studies in this group of organisms suggest strong limits on the rate and magnitude of contemporary thermal adaptation. Here, we explore this discrepancy by examining the potential for rapid evolutionary divergence between wild populations of Drosophila tripunctata Loew, 1862 from rural and urban sites. We performed a multi-generation common garden study and found evidence for the evolution of higher heat tolerance (critical thermal maximum) in flies from urban populations. We also detected evolutionary divergence in cold resistance (chill coma recovery time), with diminished cold resistance in flies from urban populations, although the effect was weaker than the shift in heat tolerance. Our study provides evidence of contemporary urban thermal adaptation, although the magnitude of phenotypic change lagged the magnitude of environmental temperature change across the urbanization gradient, suggesting potential limits on the evolution of urban thermal physiology.

Body size responses to the combined effects of climate and land use changes within an urban framework

Alterations in body size can have profound impacts on an organism's life history and ecology with long-lasting effects that span multiple biological scales. Animal body size is influenced by environmental drivers, including climate change and land use change, the two largest current threats to biodiversity. Climate warming has led to smaller body sizes of many species due to impacts on growth (i.e., Bergmann's rule and temperature-size rule). Conversely, urbanization, which serves as a model for investigating the effects of land use changes, has largely been demonstrated to cause size increases, but few studies have examined the combined influences of climate and land use changes on organism size. We present here the background theory on how each of these factors is expected to influence body size, summarize existing evidence of how size has recently been impacted by climate and land use changes, and make several recommendations to guide future research uniting these areas of focus. Given the rapid pace of climate change and urbanization, understanding the combined effects of climate and land use changes on body size is imperative for biodiversity preservation.

Daphnia as a versatile model system in ecology and evolution

Water fleas of the genus Daphnia have been a model system for hundreds of years and is among the best studied ecological model organisms to date. Daphnia are planktonic crustaceans with a cyclic parthenogenetic life-cycle. They have a nearly worldwide distribution, inhabiting standing fresh- and brackish water bodies, from small temporary pools to large lakes. Their predominantly asexual reproduction allows for the study of phenotypes excluding genetic variation, enabling us to separate genetic from non-genetic effects. Daphnia are often used in studies related to ecotoxicology, predator-induced defence, host–parasite interactions, phenotypic plasticity and, increasingly, in evolutionary genomics. The most commonly studied species are Daphnia magna and D. pulex, for which a rapidly increasing number of genetic and genomic tools are available. Here, I review current research topics, where the Daphnia model system plays a critical role.

Extreme Hot Weather Has Stronger Impacts on Avian Reproduction in Forests Than in Cities

Climate change and urbanisation are among the most salient human-induced changes affecting Earth’s biota. Extreme weather events can have high biological impacts and are becoming more frequent recently. In cities, the urban heat island can amplify the intensity and frequency of hot weather events. However, the joint effects of heat events and urban microclimate on wildlife are unclear, as urban populations may either suffer more from increased heat stress or may adapt to tolerate warmer temperatures. Here, we test whether the effects of hot weather on reproductive success of great tits (Parus major) are exacerbated or dampened in urban environments compared to forest habitats. By studying 760 broods from two urban and two forest populations over 6 years, we show that 14–16 days-old nestlings have smaller body mass and tarsus length, and suffer increased mortality when they experience a higher number of hot days during the nestling period. The negative effects of hot weather on body mass and survival are significantly stronger in forests than in urban areas, where these effects are dampened or even reversed. These results suggest that urban nestlings are less vulnerable to extreme hot weather conditions than their non-urban conspecifics. This difference might be the result of adaptations that facilitate heat dissipation, including smaller body size, altered plumage and reduced brood size. Alternatively or additionally, parental provisioning and food availability may be less affected by heat in urban areas. Our findings suggest that adaptation to heat stress may help birds cope with the joint challenges of climate change and urbanisation.

A Theory of City Biogeography and the Origin of Urban Species

Many of the choices humans make with regard to infrastructure, urban planning and other phenomena have impacts that will last thousands of years. This can readily be seen in modern cities in which contemporary streets run along street grids that were laid out thousands of years prior or even in which ancient viaducts still play a role. However, rarely do evolutionary biologists explicitly consider the future of life likely to be associated with the decisions we are making today. Here, we consider the evolutionary future of species in cities with a focus on the origin of lineages and species. We do so by adjusting evolutionary predictions from the theory of island biogeography so as to correspond to the unique features of cities as islands. Specifically, the species endemic to cities tend to be associated with the gray habitats in cities. Those habitats tend to be dominated by human bodies, pet bodies and stored food. It is among such species where the origin of new lineages is most likely, although most research on evolution in cities has focused on green habitats. We conclude by considering a range of scenarios for the far future and their implications for the origin of lineages and species.

Stress-induced changes in body surface temperature are repeatable, but do not differ between urban and rural birds

Urbanisation can alter local microclimates, thus creating new thermal challenges for resident species. However, urban environments also present residents with frequent, novel stressors (e.g., noise, human interaction) which may demand investment in costly, self-preserving responses (e.g., the fight-or-flight response). One way that urban residents might cope with this combination of demands is by using regional heterothermy to reduce costs of thermoregulation during the stress response. In this study, we used black-capped chickadees (n_urban = 9; n_rural = 10) to test whether known heterothermic responses to stress exposure (here, at the bare skin around the eye): (1) varied consistently among individuals (i.e., were repeatable), and (2) were most pronounced among urban individuals compared with rural individuals. Further, to gather evidence for selection on stress-induced heterothermic responses in urban settings, we tested: (3) whether repeatability of this response was lower among birds sampled from urban environments compared with those sampled from rural environments. For the first time, we show that heterothermic responses to stress exposures (i.e. changes in body surface temperature) were highly repeatable across chronic time periods (R = 0.58) but not acute time periods (R = 0.13). However, we also show that these responses did not differ between urban and rural birds, nor were our repeatability estimates any lower in our urban sample. Thus, while regional heterothermy during stress exposure may provide energetic benefits to some, but not all, individuals, enhanced use of this response to cope with urban pressures appears unlikely in our study species.

Differential local genetic adaptation to pesticide use in organic and conventional agriculture in an aquatic non-target species

Pesticide application is an important stressor to non-target species and can profoundly affect ecosystem functioning. Debates continue on the choice of agricultural practices regarding their environmental impact, and organic farming is considered less detrimental compared to conventional practices. Nevertheless, comparative studies on the impacts of both agricultural approaches on the genetic adaptation of non-target species are lacking. We assessed to what extent organic and conventional agriculture elicit local genetic adaptation of populations of a non-target aquatic species, Daphnia magna. We tested for genetic differences in sensitivity of different D. magna populations (n = 7), originating from ponds surrounded by conventional and organic agriculture as well as nature reserves, to pesticides used either in conventional (chlorpyrifos) or organic agriculture (deltamethrin and copper sulfate). The results indicate that D. magna populations differentially adapt to local pesticide use. Populations show increased resistance to chlorpyrifos as the percentage of conventional agriculture in the surrounding landscape increases, whereas populations from organic agriculture sites are more resistant to deltamethrin. While organic agriculture is considered less harmful for non-target species than conventional, both types of agriculture shape the evolution of pesticide resistance in non-target species in a specific manner, reflecting the differences in selection pressure.

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.

Selection on adaptive and maladaptive gene expression plasticity during thermal adaptation to urban heat islands

Phenotypic plasticity enables a single genotype to produce multiple phenotypes in response to environmental variation. Plasticity may play a critical role in the colonization of novel environments, but its role in adaptive evolution is controversial. Here we suggest that rapid parallel regulatory adaptation of Anolis lizards to urban heat islands is due primarily to selection for reduced and/or reversed heat-induced plasticity that is maladaptive in urban thermal conditions. We identify evidence for polygenic selection across genes of the skeletal muscle transcriptome associated with heat tolerance. Forest lizards raised in common garden conditions exhibit heat-induced changes in expression of these genes that largely correlate with decreased heat tolerance, consistent with maladaptive regulatory response to high-temperature environments. In contrast, urban lizards display reduced gene expression plasticity after heat challenge in common garden and a significant increase in gene expression change that is congruent with greater heat tolerance, a putatively adaptive state in warmer urban environments. Genes displaying maladaptive heat-induced plasticity repeatedly show greater genetic divergence between urban and forest habitats than those displaying adaptive plasticity. These results highlight the role of selection against maladaptive regulatory plasticity during rapid adaptive modification of complex systems in the wild.

Growing up in a new world: trait divergence between rural, urban, and invasive populations of an amphibian urban invader

Cities are focal points of introduction for invasive species. Urban evolution might facilitate the success of invasive species in recipient urban habitats. Here we test this hypothesis by rearing tadpoles of a successful amphibian urban coloniser and invader in a common garden environment. We compared growth rate, morphological traits, swimming performance, and developmental rate of guttural toad tadpoles (Sclerophrys gutturalis) from native rural, native urban, and non-native urban habitats. By measuring these traits across ontogeny, we were also able to compare divergence across different origins as the tadpoles develop. The tadpoles of non-native urban origin showed significantly slower developmental rate (e.g., the proportion of tadpoles reaching Gosner stage 31 or higher was lower at age 40 days) than tadpoles of native urban origin. Yet, tadpoles did not differ in growth rate or any morphological or performance trait examined, and none of these traits showed divergent ontogenetic changes between tadpoles of different origin. These findings suggest that prior adaptation to urban habitats in larval traits likely does not play an important role in facilitating the invasion success of guttural toads into other urban habitats. Instead, we suggest that evolutionary changes in larval traits after colonization (e.g., developmental rate), together with decoupling of other traits and phenotypic plasticity might explain how this species succeeded in colonising extra-limital urban habitats.

Urbanization extends flight phenology and leads to local adaptation of seasonal plasticity in Lepidoptera

Cities represent novel environments with altered seasonality; they are warmer, which may accelerate growth, but light pollution can also lengthen days, misleading organisms that use daylength to predict seasonal change. Using long-term observational data, we show that urban populations of a butterfly and a moth have longer flight seasons than neighboring rural populations for six Nordic city regions. Next, using laboratory experiments, we show that the induction of diapause by daylength has evolved in urban populations in the direction predicted by urban warming. We thus show that the altered seasonality of urban environments can lead to corresponding evolutionary changes in the seasonal responses of urban populations, a pattern that may be repeated in other species.

Urbanization is gaining force globally, which challenges biodiversity, and it has recently also emerged as an agent of evolutionary change. Seasonal phenology and life cycle regulation are essential processes that urbanization is likely to alter through both the urban heat island effect (UHI) and artificial light at night (ALAN). However, how UHI and ALAN affect the evolution of seasonal adaptations has received little attention. Here, we test for the urban evolution of seasonal life-history plasticity, specifically changes in the photoperiodic induction of diapause in two lepidopterans, Pieris napi (Pieridae) and Chiasmia clathrata (Geometridae). We used long-term data from standardized monitoring and citizen science observation schemes to compare yearly phenological flight curves in six cities in Finland and Sweden to those of adjacent rural populations. This analysis showed for both species that flight seasons are longer and end later in most cities, suggesting a difference in the timing of diapause induction. Then, we used common garden experiments to test whether the evolution of the photoperiodic reaction norm for diapause could explain these phenological changes for a subset of these cities. These experiments demonstrated a genetic shift for both species in urban areas toward a lower daylength threshold for direct development, consistent with predictions based on the UHI but not ALAN. The correspondence of this genetic change to the results of our larger-scale observational analysis of in situ flight phenology indicates that it may be widespread. These findings suggest that seasonal life cycle regulation evolves in urban ectotherms and may contribute to ecoevolutionary dynamics in cities.

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.

Thermal tolerance of cyprinids along an urban-rural gradient: Plasticity, repeatability and effects of swimming and temperature shock

Urbanization changes the thermal profile of streams in much the same way that climate change is predicted to with higher temperatures, more varied flow and rapid temperature pulses with precipitation events. Whether exceptional tolerance to these altered thermal conditions is a pre-requisite for a fish species to inhabit urban streams or if urbanization has changed the thermal physiology of those fish species that persist in urban streams is unknown, but could help predict the outcome of future climate disruption. To test whether residence in urban streams is associated with altered thermal tolerance, we compared thermal tolerance (CT_Max) and phenotypic plasticity of thermal tolerance (ΔCT_Max/Δ acclimation temperature) in five populations of an urban-tolerant cyprinid, the blacknose dace (Rhinichthys atratulus), from multiple watersheds along an urban/rural gradient. Thermal tolerance of these stream fish was tested while swimming at 10 cm*s^-1 but also in static water and after thermal shocks of 4°-6 °C simulating precipitation events. To test whether blacknose dace as a species has unusual thermal tolerance or thermal plasticity, we also compared two blacknose dace populations with two co-resident, co-familiars (creek chub (Semotilus atromaculatus) and rosyside dace (Clinostomus funduloides), that don't persist in urban streams at three different acclimation temperatures. Thermal tolerance of blacknose dace, as measured by a critical thermal maximum test (CT_Max), was independent of size and activity level, i.e. individuals had identical thermal tolerance whether swimming or resting and CT_Max was significantly repeatable across two levels of activity. Although there was some variance among populations, blacknose dace from streams of varied urbanization generally exhibited comparable thermal tolerances, ability to acclimate to different temperatures and were unaffected by thermal shocks. Rosyside dace had significantly lower thermal tolerance than the other two species but plasticity of thermal tolerance was uniform across the three cyprinid species. Our conclusions are that exceptional thermal tolerance or ability to thermally acclimate are not pre-requisite characters for a given cyprinid species to survive in urban streams, nor has thermal tolerance undergone directional selection in this urban environment.

Disentangling the roles of social and individual effects on cadmium tolerance in the ant Temnothorax nylanderi

Urbanization brings new pressures for individuals. Among them, trace elements, such as cadmium, are important stressors. A recent study highlights a weaker negative effect of cadmium on city colonies relative to their forest counterparts in the ant Temnothorax nylanderi. Here, we aim to test whether the better tolerance of city colonies in this species results from a better ability of workers to rear larvae despite stressful conditions and/or a better ability of larvae to develop properly despite stressful conditions. We performed a cross-fostering experiment of workers and larvae from city and forest colonies, in common garden conditions in the laboratory. Colonies were fed using cadmium-enriched or cadmium-free food for 2 months, and we measured four life-history traits. As expected, cadmium had a negative impact on all traits. Unexpectedly, we did not observe a better tolerance of city colonies to cadmium, contrary to our previous study, which prevented us from disentangling the respective contributions of workers and larvae to cadmium tolerance. Interestingly, forest colonies seemed to be of better quality in our laboratory conditions. Finally, colony size increased adult survival, but only in the absence of cadmium, suggesting that social buffering could collapse with strong external disturbances.

Late-stage pregnancy reduces upper thermal tolerance in a live-bearing fish

Upper thermal limits are considered a key determinant of a population's ability to persist in the face of extreme heat events. However, these limits differ considerably among individuals within a population, and the mechanisms underlying this differential sensitivity are not well understood. Upper thermal tolerance in aquatic ectotherms is thought to be determined by a mismatch between oxygen supply and the increased metabolic demands associated with warmer waters. As such, tolerance is expected to decline during reproduction given the heightened oxygen demand for gamete production and maintenance. Among live-bearing species, upper thermal tolerance of reproductive adults may decline even further after fertilization due to the cost of meeting the increasing oxygen demands of developing embryos. We examined the upper thermal tolerance of live-bearing female Trinidadian guppies at different stages of reproduction and found that critical thermal maximum was similar during the egg yolking and early embryos stage but then declined by almost 0.5 °C during late pregnancy when oxygen demands are the greatest. These results are consistent with the hypothesis that oxygen limitation sets thermal limits and show that reproduction is associated with a decline in upper thermal tolerance.

Impact of intraspecific variation on measurements of thermal tolerance in bumble bees

Climate change is an important driver of bee decline despite the fact that many species might respond to climate change differently. One method to predict how a species will respond to climate change is to identify its thermal tolerance limits. However, differences in thermal tolerance might also occur among distant populations of the same species based on their local environment or even among castes of social insects. Here, we investigated intraspecific differences in thermal tolerance among subspecies of the large earth bumble bee, Bombus terrestris (Apidae). We determined the critical thermal minima and maxima (CT_min and CT_max, respectively) of workers and queens from three lab-reared B. terrestris subspecies (B. t. terrestris, B. t. audax, and B. t. canariensis) which originated from different thermal environments. Our results showed that caste has an influence on critical thermal minima, with queens being most cold-tolerant, but the values of critical thermal maxima were not correlated to caste or size. The thermal tolerance of workers did not differ among the subspecies. Although heat tolerance was similar in queens, B. t. canariensis queens (originating from the warmest environments) were the least cold tolerant. Overall, we showed that B. terrestris may be generally robust against climate warming, but that particular subspecies and/or populations may be more vulnerable to extreme temperature variability. Future research should focus on responses of B. terrestris populations to short, extreme thermal events.

Potential local adaptation in populations of invasive reed canary grass (Phalaris arundinacea) across an urbanization gradient

Urban stressors represent strong selective gradients that can elicit evolutionary change, especially in non-native species that may harbor substantial within-population variability. To test whether urban stressors drive phenotypic differentiation and influence local adaptation, we compared stress responses of populations of a ubiquitous invader, reed canary grass (Phalaris arundinacea). Specifically, we quantified responses to salt, copper, and zinc additions by reed canary grass collected from four populations spanning an urbanization gradient (natural, rural, moderate urban, and intense urban). We measured ten phenotypic traits and trait plasticities, because reed canary grass is known to be highly plastic and because plasticity may enhance invasion success. We tested the following hypotheses: (a) Source populations vary systematically in their stress response, with the intense urban population least sensitive and the natural population most sensitive, and (b) plastic responses are adaptive under stressful conditions. We found clear trait variation among populations, with the greatest divergence in traits and trait plasticities between the natural and intense urban populations. The intense urban population showed stress tolerator characteristics for resource acquisition traits including leaf dry matter content and specific root length. Trait plasticity varied among populations for over half the traits measured, highlighting that plasticity differences were as common as trait differences. Plasticity in root mass ratio and specific root length were adaptive in some contexts, suggesting that natural selection by anthropogenic stressors may have contributed to root trait differences. Reed canary grass populations in highly urbanized wetlands may therefore be evolving enhanced tolerance to urban stressors, suggesting a mechanism by which invasive species may proliferate across urban wetland systems generally.

Intra-population variation in reproductive timing covaries with thermal plasticity of offspring performance in perch Perca fluviatilis

Life history theory posits that organisms should time their reproduction to coincide with environmental conditions that maximize their fitness. Population-level comparisons have contributed important insights on the adaptive value of reproductive timing and its association to environmental variation. Yet, despite its central role to ecology and evolution, the causes and consequences of variation in reproductive timing among individuals within populations are poorly understood in vertebrates other than birds. Using a combination of observational field studies and a split-brood experiment, we investigated whether differences in breeding time were associated with changes in hatching success, reproductive allocation and reaction norms linking offspring performance to temperature within an anadromous Baltic Sea population of perch Perca fluviatilis. Field observations revealed substantial variation in reproductive timing, with the breeding period lasting almost 2 months and occurring in temperatures ranging from 10 to 21℃. The hatching success of perch decreased as the reproductive season progressed. At the same time, the reproductive allocation strategy changed over the season, late breeders (the offspring of which were introduced into a high resource environment and increased predation pressure) produced more and smaller eggs that resulted in smaller larvae, compared with early breeders. The split-brood experiment in which eggs were incubated in different temperatures (10, 12, 15, 18°C) showed that differences in reproductive timing were associated with a change in the shape of the reaction norm linking offspring performance to water temperature indicative of adaptive phenotypic plasticity, with the offspring of early breeders performing best in low temperatures and the offspring of late breeders performing best in high temperatures. The seasonal changes in reproductive traits and the shape of the thermal performance suggest time-dependent adaptive differences among individuals within the population. Management actions aimed at preserving and restoring variation in the timing of reproductive events will thus likely also influence variation in associated life history traits and thermal performance curves, which could safeguard populations against environmental challenges and changes associated with exploitation and global warming.

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.

The Biological Deserts Fallacy: Cities in Their Landscapes Contribute More than We Think to Regional Biodiversity

Cities are both embedded within and ecologically linked to their surrounding landscapes. Although urbanization poses a substantial threat to biodiversity, cities also support many species, some of which have larger populations, faster growth rates, and higher productivity in cities than outside of them. Despite this fact, surprisingly little attention has been paid to the potentially beneficial links between cities and their surroundings. We identify five pathways by which cities can benefit regional ecosystems by releasing species from threats in the larger landscape, increasing regional habitat heterogeneity and genetic diversity, acting as migratory stopovers, preadapting species to climate change, and enhancing public engagement and environmental stewardship. Increasing recognition of these pathways could help cities identify effective strategies for supporting regional biodiversity conservation and could provide a science-based platform for incorporating biodiversity alongside other urban greening goals.

Variation in upper thermal tolerance among 19 species from temperate wetlands

Communities usually possess a multitude of interconnected trophic interactions within food webs. Their regulation generally depends on a balance between bottom-up and top-down effects. However, if sensitivity to temperature varies among species, rising temperatures may change trophic interactions via direct and indirect effects. We examined the critical thermal maximum (CT_max) of 19 species from temperate wetlands (insect predators, amphibian larvae, zooplankton and amphipods) and determined if they vary in their sensitivity to warming temperatures. CT_max differed between the groups, with predatory insects having higher CT_max than amphibians (both herbivorous larval anurans and predatory larval salamanders), amphipods and zooplankton. In a scenario of global warming, these differences in thermal tolerance may affect top-down and bottom-up processes, particularly considering that insect predators are more likely to maintain or improve their performance at higher temperatures, which could lead to increased predation rates on the herbivores in the food web. Further studies are needed to understand how the energy flows through communities, how species' energy budgets may change and whether other physiological and behavioral responses (such as phenotypic plasticity and thermoregulation) can buffer or increase these changes in the top-down regulation of wetland food webs.

Socio-eco-evolutionary dynamics in cities

Cities are uniquely complex systems regulated by interactions and feedbacks between nature and human society. Characteristics of human society-including culture, economics, technology and politics-underlie social patterns and activity, creating a heterogeneous environment that can influence and be influenced by both ecological and evolutionary processes. Increasing research on urban ecology and evolutionary biology has coincided with growing interest in eco-evolutionary dynamics, which encompasses the interactions and reciprocal feedbacks between evolution and ecology. Research on both urban evolutionary biology and eco-evolutionary dynamics frequently focuses on contemporary evolution of species that have potentially substantial ecological-and even social-significance. Still, little work fully integrates urban evolutionary biology and eco-evolutionary dynamics, and rarely do researchers in either of these fields fully consider the role of human social patterns and processes. Because cities are fundamentally regulated by human activities, are inherently interconnected and are frequently undergoing social and economic transformation, they represent an opportunity for ecologists and evolutionary biologists to study urban "socio-eco-evolutionary dynamics." Through this new framework, we encourage researchers of urban ecology and evolution to fully integrate human social drivers and feedbacks to increase understanding and conservation of ecosystems, their functions and their contributions to people within and outside cities.

Growing in the city: Urban evolutionary ecology of avian growth rates

INTRODUCTION

Rapid environmental change driven by urbanization offers a unique insight into the adaptive potential of urban-dwelling organisms. Urban-driven phenotypic differentiation is increasingly often demonstrated, but the impact of urbanization (here modelled as the percentage of impervious surface (ISA) around each nestbox) on offspring developmental rates and subsequent survival remains poorly understood. Furthermore, the role of selection on urban-driven phenotypic divergence was rarely investigated to date.

METHODS AND RESULTS

Data on nestling development and body mass were analysed in a gradient of urbanization set in Warsaw, Poland, in two passerine species: great tits (Parus major) and blue tits (Cyanistes caeruleus). Increasing levels of impervious surface area (ISA) delayed the age of fastest growth in blue tits. Nestling body mass was also negatively affected by increasing ISA 5 and 10 days after hatching in great tits, and 10 and 15 days in blue tits, respectively. High levels of ISA also increased nestling mortality 5 and 10 days after hatching in both species. An analysis of selection differentials performed for two levels of urbanization (low and high ISA) revealed a positive association between mass at day 2 and survival at fledging.

DISCUSSION

This study confirms the considerable negative impact of imperviousness-a proxy for urbanization level-on offspring development, body mass and survival, and highlights increased selection on avian mass at hatching in a high ISA environment.

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.

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.

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.

Low potential for evolutionary rescue from climate change in a tropical fish

Climate change is increasing global temperatures and intensifying the frequency and severity of extreme heat waves. How organisms will cope with these changes depends on their inherent thermal tolerance, acclimation capacity, and ability for evolutionary adaptation. Yet, the potential for adaptation of upper thermal tolerance in vertebrates is largely unknown. We artificially selected offspring from wild-caught zebrafish (Danio rerio) to increase (Up-selected) or decrease (Down-selected) upper thermal tolerance over six generations. Selection to increase upper thermal tolerance was also performed on warm-acclimated fish to test whether plasticity in the form of inducible warm tolerance also evolved. Upper thermal tolerance responded to selection in the predicted directions. However, compared to the control lines, the response was stronger in the Down-selected than in the Up-selected lines in which evolution toward higher upper thermal tolerance was slow (0.04 ± 0.008 °C per generation). Furthermore, the scope for plasticity resulting from warm acclimation decreased in the Up-selected lines. These results suggest the existence of a hard limit in upper thermal tolerance. Considering the rate at which global temperatures are increasing, the observed rates of adaptation and the possible hard limit in upper thermal tolerance suggest a low potential for evolutionary rescue in tropical fish living at the edge of their thermal limits.

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.

Big City Living: A Global Meta-Analysis Reveals Positive Impact of Urbanization on Body Size in Lizards

Urban environments pose different selective pressures than natural ones, leading to changes in animal behavior, physiology, and morphology. Understanding how animals respond to urbanization could inform the management of urban habitats. Non-avian reptiles have important roles in ecosystems worldwide, yet their responses to urbanization have not been as comprehensively studied as those of mammals and birds. However, unlike mammals and birds, most reptiles cannot easily move away from disturbances, making the selective pressure to adapt to urban environments especially strong. In recent years, there has been a surge in research on the responses of lizards to urbanization, yet no formal synthesis has determined what makes an urban lizard, in other words, which phenotypic traits are most likely to change with urbanization and in which direction? Here, we present a qualitative synthesis of the literature and a quantitative phylogenetic meta-analysis comparing phenotypic traits between urban and non-urban lizard populations. The most robust finding from our analysis is that urban lizards are larger than their non-urban counterparts. This result remained consistent between sexes and taxonomic groups. Hence, lizards that pass through the urban filter have access to better resources, more time for foraging, and/or there is selection on attaining a larger body size. Other results included an increase in the diameters of perches used and longer limb and digit lengths, although this may be a result of increased body size. Urban lizards were not bolder, more active or exploratory, and did not differ in immune responses than non-urban populations. Overall, studies are biased to a few geographic regions and taxa. More than 70% of all data came from three species of anoles in the family Dactyloidae, making it difficult to generalize patterns to other clades. Thus, more studies are needed across multiple taxa and habitats to produce meaningful predictions that could help inform conservation and management of urban ecological communities.