Special Feature: Intraspecific variation in ecology & evolution

Abstract

Species across the tree of life differ in many aspects. Comparative analyses and meta-analyses in both animal and plant kingdoms have brought significant insights on interspecific variation (e.g. Raffard et al., 2019; Siefert et al., 2015). As ecologists, we often work on the assumption that interspecific variation is greater than intraspecific variation, despite the central role intraspecific variation plays in evolutionary theory (Darwin, 1859). Yet, intraspecific variation may improve species' ability to respond and adapt to new environmental conditions, which is pivotal in the current context of global changes where both the mean and the variance of environmental conditions are changing. Therefore, it is surprising that the use of single average values per species has persisted across ecological research, from using mean values of ecological traits in community ecology (e.g. McGill et al., 2006), to mean values of vital rates in population projection models (e.g. Caswell, 2006; Vindenes et al., 2021). Doing so ignores the genetic and phenotypic variation present within species, from individuals through to populations, often over-simplifying the complexity that exists in nature. In this special feature, Rosa et al. (2025) nicely exemplifies how ignoring intraspecific variation in vital rates can bias predictions of population growth rates from demographic models. The authors show that mixing vital rates (e.g. survival, recruitment) from different yellow-bellied toad (Bombina variegata) populations, inhabiting contrasting ecological contexts, in hybrid ‘Frankenstein’ matrices, would rely on the assumption that all populations have similar vital rates, independently on their habitats or encountered weather conditions. This strong and somewhat unrealistic biological assumption can lead hybrid matrices to produce biased predictions of population growth rates with potentially deleterious implications for conservation or management.

Up to now, the literature on intraspecific variation has focused on genetic diversity (Bolnick et al., 2011; Hughes et al., 2008). However, intraspecific variation may also be stochastic or environmentally induced (Fox & Kendall, 2002; West-Eberhard, 2003) and emerge as individual differences in behaviour (Réale et al., 2010), phenotype (Douhard et al., 2013), or demographic rates (Vindenes et al., 2008), which can ultimately impact species interactions and coexistence (Bolnick et al., 2011; Hart et al., 2016; Violle et al., 2012). Thus, intraspecific variation at the individual level can propagate across levels of biological organisation through to the whole ecosystem. For example, intraspecific variation has important ecological effects and its degradation can impact ecosystem processing and functioning (Des Roches et al., 2018) as well as nature's contributions to people (Des Roches et al., 2021). Therefore, integrating intraspecific variation into biodiversity research and conservation is crucial.

The articles in this special feature include research articles, concept papers and method papers that use both experimental and observational approaches. They explore intraspecific variation across fields within ecology and evolution, including animal behaviour, population ecology, and functional ecology investigated in a myriad of taxa, such as mammals (marmots, sheep, Northern Elephant seals, mice, etc.), insects (e.g. bumblebees, mosquitoes), birds (e.g. blue tit, superb starling, red knots), fish (e.g. guppies) and amphibians (e.g. boreal toads). This impressive corpus of studies clearly demonstrates that intraspecific variation is the rule rather than the exception and is widespread across the entire Tree of Life. We identified three dimensions within which intraspecific variation can be investigated and the contributions to this special feature show how these dimensions can uncover insights into intraspecific variation significance in ecological and evolutionary contexts (Figure 1): (Dimension 1: Organismal scales) how to look at intraspecific variation? Intraspecific variation has been explored both within and among populations, based either on the monitoring of individuals throughout their life or on among-population comparisons; (Dimension 2: Biological scales) where to look at intraspecific variation? Intraspecific variation can be studied at different levels of biological organisation, from molecule to population levels; and (Dimension 3: Environmental gradient) in which ecological contexts? Intraspecific variation may imply different abilities for individuals to cope with environmental stressors, with consequences for individual fitness and population persistence. Each of these dimensions enable us to put intraspecific variation into broader eco-evolutionary contexts. By looking at these three dimensions, future work will advance our understanding of how intraspecific competition, a prerequisite for evolution, shapes the striking biodiversity of our planet.

One way to explore intraspecific variation is to compare multiple populations within the same species under contrasting ecological conditions. In this special feature, several studies have compared populations across a gradient of environmental conditions; the comparison of urban versus forest blue tits Cyanistes caeruleus (Pitt et al., 2025), or of mosquitos Aedes sierrensis along a latitudinal gradient (Lyberger et al., 2025), shed new light on intraspecific variation in phenotypic traits in nature. In particular, in tits, the removal of eggs cannot be compensated by the production of a new egg in an urban environment, whereas it can be in the forest blue tit, highlighting intraspecific variation in the ability to deal with artificially reduced reproduction. In mosquitoes, body size increases with latitude and decreases with increasing temperature, providing clear evidence for intraspecific variation in this key phenotypic trait. Comparative analyses involving multiple populations (i.e. among-population comparisons) are powerful tools to tackle the question of intraspecific variation across space and possibly time. Intraspecific variation can also be explored through the direct comparison of different individuals within a given population. In mice and voles for example, Humphreys and Mortelliti (2025) demonstrate high among-individual differences in pilfering personalities within a population. Another study in this issue has investigated how intraspecific variation in sleep varies throughout ontogeny by comparing different individuals in a wild population of fallow deer Dama dama whilst accounting for age and environmental conditions (Mortlock et al. 2025).

Our knowledge on intraspecific variation can also be improved by looking more closely at within-individual variation. For instance, thanks to their longitudinal long-term monitoring of female elephant seals Mirounga angustirostris throughout their (whole) life in the wild, Payne et al. (2024) show that among-individual differences in reproductive success observed at the population level are due to a decline of reproductive performance with increasing age. This demonstration of reproductive senescence in such a long-lived species shows that intraspecific variation in reproduction output observed at the population level partly results from an individual decrease of performance with increasing age. Likewise, within-individual variation in mating tactics has been highlighted in male Trinidadian guppies (Poecilia reticulata), depending on the predation regimes they were adapted to (Yang et al., 2025), demonstrating experimentally clear developmental plasticity in brain morphology.

The contributing studies to this special feature highlight the range of levels of biological organisation at which intraspecific variation can be observed and measured, going from differences reported at the molecular level to trait heterogeneities identified at the population level. Proximate mechanisms underlying observed intraspecific variation are, in general, poorly studied. Wolf et al. (2025) address this gap by demonstrating that shelterin protein gene expression varies latitudinally and correlates more strongly with body size differences than telomere length across populations of tree swallow (Tachycineta bicolor). Castellanos-Labarcena et al. (2025) also use a molecular approach to investigate latitudinal gradients, showing that in parasitic wasps, the unusual pattern of high species richness in temperate regions has a weak, positive relationship with intraspecific diversity determined by mitochondrial DNA. These studies demonstrate that understanding intraspecific variation at a molecular level can contribute towards our knowledge of processes leading to spatial patterns of taxonomic and phenotypic diversity at higher levels of organisation.

Intraspecific variation is often also investigated by measuring differences in phenotypic traits among individuals. Life-history and morphological traits are commonly studied individual characteristics in this special feature. They include body size/mass (e.g. Fitzgerald et al., 2025; Lyberger et al., 2025), clutch size (e.g. Pitt et al., 2025), and reproductive traits (e.g. Aich et al., 2024) to name just a few examples. Behavioural traits are also well represented in this special feature, with pilfering personalities in small mammals (Humphreys & Mortelliti, 2025), and activity (Aich et al., 2024) or mating tactics in guppies (Yang et al., 2025), as well as sleep behaviour in fallow deer (Mortlock et al. 2025).

Moving from individual to group and then population level characteristics, studies often investigate ultimate mechanisms shaping intraspecific variation (e.g. environment, population density). For example, Shah and Rubenstein (2025) show how variation in within-population group structure of superb starling (Lamprotornis superbus) is related to habitat through directional dispersal from low to high habitat quality, while Fitzgerald et al. (2025) demonstrate that environmental filtering of body size in multiple bumblebee species acts across multiple moments of trait distributions, with wider ecological consequences on variation in diet breadth and phenology. Vital rates, such as survival and fecundity, often estimated for a category of individuals (e.g. of a given age, of a given sex) can also shed new light on intraspecific variation. For instance, among-individual differences related to age in fecundity lead to intraspecific variation in fecundity rates in a strongly age-structured population such as elephant seals (Payne et al., 2024). Along the same line, different populations of the same species inhabiting contrasting habitat types can show markedly different vital rates (Rosa et al., 2025), reflecting intraspecific variation (i.e. among-population differences).

One important motivation for investigating intraspecific variation is how individuals of a species respond differently to environmental stress, with implications for population persistence under global environmental changes. The contributions to this special feature highlight three key classes of environmental stressors within which intraspecific variation has been investigated: biotic (e.g. pathogens, predators), abiotic (climate or weather, latitudinal gradient) and anthropogenic (e.g. pollution, urbanisation) factors.

For example, within disease ecology research, traditional labelling of an entire host species as tolerant or resistant obscures the intraspecific variation of host susceptibility. Hardy et al. (2025) suggest that differences across populations can help inform conservation activities such as targeted translocations after demonstrating differences in tolerance and resistance of boreal toad (Anaxyrus boreas boreas) populations to the fungal pathogen Batrachochytrium dendrobatidis. Similarly, when facing various climatic conditions, high intraspecific variation in body size can be observed in mosquitoes along a latitudinal gradient (Lyberger et al., 2025) or in bumblebees (Fitzgerald et al., 2025). These studies linking climatic conditions to intraspecific variation have strong implications in the context of climate changes, as they can inform on individuals' ability to respond to new environmental conditions. Furthermore, it is important to consider intraspecific variation in the context of both biotic and abiotic factors together, as encouraged by Costa-Pereira and Shaner (2025) in their concept paper.

Another example of the importance of accounting for intraspecific variation when assessing the effect of an environmental stressor in the wild is provided by Aich et al. (2024). They exposed the progeny of wild caught guppies to different ecologically relevant concentrations of a pharmaceutical pollutant to show that not only does this stressor impact individual behaviour, life history and reproductive traits but also individual level correlations between sets of traits. This between-individual variation in trade-offs can have important consequences for population and individual resilience to environmental stressors. Individual level correlations between sets of traits, also called life-history trade-offs, have been studied experimentally in ‘controlled’ studies, but it remains hard to detect them in the wild. One of the reasons is that trade-off expressions are context-dependent, that is they may depend on environmental conditions. To account for that, Bliard et al. (2025) propose a novel approach they apply to long-term data collected on yellow-bellied marmots (Marmota flaviventris) and Soay sheep (Ovis aries), which allows detecting context dependence in the expression of trade-offs. As trade-off expressions can vary over time within a population due to changes of the environmental context, the question ‘in which ecological context’ is crucial.

Although the concept of individual heterogeneity has been intensively studied in population ecology and demography research (e.g. Cam et al., 2016; Forsythe et al., 2021; Gimenez et al., 2018; Stover et al., 2012; Tuljapurkar et al., 2009; Van Daalen & Caswell, 2020; Wilson & Nussey, 2010), as also well highlighted by the contributions of this special feature, the identification and quantification of intraspecific variation in other fields of ecology and evolution such as community ecology and macroecology has lagged behind. This is true even in plant-focused studies, despite the history of trait research in plant ecology (Schleuning et al., 2023; Westerband et al., 2021). One challenge has been the availability of individual-level data, which is required to assess reliably intra-specific variability. Although there have been substantial efforts to address this topic in plants, with large scale databases holding individual observations of plant ecological traits (e.g., TRY, Kattge et al., 2011), many animal trait databases are still only available at the species level e.g., PanTHERIA (Jones et al., 2009) and EltonTraits (Wilman et al., 2014). The publication of AVONET (Tobias et al., 2022), which includes individual-level data for lots of bird species is a significant advance in addressing this shortfall, although the challenge remains for less well-studied taxa, and even for small scale studies, measuring intraspecific variation is generally acknowledged as a caveat in community ecology research (Violle et al., 2012). Yet we know that across spatiotemporal scales, intraspecific trait variation has an important role to play in ecological patterns and processes. For example, at large spatial scales, intraspecific variation in species traits may explain differences in population responses to habitat transformation across their ranges (Banks-Leite et al., 2022).

This special feature shows that this challenge remains, with substantially fewer papers measuring and investigating intraspecific variation of ecological and life history traits than behavioural and population ecology studies. One trait, however, that has facilitated investigation into intraspecific trait variation in animal ecology is body size and/or mass as shown in this special feature by Fitzgerald et al. (2025), Lyberger et al. (2025) and Wolf et al. (2025). Yet moving beyond body mass has proved challenging, and even at the species level, the only complete trait data within many trait databases is body mass (Tobias et al., 2022). Expanding the number of ecological and life history traits measured at the individual level will further facilitate the integration of intraspecific variation into a broader range of fields within ecology and evolution, including functional and community ecology, and provide a more holistic understanding of intraspecific trait variation.

The special feature also highlights the need to consider multiple moments of intraspecific variation. Beyond the consideration of single average values across a species, even when investigating intraspecific variation, we have often focused on average values within, for example, a population. This, however, ignores other aspects of the distribution of values within a species such as variance and kurtosis. Therefore, a key methodological approach going forwards in the field of intraspecific variation is the quantification of multiple moments to reveal, for example, information relating to environmental filtering and impacts on ecosystem processes, as demonstrated by Fitzgerald et al. (2025).

As shown in this special feature, intraspecific variation can be classified in many ways, and although almost ubiquitous across fields of study in ecology and evolution, its definition, classification and quantification vary greatly among studies. Costa-Pereira and Shaner (2025) argue that variation in individual niche specialisation has primarily used resource niche axes for its quantification (e.g. Arroyo-Correa et al., 2023; Costa-Pereira et al., 2019), often ignoring intraspecific variation in environmental associations. They propose a multidimensional approach to study individual niche specialisation by incorporating both biotic and abiotic niche axes and call for both empirical ecologists and theoreticians to integrate resource use and environmental associations across space and time to show how intraspecific variation can lead to emergent properties at higher levels of biological organisation, and maximise the data available to quantify n-dimensional niches.

This emphasises how intraspecific variation may lead to emergent properties at higher biological levels of organisation. For example, Castellanos-Labarcena et al. (2025) demonstrate in this special feature that variation at the genetic level can contribute towards our knowledge of emergent patterns at the species and community scales. Furthermore, intraspecific variation can have broader ecological impacts. Humphreys and Mortelliti (2025) suggest that variation in caching behaviour of small mammals has important implications for the ecosystem service of seed dispersal, and thus forest regeneration. We believe that understanding how intraspecific variation can lead to broader scale ecological patterns is an important direction for future research in this field, particularly due to its potential impacts on ecological processes and nature's contributions to people (Des Roches et al., 2018; Des Roches et al., 2021).

Intraspecific variation is a key process underpinning evolution. As a result, it is ubiquitous across research areas in ecology and evolution. This special feature highlights the diversity and breadth of approaches that can be used to uncover insights into intraspecific variation. We identified three key dimensions upon which contributions in this special feature to our understanding of intraspecific variation belong (Figure 1). First, we identified the axis of organismal scale where intraspecific variation can be investigated within an individual right through to among populations. Second, we identified the axis of biological scale where intraspecific variation occurs from the molecular to the among-population levels, with implications for emergent patterns at higher levels of biological organisation such as at the community scale, although the paucity of studies at this scale highlights the remaining challenges in this field. Finally, we identified the environmental gradient, which is the ecological context within which intraspecific variation is placed, which includes biotic and abiotic interactions. We call, along with the authors of the papers featured here, for more consideration of intraspecific variation in studies in ecology and evolution, as the use of single-value averages for species has restricted the capacity for comparative analyses at multiple levels, from molecular patterns, organismal processes and through to populations and community assemblages.

Marlene Gamelon and Hannah J. White wrote the first draft of the manuscript. All authors provided inputs for the figure, revised the manuscript and approved its submission.

The authors have no conflict of interest to declare.