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The heritability (h2) of fitness traits is often low. Although this has been attributed to directional selection having eroded genetic variation in direct proportion to the strength of selection, heritability does not necessarily reflect a trait's additive genetic variance and evolutionary potential (\"evolvability\"). Recent studies suggest that the low h2 of fitness traits in wild populations is caused not by a paucity of additive genetic variance (VA) but by greater environmental or nonadditive genetic variance (VR). We examined the relationship between h2 and variance-standardized selection intensities (i or βσ), and between evolvability (IA:VA divided by squared phenotypic trait mean) and mean-standardized selection gradients (βμ). Using 24 years of data from an island population of Savannah sparrows, we show that, across diverse traits, h2 declines with the strength of selection, whereas IA and IR (VR divided by squared trait mean) are independent of the strength of selection. Within trait types (morphological, reproductive, life-history), h2, IA, and IR are all independent of the strength of selection. This indicates that certain traits have low heritability because of increased residual variance due to the age at which they are expressed or the multiple factors influencing their expression, rather than their association with fitness.

The fundamental equation in evolutionary quantitative genetics, the Lande equation, describes the response to directional selection as a product of the additive genetic variance and the selection gradient of trait value on relative fitness. Comparisons of both genetic variances and selection gradients across traits or populations require standardization, as both are scale dependent. The Lande equation can be standardized in two ways. Standardizing by the variance of the selected trait yields the response in units of standard deviation as the product of the heritability and the variance‐standardized selection gradient. This standardization conflates selection and variation because the phenotypic variance is a function of the genetic variance. Alternatively, one can standardize the Lande equation using the trait mean, yielding the proportional response to selection as the product of the squared coefficient of additive genetic variance and the mean‐standardized selection gradient. Mean‐standardized selection gradients are particularly useful for summarizing the strength of selection because the mean‐standardized gradient for fitness itself is one, a convenient benchmark for strong selection. We review published estimates of directional selection in natural populations using mean‐standardized selection gradients. Only 38 published studies provided all the necessary information for calculation of mean‐standardized gradients. The median absolute value of multivariate mean‐standardized gradients shows that selection is on average 54% as strong as selection on fitness. Correcting for the upward bias introduced by taking absolute values lowers the median to 31%, still very strong selection. Such large estimates clearly cannot be representative of selection on all traits. Some possible sources of overestimation of the strength of selection include confounding environmental and genotypic effects on fitness, the use of fitness components as proxies for fitness, and biases in publication or choice of traits to study.

Phenotypic plasticity is frequently assumed to be an adaptive mechanism by which organisms cope with rapid changes in their environment, such as shifts in temperature regimes owing to climate change. However, despite this adaptive assumption, the nature of selection on plasticity within populations is still poorly documented. Here, we performed a systematic review and meta-analysis of estimates of selection on thermal plasticity. Although there is a large literature on thermal plasticity, we found very few studies that estimated coefficients of selection on measures of plasticity. Those that did do not provide strong support for selection on plasticity, with the majority of estimates of directional selection on plasticity being weak and non-significant, and no evidence for selection on plasticity overall. Although further estimates are clearly needed before general conclusions can be drawn, at present there is not clear empirical support for any assumption that plasticity in response to temperature is under selection. We present a multivariate mixed model approach for robust estimation of selection on plasticity and demonstrate how it can be implemented. Finally, we highlight the need to consider the environments, traits and conditions under which plasticity is (or is not) likely to be under selection, if we are to understand phenotypic responses to rapid environmental change. This article is part of the theme issue ‘The role of plasticity in phenotypic adaptation to rapid environmental change’.

Although many selection estimates have been published, the environmental factors that cause selection to vary in space and time have rarely been identified. One way to identify these factors is by experimentally manipulating the environment and measuring selection in each treatment. We compiled and analyzed selection estimates from experimental studies. First, we tested whether the effect of manipulating the environment on selection gradients depends on taxon, trait type, or fitness component. We found that the effect of manipulating the environment was larger when selection was measured on life-history traits or via survival. Second, we tested two predictions about the environmental factors that cause variation in selection. We found support for the prediction that variation in selection is more likely to be caused by environmental factors that have a large effect on mean fitness but not for the prediction that variation is more likely to be caused by biotic factors. Third, we compared selection gradients from experimental and observational studies. We found that selection varied more among treatments in experimental studies than among spatial and temporal replicates in observational studies, suggesting that experimental studies can detect relationships between environmental factors and selection that would not be apparent in observational studies.

Evolutionary biology and ecology have a strong theoretical underpinning, and this has fostered a variety of modeling approaches. A major challenge of this theoretical work has been to unravel the tangled feedback loop between ecology and evolution. This has prompted the development of two main classes of models. While quantitative genetics models jointly consider the ecological and evolutionary dynamics of a focal population, a separation of timescales between ecology and evolution is assumed by evolutionary game theory, adaptive dynamics, and inclusive fitness theory. As a result, theoretical evolutionary ecology tends to be divided among different schools of thought, with different toolboxes and motivations. My aim in this synthesis is to highlight the connections between these different approaches and clarify the current state of theory in evolutionary ecology. Central to this approach is to make explicit the dependence on environmental dynamics of the population and evolutionary dynamics, thereby materializing the eco-evolutionary feedback loop. This perspective sheds light on the interplay between environmental feedback and the timescales of ecological and evolutionary processes. I conclude by discussing some potential extensions and challenges to our current theoretical understanding of eco-evolutionary dynamics.

Cope's rule, the tendency for species within a lineage to evolve towards larger body size, has been widely reported in the fossil record, but the mechanisms leading to such phyletic size increase remain unclear. Here we show that selection acting on individual organisms generally favors larger body size. We performed an analysis of the strength of directional selection on size compared with other quantitative traits by evaluating 854 selection estimates from 42 studies of contemporaneous natural populations. For size, more than 79% of selection estimates exceed zero, whereas for other morphological traits positive and negative values are similar in frequency. The selective advantage of increased size occurs for traits implicated in both natural selection (e.g., differences in survival) and sexual selection (e.g., differences in mating success). The predominance of positive directional selection on size within populations could translate into a macroevolutionary trend toward increased size and thereby explain Cope's rule.

Despite the potential for rapid evolution, stasis is commonly observed over geological timescales—the so-called \"paradox of stasis.\" This paradox would be resolved if stabilizing selection were common, but stabilizing selection is infrequently detected in natural populations. We hypothesize a simple solution to this apparent disconnect: stabilizing selection is hard to detect empirically once populations have adapted to a fitness peak. To test this hypothesis, we developed an individual-based model of a population evolving under an invariant stabilizing fitness function. Stabilizing selection on the population was infrequently detected in an \"empirical\" sampling protocol, because (1) trait variation was low relative to the fitness peak breadth; (2) nonselective deaths masked selection; (3) populations wandered around the fitness peak; and (4) sample sizes were typically too small. Moreover, the addition of negative frequency-dependent selection further hindered detection by flattening or even dimpling the fitness peak, a phenomenon we term \"squashed stabilizing selection.\" Our model demonstrates that stabilizing selection provides a plausible resolution to the paradox of stasis despite its infrequent detection in nature. The key reason is that selection \"erases its traces\": once populations have adapted to a fitness peak, they are no longer expected to exhibit detectable stabilizing selection.

Populations are shifting their phenology in response to climate change, but these shifts are often asynchronous among interacting species. Resulting phenological mismatches can drive simultaneous changes in natural selection and population demography, but the links between these interacting processes are poorly understood. Here we analyse 37 years of data from an individual‐based study of great tits (Parus major) in the Netherlands and use mixed‐effects models to separate the within‐ and across‐year effects of phenological mismatch between great tits and caterpillars (a key food source for developing nestlings) on components of fitness at the individual and population levels. Several components of individual fitness were affected by individual mismatch (i.e. late breeding relative to the caterpillar food peak date), including the probability of double‐brooding, fledgling success, offspring recruitment probability and the number of recruits. Together these effects contributed to an overall negative relationship between relative fitness and laying dates, that is, selection for earlier laying on average. Directional selection for earlier laying was stronger in years where birds bred on average later than the food peak, but was weak or absent in years where the phenology of birds and caterpillars matched (i.e. no population mismatch). The mean number of fledglings per female was lower in years when population mismatch was high, in part because fewer second broods were produced. Population mismatch had a weak effect on the mean number of recruits per female, and no effect on mean adult survival, after controlling for the effects of breeding density and the quality of the autumnal beech (Fagus sylvatica) crop. These findings illustrate how climate change‐induced mismatch can have strong effects on the relative fitness of phenotypes within years, but weak effects on mean demographic rates across years. We discuss various general mechanisms that influence the extent of coupling between breeding phenology, selection and population dynamics in open populations subject to strong density regulation and stochasticity.

Standard metabolic rate (SMR), defined as the minimal energy expenditure required for self-maintenance, is a key physiological trait. Few studies have estimated its relationship with fitness, most notably in insects. This is presumably due to the difficulty of measuring SMR in a large number of very small individuals. Using high-throughput flow-through respirometry and a Drosophila melanogaster laboratory population adapted to a life cycle that facilitates fitness measures, we quantified SMR, body mass, and fitness in 515 female and 522 male adults. We used a novel multivariate approach to estimate linear and nonlinear selection differentials and gradients from the variance-covariance matrix of fitness, SMR, and body mass, allowing traits specific covariates to be accommodated within a single model. In males, linear selection differentials for mass and SMR were positive and individually significant. Selection gradients were also positive but, despite substantial sample sizes, were nonsignificant due to increased uncertainty given strong SMR-mass collinearity. In females, only nonlinear selection was detected and it appeared to act primarily on body size, although the individual gradients were again nonsignificant. Selection did not differ significantly between sexes although differences in the fitness surfaces suggest sex-specific selection as an important topic for further study.

Few facets of biology vary more than functional traits and life‐history traits. To explore this vast variation, functional ecologists and population ecologists have developed independent approaches that identify the mechanisms behind and consequences of trait variation. Collaborative research between researchers using trait‐based and demographic approaches remains scarce. We argue that this is a missed opportunity, as the strengths of both approaches could help boost the research agendas of functional ecology and population ecology. This special feature, which spans three journals of the British Ecological Society due to its interdisciplinary nature, showcases state‐of‐the‐art research applying trait‐based and demographic approaches to examine relationships between organismal function, life history strategies and population performance across multiple kingdoms. Examples include the exploration of how functional trait × environment interactions affect vital rates and thus explain population trends and species occurrence; the coordination of seed traits and dispersal ability with the pace of life in plants; the incorporation of functional traits in dynamic energy budget models; or the discovery of linkages between microbial functional traits and the fast–slow continuum. Despite their historical isolation, collaborative work between functional ecologists and population ecologists could unlock novel research pathways. We call for an integrative research agenda to evaluate which and when traits are functional, as well as their ability to describe and predict life history strategies and population dynamics. We highlight promising, complementary research avenues to overcome current limitations. These include a more explicit linkage of selection gradients in the context of functional trait–vital rate relationships, and the implementation of standardised protocols to track changes in traits and vital rates over time at the same location and individuals, thus allowing for the explicit incorporation of trade‐offs in analyses of covariation of functional traits and life‐history traits. Sumario Pocos aspectos varían más en biología que los caracteres funcionales y de historia vital. Para explorar esta vasta variación, los ecólogos funcionales y de poblaciones han desarrollado independientemente métodos que identifican los mecanismos y las consecuencias de dicha variabilidad. Las colaboraciones entre investigadores que utilizan métodos basados en caracteres funcionales y de historia vital son bastante limitadas a día de hoy. Aquí argumentamos que éllo conlleva una gran oportunidad aún por explotar, ya que las fortalezas de ambas metodologías podrían revolucionar las agendas investigadoras de la ecología funcional y de poblaciones. Este número especial, el cual incorpora tres de las revistas de la Sociedad Ecológica Británica debido a su carácter interdisciplinar, contiene investigaciones punteras en la aplicación de metodologías de caracteres funcionales y métodos demográficos para examinar relaciones entre funciones del organismo, estrategias de historia vital, y el rendimiento poblacional en varios reinos. Algunos ejemplos incluyen la exploración de cómo las interacciones de carácter funcional~ambiente afectan las tasas vitales para así explicar tendencias demográficas y ocurrencia de especies; la coordinación de algunas características de las semillas, su habilidad dispersora, y el ritmo vital en el reino vegetal; la incorporación de caracteres funcionales en modelos de presupuesto dinámico de energía; o el descubrimiento de enlaces entre caracteres funcionales en microbios y el continuo rápido‐lento. A pesar del aislamiento histórico, las colaboraciones entre la ecología funcional y poblacional podrían abrir novedosas rutas de investigación. Hacemos una llamada para el desarrollo de una agenda investigadora integradora que evalúe cuáles y cuándo los caracteres son funcionales, así como su habilidad para predecir estrategias de historia vital y dinámicas poblacionales. Asimismo, resaltamos aproximaciones investigadoras complementarias y prometedoras para superar las limitaciones actuales, incluyendo una vinculación más explícita de los gradientes de selección en las relaciones carácter funcional~tasa vital, o protocolos estandarizados para examinar cambios temporales en caracteres funcionales y tasas vitales en la misma población e individuos. Éllo permitirá incorporar explícitamente los compromisos energéticos en los análisis de co‐variación de caracteres funcionales y de historia vital. Résumé Peu de facettes de la biologie varient autant que les traits fonctionnels et les traits d'histoire de vie. Pour explorer cette grande variation, les écologues fonctionnels et les écologues des populations ont développé des approches indépendantes pour identifier les mécanismes sous‐jacents et les conséquences des variations de ces traits. Les interactions entre les chercheurs utilisant les approches soit basées sur les traits fonctionnels soit sur la démographie sont rares. Il s'agit sans aucun doute d'une opportunité manquée car les forces combinées des deux approches pourraient accélérer les agendas de recherches aussi bien de l'écologie fonctionnelle que de l'écologie des populations. Ce numéro spécial, à cheval sur trois journaux de la Société Britannique d'Ecologie, illustre les dernières recherches appliquant les approches qui s'appuient sur les traits et la démographie pour examiner les relations entre les fonctions des organismes, les stratégies d'histoire de vie, et les performances des populations pour différents groupes taxonomiques. Les exemples incluent: (i) l'exploration de la façon dont les interactions traits fonctionnels x environnement affectent les taux vitaux et expliquent l'occurrence des espèces et les tendances populationnelles; (ii) la façon dont le rythme de vie des plantes covarie avec les traits des graines et la capacité de dispersion de celles‐ci; (iii) l'incorporation des traits fonctionnels dans les modèles de budget énergétique; (iv) ou encore la découverte d'un lien entre les traits fonctionnels des microbes et le gradient lent‐rapide. Malgré leur isolement historique, le travail collaboratif entre les écologues fonctionnels et les écologues des populations pourrait ouvrir de nouvelles voies de recherche. Nous appelons ainsi à une recherche intégrative pour évaluer quels traits sont fonctionnels, et dans quelles mesures, ainsi que leur capacité à décrire et prédire les stratégies d'histoire de vie et la dynamique des populations. Nous mettons en lumière des pistes de recherche prometteuses et complémentaires pour dépasser les limites actuelles. Ces pistes incluent une liaison plus explicite des gradients de sélection dans le contexte de la relation trait fonctionnel~taux vitaux et l'implémentation de protocoles standardisés pour suivre les variations temporelles des traits fonctionnels et les taux vitaux en un même lieu et sur les mêmes individus, permettant ainsi la prise en compte explicite des compromis dans l'analyse de la covariation entre les traits fonctionnels et les traits d'histoire de vie.