Plasticity of size and growth in fluctuating thermal environments: comparing reaction norms and performance curves

Ectothermic animals exhibit two distinct kinds of plasticityin response to temperature: Thermal performance curves (TPCs),in which an individual's performance (e.g., growth rate) variesin response to current temperature; and developmental reactionnorms (DRNs), in which the trait value (e.g., adult body sizeor development time) of a genotype varies in response to developmentaltemperatures experienced over some time period during development.Here we explore patterns of genetic variation and selectionon TPCs and DRNs for insects in fluctuating thermal environments.First, we describe two statistical methods for partitioningtotal genetic variation into variation for overall size or performanceand variation in plasticity, and apply these methods to availabledatasets on DRNs and TPCs for insect growth and size. Our resultsindicate that for the datasets we considered, genetic variationin plasticity represents a larger proportion of the total geneticvariation in TPCs compared to DRNs, for the available datasets.Simulations suggest that estimates of the genetic variationin plasticity are strongly affected by the number and rangeof temperatures considered, and by the degree of nonlinearityin the TPC or DRN. Second, we review a recent analysis of fieldselection studies which indicates that directional selectionfavoring increased overall size is common in many systems—thatbigger is frequently fitter. Third, we use a recent theoreticalmodel to examine how selection on thermal performance curvesrelates to environmental temperatures during selection. Themodel predicts that if selection acts primarily on adult sizeor development time, then selection on thermal performance curvesfor larval growth or development rates is directly related tothe frequency distribution of temperatures experienced duringlarval development. Using data on caterpillar temperatures inthe field, we show that the strength of directional selectionon growth rate is predicted to be greater at the modal (mostfrequent) temperatures, not at the mean temperature or at temperaturesat which growth rate is maximized. Our results illustrate someof the differences in genetic architecture and patterns of selectionbetween thermal performance curves and developmental reactionnorms.     

Sources of Variation in Physiological Phenotypes and Their Evolutionary Significance

We offer the thesis that environmental physiologists and evolutionarybiologists can find fertile common ground in the study of howindividual variation in physiological phenotypes originatesand develops. The sources of such individual variation are oftencomplex; the consequences affect how natural selection willact on a suite of traits, of which some may seem, at first glance,far removed from the usual domain of environmental physiology.We illustrate our thesis in two ways. First, we offer two examplesdrawn from studies of thermal tolerance in the poeciliid fishHeterandria formosa. We show how fitness variation can be acomplex function of the gestational temperature and thermaltolerance and how these effects can produce environmentallyinduced variation among populations in thermal tolerance thatmimics a pattern of adaptive variation. Second, we review twocase studies that illuminate how environmental effects on amultivariate phenotype can channel the action of natural selection.The phenotypic plasticity of male life history in Poecilia latipinnain response to temperature embraces a spectrum of traits; theeffects of each one upon fitness will influence the abilityof selection to mold the response of any one of them to temperature.The phenotypic covariances in thermal tolerance and life-historytraits in Heterandria formosa differ slightly between populationsfrom different parts of the species range, apparently becauseof differences between them in thermal sensitivity; this differenceinsures that the multivariate nature of selection will be correspondinglydifferent in those different populations.     

Variation,selection and evolution of function-valued traits

We describe an emerging framework for understanding variation, selection and evolution of phenotypic traits that are mathematical functions. We use one specific empirical example – thermal performance curves (TPCs) for growth rates of caterpillars – to demonstrate how models for function-valued traits are natural extensions of more familiar, multivariate models for correlated, quantitative traits. We emphasize three main points. First, because function-valued traits are continuous functions, there are important constraints on their patterns of variation that are not captured by multivariate models. Phenotypic and genetic variation in function-valued traits can be quantified in terms of variance-covariance functions and their associated eigenfunctions: we illustrate how these are estimated as well as their biological interpretations for TPCs. Second, selection on a function-valued trait is itself a function, defined in terms of selection gradient functions. For TPCs, the selection gradient describes how the relationship between an organism's performance and its fitness varies as a function of its temperature. We show how the form of the selection gradient function for TPCs relates to the frequency distribution of environmental states (caterpillar temperatures) during selection. Third, we can predict evolutionary responses of function-valued traits in terms of the genetic variance-covariance and the selection gradient functions. We illustrate how non-linear evolutionary responses of TPCs may occur even when the mean phenotype and the selection gradient are themselves linear functions of temperature. Finally, we discuss some of the methodological and empirical challenges for future studies of the evolution of function-valued traits.     

A Compartment Model of the Effect of Early-season Temperatures on Potential Size and Growth of 'Delicious' Apple Fruits

A compartmental growth model was developed to describe expansionof ‘Delicious’ apple fruit diameter and the effectof early-season temperatures on potential size at harvest. Themodel was based on the assumption that growth may be describedas a function of transfer between two conceptual compartments.Under this scheme, the first compartment represented all tissuecontributing to the setting of potential fruit size (determinedas the integral of its output) whereas the second compartmentrepresented all other fruit tissue whose growth actualized thatpotential. Expansion of both compartments was assumed to havea temperature response with an optimum, whereas an aging processwith an asymptotic temperature response controlled transferto the second compartment. Model parameters were estimated byfitting to data from controlled environment experiments in whichearly-season temperature conditions were varied. Predicted fruitgrowth curves showed close agreement with measured diameterdata. The results were consistent with a two-fold impact ofearly-season temperatures on apple fruit size: an immediate,direct effect on growth rate and an enduring effect, mediatedthrough fruit cell number or resource allocation to young fruit,reflecting the establishment of a potential that subsequentgrowth actualizes.Copyright 1999 Annals of Botany Company. Malus domesticaBorkh., apples, ‘Delicious’, fruit growth, models, temperature, potential size, cell division.     

Immunity in a variable world

Immune function is likely to be a critical determinant of an organism's fitness, yet most natural animal and plant populations exhibit tremendous genetic variation for immune traits. Accumulating evidence suggests that environmental heterogeneity may retard the long-term efficiency of natural selection and even maintain polymorphism, provided alternative host genotypes are favoured under different environmental conditions. 'Environment' in this context refers to abiotic factors such as ambient temperature or availability of nutrient resources, genetic diversity of pathogens or competing physiological demands on the host. These factors are generally controlled in laboratory experiments measuring immune performance, but variation in them is likely to be very important in the evolution of resistance to infection. Here, we review some of the literature emphasizing the complexity of natural selection on immunity. Our aim is to describe how environmental and genetic heterogeneities, often excluded from experimentation as 'noise', may determine the evolutionary potential of populations or the potential for interacting species to coevolve.     

Phytoplankton thermal responses adapt in the absence of hard thermodynamic constraints

To better predict how populations and communities respond to climatic temperature variation, it is necessary to understand how the shape of the response of fitness-related rates to temperature evolves (the thermal performance curve). Currently, there is disagreement about the extent to which the evolution of thermal performance curves is constrained. One school of thought has argued for the prevalence of thermodynamic constraints through enzyme kinetics, whereas another argues that adaptation can—at least partly—overcome such constraints. To shed further light on this debate, we perform a phylogenetic meta-analysis of the thermal performance curves of growth rate of phytoplankton—a globally important functional group—controlling for environmental effects (habitat type and thermal regime). We find that thermodynamic constraints have a minor influence on the shape of the curve. In particular, we detect a very weak increase of maximum performance with the temperature at which the curve peaks, suggesting a weak “hotter-is-better” constraint. Also, instead of a constant thermal sensitivity of growth across species, as might be expected from strong constraints, we find that all aspects of the thermal performance curve evolve along the phylogeny. Our results suggest that phytoplankton thermal performance curves adapt to thermal environments largely in the absence of hard thermodynamic constraints.     

Quantitative genetics of temperature performance curves of Neurospora crassa

Earth's temperature is increasing due to anthropogenic CO emissions; and organisms need either to adapt to higher temperatures, migrate into colder areas, or face extinction. Temperature affects nearly all aspects of an organism's physiology via its influence on metabolic rate and protein structure, therefore genetic adaptation to increased temperature may be much harder to achieve compared to other abiotic stresses. There is still much to be learned about the evolutionary potential for adaptation to higher temperatures, therefore we studied the quantitative genetics of growth rates in different temperatures that make up the thermal performance curve of the fungal model system Neurospora crassa. We studied the amount of genetic variation for thermal performance curves and examined possible genetic constraints by estimating the G -matrix. We observed a substantial amount of genetic variation for growth in different temperatures, and most genetic variation was for performance curve elevation. Contrary to common theoretical assumptions, we did not find strong evidence for genetic trade-offs for growth between hotter and colder temperatures. We also simulated short-term evolution of thermal performance curves of N. crassa, and suggest that they can have versatile responses to selection.     

An architectural analysis of the elongation of field-grown sunflower root systems. Elements for modelling the effects of temperature and intercepted radiation

The effects of photosynthetic photon flux density (PPFD) andsoil temperature on root system elongation rate have been analysedby using an architectural framework. Root elongation rate wasanalysed by considering three terms, (i) the branch appearancerate, (ii) the individual elongation rates of the taproot andbranches and (iii) the proportion of branches which stop elongating.Large ranges ofPPFD and soil temperature were obtained in aseries of field and growth chamber experiments. In the field,the growth of root systems experiencing day-to-day natural fluctuationof PPFD and temperature was followed, and some of the plantsunder study were shaded. In the growth chamber, plants experiencedcontrasting and constant PPFDs and root temperatures. The directeffect of apex temperature on individual root elongation ratewas surprisingly low in the range 13–25C, except forthe first days after germination. Root elongation rate was essentiallyrelated to intercepted PPFD and to distance to the source, bothin the field and in the growth chamber. Branch appearance ratesubstantially varied among days and environmental conditions.It was essentially linked to taproot elongation rate, as theprofile of branch density along the taproot was quite stable.The length of the taproot segment carrying newly appeared brancheson a given day was equal to taproot elongation on this day,plus a 'buffering term' which transiently increased if taprootelongation rate slowed down. The proportion of branches whichstopped elongating a short distance from the taproot rangedfrom 50–80% and was, therefore, a major architecturalvariable, although it is not taken into account in current architecturalmodels. A set of equations accounting for the variabilitiesin elongation rate, branch appearance rate and proportion ofbranches which stop elongating, as a function of interceptedPPFD and apex temperature is proposed. These equations applyfor both field and growth chamber experiments. Key words: Sunflower, root system, model, temperature, radiation     

Developmental evolution of sexual ornamentation: model and a test of feather growth and pigmentation

A tremendous diversity of avian color displays has stimulatednumerous studies of natural and sexual selection. Yet, the developmentalmechanisms that produce such diversification, and thus the proximatetargets of selection pressures, are rarely addressed and poorlyunderstood. In particular, because feathers are colored duringgrowth, the dynamics of feather growth play a deterministicrole in the variation in ornamentation. No study to date, however,has addressed the contribution of feather growth to the expressionof carotenoid-based ornamentation. Here, we examine the developmentalbasis of variation in ornamental feather shapes in male housefinches (Carpodacus mexicanus)—a species in which carotenoiddisplays are under strong natural and sexual selection. First,we use geometric morphometrics to partition the observed shapevariation in fully grown feathers among populations, ages, degreesof elaboration, ornamental body parts, and individuals. Second,we use a biologically informed mathematical model of feathergrowth to predict variation in shape of ornamental feathersdue to simulated growth rate, angle of helical growth of featherbarbs, initial number of barb ridges, rate of addition of newbarbs, barb diameter, and ramus-expansion angle. We find closeconcordance between among-individual variation in feather shapeand hue of entire ornament, and show that this concordance canbe attributed to a shared mechanism—growth rate of featherbarbs. Predicted differences in feather shape due to rate ofaddition of barbs and helical angle of feather growth explainedobserved variation in ornamental area both among individualsand between populations, whereas differences in helical angleof growth and the number of barbs in the feather follicle explaineddifferences in feather shape between ornamental parts and amongmales of different ages. The findings of a close associationof feather growth dynamics and overall ornamentation identifythe proximate targets of selection for elaboration of sexualdisplays. Moreover, the close association of feather growthand pigmentation not only can reinforce condition-dependencein color displays, but can also enable phenotypic and geneticaccommodation of novel pigments into plumage displays providinga mechanism for the observed concordance of within-populationdevelopmental processes and between-population diversificationof color displays.     

Physiological Variation in Clonal Anemones: Energy Balance and Quantitative Genetics

Our understanding of evolutionary mechanisms leading to populationdifferences in mean performance values relies on understandingperformance variation within single populations. Unfortunately,relatively little information about physiological variabilitywithin natural populations of organisms is available. In particular,to begin to understand how physiological traits evolve we needinformation on the extent of physiological variability relatedto the extent of genetic variability over a range of environmentalconditions experienced by individual populations. Clonal organismsmay be particularly well-suited to such studies because theyprovide an opportunity to use replicated genotypes (i.e., clonemates)in controlled experiments. We are using the cosmopolitan seaanemone Haliplanella lineata to explore physiological variancein natural populations. Growth, absorption and routine ratesof oxygen uptake do not vary among three clones from a singlepopulation when measured at 15°C, the approximate midpointin the seasonal range of water temperatures experienced by thispopulation. Broad-sense heritabilities for routine rates ofoxygen consumption and ammonia excretion (0.14 and 0.09, respectively),indicate a relatively low fraction of variance in these physiologicalrates is attributable to genetic variation among five clonesin this population. Although some literature indicates thatsuch low heritabilities may be expected when physiological traitsare measured at environmental mid-range as opposed to extremes,other evidence indicates that it will be difficult to predictthe trend between environmental stress and genetic variancein physiological performance.     

The Effect of Temperature on the Growth of Individual Leaves of Vicia faba L. in the Field

The generalized logistic curve was used to describe the growthof individual leaves in crops of Vicia faba L. Durations of.expansionand mean absolute growth rates were derived from these curves.The duration of expansion was inversely related to temperatureaveraged over four days from unfolding. This relationship wasindependent of leaf position except for the lowest leaves. Theduration of expansion of a leaf was related to the rate of productionof new leaves, the number of expanding leaves remaining relativelyconstant. Absolute growth rates varied with leaf position upto leaf 10. At higher leaves, in the absence of water stress,absolute growth rate was a function of temperature and radiation. Vicia faba L., field bean, leaf growth, temperature     

Interactions between temperature and light intensity on growth and photosynthesis of the cyanobacterium Oscillatoria agardhii

Oscillatoria agardhii was grown in turbidostat cultures undera 16/8 h light/dark cycle at various combinations of light intensityand temperature. Temperature was found to influence only themaximal growth rate; this relationship was linear over the temperaturerange studied. An equation was derived describing the growthrate (µ) as a continuous function of light intensity andtemperature. The light harvesting pigments chlorophyll a andC-phycocyanin increased in concentration when growth becamelight limited. The regulation patterns observed did not suggestany influence of temperature on their steady state concentrations.The initial slope of the P versus I curves (     

Water temperature, predation, and the neglected role of physiological rate effects in rocky intertidal communities

Ecologists and physiologists working on rocky shores have emphasizedthe effects of environmental stress on the distribution of intertidalorganisms. Although consumer stress models suggest that physicalextremes may often reduce predation and herbivory through negativeimpacts on the physiological performance of consumers, few fieldstudies have rigorously tested how environmental variation affectsfeeding rates. I review and analyze field experiments that quantifiedper capita feeding rates of a keystone predator, the sea starPisaster ochraceus, in relation to aerial heat stress, waveforces, and water temperature at three rocky intertidal siteson the Oregon coast. Predation rates during 14-day periods wereunrelated to aerial temperature, but decreased significantlywith decreasing water temperature. There was suggestive butinconclusive evidence that predation rates also declined withincreasing wave forces. Data-logger records suggested that thermalstress was rare in the wave-exposed habitats that I studied;sea star body temperatures likely reached warm levels (>24°C)on only 9 dates in 3 yr. In contrast, wind-driven upwellingregularly generated 3 to 5°C fluctuations in water temperature,and field and laboratory results suggest that such changes significantlyalter feeding rates of Pisaster. These physiological rate effects,near the center of an organism's thermal range, may not reducegrowth or fitness, and thus are distinct from the effects ofenvironmental stress. This study underscores the need to considerorganismal responses both under "normal" conditions, as wellas under extreme conditions. Examining both kinds of responsesis necessary to understand how different components of environmentalvariation regulate physiological performance and the strengthof species interactions in intertidal communities.     

Predicting population genetic change in an autocorrelated random environment: Insights from a large automated experiment

Most natural environments exhibit a substantial component of random variation, with a degree of temporal autocorrelation that defines the color of environmental noise. Such environmental fluctuations cause random fluctuations in natural selection, affecting the predictability of evolution. But despite long-standing theoretical interest in population genetics in stochastic environments, there is a dearth of empirical estimation of underlying parameters of this theory. More importantly, it is still an open question whether evolution in fluctuating environments can be predicted indirectly using simpler measures, which combine environmental time series with population estimates in constant environments. Here we address these questions by using an automated experimental evolution approach. We used a liquid-handling robot to expose over a hundred lines of the micro-alga Dunaliella salina to randomly fluctuating salinity over a continuous range, with controlled mean, variance, and autocorrelation. We then tracked the frequencies of two competing strains through amplicon sequencing of nuclear and choloroplastic barcode sequences. We show that the magnitude of environmental fluctuations (determined by their variance), but also their predictability (determined by their autocorrelation), had large impacts on the average selection coefficient. The variance in frequency change, which quantifies randomness in population genetics, was substantially higher in a fluctuating environment. The reaction norm of selection coefficients against constant salinity yielded accurate predictions for the mean selection coefficient in a fluctuating environment. This selection reaction norm was in turn well predicted by environmental tolerance curves, with population growth rate against salinity. However, both the selection reaction norm and tolerance curves underestimated the variance in selection caused by random environmental fluctuations. Overall, our results provide exceptional insights into the prospects for understanding and predicting genetic evolution in randomly fluctuating environments.     

Relating environmental variation to selection on reaction norms: an experimental test

Theoretical models predict that selection on reaction norms should depend on the relative frequency of environmental states experienced by a population. We report a laboratory experimental test of this prediction for thermal performance curves of larval growth rate in Pieris rapae in relation to their thermal environment. We measured short-term relative growth rate (RGR) for each individual at a series of five temperatures, and then we assigned individuals randomly to warm or cool selection treatments, which differ in the frequency distributions of environmental temperatures. Selection gradient analyses of two independent experiments demonstrated significant positive selection for increasing RGR, primarily through its effects on survival to adulthood and on development rate. In both the warm and cool selection treatments, the magnitude of directional selection on RGR was consistently greater at lower (suboptimal) temperatures than at higher temperatures; differences in selection between the treatments did not match model predictions. The temporal order and duration of environmental conditions may affect patterns of selection on thermal performance curves and other continuous reaction norms, complicating the connections between variation in environment, phenotype, and fitness.     

Food availability and parasite infection influence mating tactics in guppies (Poecilia reticulata)

Despite the important effects of diet and parasite infectionon male reproductive behavior, few studies have simultaneouslyaddressed their influence on intrasexual selection (male–malecompetition). We examined the synergistic effects of 2 naturallyvarying environmental factors, lifetime food intake and infection,with the monogenean parasite Gyrodactylus turnbulli on the matingtactics and foraging behavior of male guppies (Poecilia reticulata).We allowed fish to interact directly with each other duringobservations and found that unparasitized males won more intermalecontests, courted females more frequently, and received positiveresponses to courtship displays more frequently than males thathad been infected. Infected males devoted more time to foragingand less time to courtship and competition than uninfected males,suggesting that they were energetically limited and could notincrease reproductive effort despite their reduced expectedlifespan. This interpretation was supported by the observationthat greater food intake ameliorated the negative effects ofparasite infection on courtship effort. Our results have bearingon how natural variation in food availability and parasite prevalenceinfluence geographic variation in reproductive behavior.     

Variation in heritability of tadpole growth: an experimental analysis

Heritability characteristically shows large variation between traits, among populations and species, and through time. One of the reasons for this is its dependence on gene frequencies and how these are altered by selection and drift through the evolutionary process. We studied variation in heritability of tadpole growth rate in populations of the Swedish common frog, Rana temporaria. In populations evolving under warmer conditions, we have demonstrated elsewhere that tadpoles show better growth and physiological performance at relatively higher temperatures than tadpoles with an evolutionary history in a relatively cooler part of the distribution range. In the current study, we ask whether this process of divergence under natural selection has influenced the genetic architecture as visualised in estimates of heritability of growth rate at different temperature treatments under laboratory conditions. The results suggest that the additive genetic variance varies between treatments and is highest in a treatment that is common to both populations. Our estimates of narrow sense heritability are generally higher in the thermal regime that dominates in the natural environment. The reason for this appears not primarily to be because the component of additive genetic variation is higher in relation to the total phenotypic variation under these conditions, but because the part of the phenotypic variance explained by environmental variation increases at temperatures to which the current populations has been less frequently under selection.     

A framework for the study of genetic variation in migratory behaviour

Evolutionary change results from selection acting on genetic variation. For migration to be successful, many different aspects of an animal’s physiology and behaviour need to function in a co-coordinated way. Changes in one migratory trait are therefore likely to be accompanied by changes in other migratory and life-history traits. At present, we have some knowledge of the pressures that operate at the various stages of migration, but we know very little about the extent of genetic variation in various aspects of the migratory syndrome. As a consequence, our ability to predict which species is capable of what kind of evolutionary change, and at which rate, is limited. Here, we review how our evolutionary understanding of migration may benefit from taking a quantitative-genetic approach and present a framework for studying the causes of phenotypic variation. We review past research, that has mainly studied single migratory traits in captive birds, and discuss how this work could be extended to study genetic variation in the wild and to account for genetic correlations and correlated selection. In the future, reaction-norm approaches may become very important, as they allow the study of genetic and environmental effects on phenotypic expression within a single framework, as well as of their interactions. We advocate making more use of repeated measurements on single individuals to study the causes of among-individual variation in the wild, as they are easier to obtain than data on relatives and can provide valuable information for identifying and selecting traits. This approach will be particularly informative if it involves systematic testing of individuals under different environmental conditions. We propose extending this research agenda by using optimality models to predict levels of variation and covariation among traits and constraints. This may help us to select traits in which we might expect genetic variation, and to identify the most informative environmental axes. We also recommend an expansion of the passerine model, as this model does not apply to birds, like geese, where cultural transmission of spatio-temporal information is an important determinant of migration patterns and their variation.     

Linking stages of life history: How larval quality translates into juvenile performance for an intertidal barnacle (Balanus glandula)

Many marine invertebrates with complex life cycles produce planktoniclarvae that experience environmental conditions different fromthose encountered by adults. Factors such as temperature andfood, known to impact the larval period, can also affect larvalsize and consequently the size of newly settled juveniles. Afterdocumenting natural variation in the size of cyprids (the finallarval stage) of the barnacle Balanus glandula, we experimentallymanipulated temperature and food given to larvae to producecyprids of differing sizes but within the size range of cypridsfound in the field. In a set of trials in which larvae of B.glandula were raised on full or reduced rations in the laboratoryand subsequently outplanted into the field as newly metamorphosedjuveniles, we explored the effects of larval nutrition and sizeon juvenile performance. Larvae that received full rations throughouttheir feeding period produced larger cyprids (with more lipidand protein). These larger cyprids grew faster as juvenilesand sometimes survived better in the field than juveniles fromlarvae that had their food ration reduced in the last feedinginstar. For naturally settling barnacles brought into the laboratorywithin 2 days of settlement and fed, we found that initial juvenilesize was a good predictor of juvenile size even after 2 weeksof growth. By manipulating food given to juveniles that werederived from larvae fed either full or reduced rations, we foundthat larval nutritional effects persisted in juveniles for 2–3times the period that larvae experienced altered food rations.     

The role of temperature variability on insect performance and population dynamics in a warming world

Despite the amount of research on the consequences of global warming on ecological systems, most studies examine the impact of increases in average temperature. However, there are few studies concerning the role of thermal variability on ecological processes. Based on insect thermal and population ecology, we propose a theoretical framework for organizing the study of the role that thermal mean and variability plays in individual performance, and how it may affect population dynamics. Starting with three predictions of global warming scenarios, we develop null models of the expected changes in individual physiological performance and population dynamics. Ecological consequences in each scenario may range from simple changes in performance to drastic changes in population fluctuations and geographic ranges. In particular, our null models show that potential changes in the intrinsic population growth rate (R_m) will depend on the interaction of mean temperature and thermal variability, and that the net effect of the interaction could be synergistic or antagonistic. To evaluate these null models, we fit performance curves to compiled data from the literature on measurements of R_m at several constant and fluctuating temperatures. The fitted models showed that several of the qualitative characteristics predicted by the null model may be found in the fitted curves. We expect that this framework will be useful as a guide to study the influence of thermal changes on the dynamics of natural populations. Synthesis Despite the common assertion that global warming impacts depend on not only the mean temperatures but also on thermal variability, theoretical approaches to explain how the interaction of thermal mean and variability determines fitness are lacking. Here we propose a framework for studying the role of thermal mean and variability on individual performance and population dynamics. We developed null models that show how changes in the intrinsic population growth rate (R_m) will depend on the interaction of mean temperature and thermal variability, and that the net effect could be synergistic or antagonistic. We expect that this framework will be useful to study the influence of thermal changes on natural populations.