The spatial scale of local adaptation in a stochastic environment

The structure of the environment influences the patterns and genetics of local adaptation

Environmental heterogeneity can lead to spatially varying selection, which can, in turn, lead to local adaptation. Population genetic models have shown that the pattern of environmental variation in space can strongly influence the evolution of local adaptation. In particular, when environmental variation is highly autocorrelated in space local adaptation will more readily evolve. However, there have been few attempts to test this prediction empirically or characterize the consequences it would have for the genetic architecture underlying local adaptation. In this study, I analyze a large-scale provenance trial conducted on lodgepole pine and find suggestive evidence that spatial autocorrelation in environmental variation is related to the strength of local adaptation that has evolved in that species. Motivated by those results, I use simulations to model local adaptation to different spatial patterns of environmental variation. The simulations confirm that local adaptation is expected to increase with the degree of spatial autocorrelation in the selective environment, but also show that highly heterogeneous environments are more likely to exhibit high variation in local adaptation, a result not previously described. I find that the spatial pattern of environmental variation influences the genetic architectures underlying local adaptation. In highly autocorrelated environments, the genetic architecture of local adaptation tends to be composed of high-frequency alleles with small phenotypic effects. In weakly autocorrelated environments, locally adaptive alleles may have larger phenotypic effects but are present at lower frequencies across species' ranges and experience more evolutionary turnover. Overall, this work emphasizes the profound importance that the spatial pattern of selection can have on the evolution of local adaptation and how spatial autocorrelation should be considered when formulating hypotheses in ecological and genetic studies.

The phylogenetic reconstruction of the Neotropical cycad genus Ceratozamia (Zamiaceae) reveals disparate patterns of niche evolution

The cycad genus Ceratozamia comprises 40 species from Mexico, Guatemala, Belize, and Honduras, where cycads occur throughout climatically varied montane habitats. Ceratozamia has the potential to reveal the history and processes of species diversification across diverse Neotropical habitats in this region. However, the species relationships within Ceratozamia and the ecological trends during its evolution remain unclear. Here, we aimed to clarify the phylogenetic relationships, the timing of clade and species divergences, and the niche evolution throughout the phylogenetic history of Ceratozamia. Genome-wide DNA sequences were obtained with MIG-seq, and multiple data-filtering steps were used to optimize the dataset used to construct an ultrametric species tree. Divergence times among branches and ancestral niches were estimated. The niche variation among species was evaluated, summarized into two principal components, and their ancestral states were reconstructed to test whether niche shifts among branches can be explained by random processes, under a Brownian Motion model. Ceratozamia comprises three main clades, and most species relationships within the clades were resolved. Ceratozamia has diversified since the Oligocene, with major branching events occurring during the Miocene. This timing is consistent with fossil evidence, the timing estimated for other Neotropical plant groups, and the major geological events that shaped the topographic and climatic variation in Mexico. Patterns of niche evolution in the genus do not accord with the Brownian Motion model. Rather, non-random evolution with shifts towards more seasonal environments at high latitudes, or shifts towards humid or dry environments at low latitudes explain the diversification of Ceratozamia. We present a comprehensive phylogenetic reconstruction for Ceratozamia and identify for the first time the environmental factors involved in clade and species diversification within the genus. This study alleviates the controversies regarding the species relationships in the genus and provides the first evidence that latitude-associated environmental factors may influence processes of niche evolution in cycads.

Space-for-time substitutions in climate change ecology and evolution

In an epoch of rapid environmental change, understanding and predicting how biodiversity will respond to a changing climate is an urgent challenge. Since we seldom have sufficient long-term biological data to use the past to anticipate the future, spatial climate-biotic relationships are often used as a proxy for predicting biotic responses to climate change over time. These 'space-for-time substitutions' (SFTS) have become near ubiquitous in global change biology, but with different subfields largely developing methods in isolation. We review how climate-focussed SFTS are used in four subfields of ecology and evolution, each focussed on a different type of biotic variable - population phenotypes, population genotypes, species' distributions, and ecological communities. We then examine the similarities and differences between subfields in terms of methods, limitations and opportunities. While SFTS are used for a wide range of applications, two main approaches are applied across the four subfields: spatial in situ gradient methods and transplant experiments. We find that SFTS methods share common limitations relating to (i) the causality of identified spatial climate-biotic relationships and (ii) the transferability of these relationships, i.e. whether climate-biotic relationships observed over space are equivalent to those occurring over time. Moreover, despite widespread application of SFTS in climate change research, key assumptions remain largely untested. We highlight opportunities to enhance the robustness of SFTS by addressing key assumptions and limitations, with a particular emphasis on where approaches could be shared between the four subfields.

Environmental variation and biotic interactions limit adaptation at ecological margins: lessons from rainforest Drosophila and European butterflies

Models of local adaptation to spatially varying selection predict that maximum rates of evolution are determined by the interaction between increased adaptive potential owing to increased genetic variation, and the cost genetic variation brings by reducing population fitness. We discuss existing and new results from our laboratory assays and field transplants of rainforest Drosophila and UK butterflies along environmental gradients, which try to test these predictions in natural populations. Our data suggest that: (i) local adaptation along ecological gradients is not consistently observed in time and space, especially where biotic and abiotic interactions affect both gradient steepness and genetic variation in fitness; (ii) genetic variation in fitness observed in the laboratory is only sometimes visible to selection in the field, suggesting that demographic costs can remain high without increasing adaptive potential; and (iii) antagonistic interactions between species reduce local productivity, especially at ecological margins. Such antagonistic interactions steepen gradients and may increase the cost of adaptation by increasing its dimensionality. However, where biotic interactions do evolve, rapid range expansion can follow. Future research should test how the environmental sensitivity of genotypes determines their ecological exposure, and its effects on genetic variation in fitness, to predict the probability of evolutionary rescue at ecological margins. This article is part of the theme issue 'Species' ranges in the face of changing environments (Part II)'.

Phenotypic plasticity is aligned with phenological adaptation on both micro- and macroevolutionary timescales

In seasonally variable environments, phenotypic plasticity in phenology may be critical for adaptation to fluctuating environmental conditions. Using an 18-generation longitudinal dataset from natural damselfly populations, we show that phenology has strongly advanced. Individual fitness data suggest this is likely an adaptive response towards a temperature-dependent optimum. A laboratory experiment revealed that developmental plasticity qualitatively matches the temperature dependence of selection, partially explaining observed advance in phenology. Expanding our analysis to the macroevolutionary level, we use a database of over 1-million occurrence records and spatiotemporally matched temperature data from 49 Swedish Odonate species to infer macroevolutionary dynamics of phenology. Phenological plasticity was more closely aligned with adaptation for species that have recently colonised northern latitudes, but with higher phenological mismatch at lower latitudes. Our results show that phenological plasticity plays a key role in microevolutionary dynamics within a single species, and such plasticity may have facilitated post-Pleistocene range expansion in this insect clade.

Spatial structure and dispersal dynamics in a house sparrow metapopulation

The effects of spatial structure on metapopulation dynamics depend upon the interaction between local population dynamics and dispersal, and how this relationship is affected by the geographical isolation and spatial heterogeneity in habitat characteristics. Our aim is to examine how emigration and immigration of house sparrows Passer domesticus in a Norwegian archipelagic metapopulation are affected by key factors predicted by classic metapopulation models to affect dispersal-spatial and temporal variation in population size, inter-island distance, local demography and habitat characteristics. This metapopulation can be divided into two major habitat types: (a) islands closer to the mainland where sparrows breed in colonies on farms, and (b) islands without farms, situated farther away from the mainland where sparrows are exposed to harsher environmental conditions. Dispersal was spatially structured within the metapopulation; there was proportionally and numerically less emigration and immigration involving farm islands, as compared to non-farm islands. Furthermore, emigration and immigration occurred mostly between nearby islands. Moreover, emigration in response to spatial differences in mean population size differed between the habitat types, but populations with large mean received more immigrants in both habitat types. The number of emigrants and immigrants was negatively related to long-term recruit production, which was not the case in non-farm islands. The proportion and number of emigrants was positively related to temporal increases in recruit production on farm islands, however not on non-farm islands. Our results demonstrate that spatial heterogeneity in environmental conditions influences how spatial variation in long-term mean population size, and temporal and spatial variation in recruit production, affects dispersal dynamics. The spatial structure of this metapopulation is therefore best described by a spatially explicit model in which the exchange of individuals within each habitat type is strongly affected by the degree of geographical isolation, population size and recruit production. However, these relationships differed between the two habitat types; non-farm islands showing similarities to a mainland-island model type of structure, whereas farm islands showed features more associated with source-sink or balanced dispersal models. Such differential dispersal dynamics between habitat types are expected to have important consequences for the ecological and evolutionary dynamics within this metapopulation.

Evolutionary mismatch along salinity gradients in a Neotropical water strider

The evolution of local adaptation is crucial for the in situ persistence of populations in changing environments. However, selection along broad environmental gradients could render local adaptation difficult, and might even result in maladaptation. We address this issue by quantifying fitness trade-offs (via common garden experiments) along a salinity gradient in two populations of the Neotropical water strider Telmatometra withei-a species found in both fresh (FW) and brackish (BW) water environments across Panama. We found evidence for local adaptation in the FW population in its home FW environment. However, the BW population showed only partial adaptation to the BW environment, with a high magnitude of maladaptation along naturally occurring salinity gradients. Indeed, its overall fitness was ~60% lower than that of the ancestral FW population in its home environment, highlighting the role of phenotypic plasticity, rather than local adaptation, in high salinity environments. This suggests that populations seemingly persisting in high salinity environments might in fact be maladapted, following drastic changes in salinity. Thus, variable selection imposed by salinization could result in evolutionary mismatch, where the fitness of a population is displaced from its optimal environment. Understanding the fitness consequences of persisting in fluctuating salinity environments is crucial to predict the persistence of populations facing increasing salinization. It will also help develop evolutionarily informed management strategies in the context of global change.

A framework for understanding gene expression plasticity and its influence on stress tolerance

Phenotypic plasticity can serve as a stepping stone towards adaptation. Recently, studies have shown that gene expression contributes to emergent stress responses such as thermal tolerance, with tolerant and susceptible populations showing distinct transcriptional profiles. However, given the dynamic nature of gene expression, interpreting transcriptomic results in a way that elucidates the functional connection between gene expression and the observed stress response is challenging. Here, we present a conceptual framework to guide interpretation of gene expression reaction norms in the context of stress tolerance. We consider the evolutionary and adaptive potential of gene expression reaction norms and discuss the influence of sampling timing, transcriptomic resilience, as well as complexities related to life history when interpreting gene expression dynamics and how these patterns relate to host tolerance. We highlight corals as a case study to demonstrate the value of this framework for non-model systems. As species face rapidly changing environmental conditions, modulating gene expression can serve as a mechanistic link from genetic and cellular processes to the physiological responses that allow organisms to thrive under novel conditions. Interpreting how or whether a species can employ gene expression plasticity to ensure short-term survival will be critical for understanding the global impacts of climate change across diverse taxa.

Rapid speciation and ecological divergence into North American alpine habitats: the Nippononebria (Coleoptera: Carabidae) species complex

The climate-driven species pump hypothesis has been supported in a number of phylogeographic studies of alpine species. Climate-driven shifts in distribution, coupled with rapid demographic change, have led to strong genetic drift and lineage diversification. Although the species pump has been linked to rapid speciation in a number of studies, few studies have demonstrated that ecological divergence accompanies rapid speciation. Here we examine genetic, morphological and physiological variation in members of the ground beetle taxon Nippononebria, to test three competing hypotheses of evolutionary diversification: isolation and incomplete lineage sorting (no speciation), recent speciation without ecological divergence, or recent speciation with ecological divergence into alpine habitats. Genetic data are consistent with recent divergence, with major lineages forming in the last million years. A species tree analysis, in conjunction with morphological divergence in male reproductive traits, support the formation of three recognized Nippononebria taxa. Furthermore, both morphological and physiological traits demonstrate ecological divergence in alpine lineages, with convergent shifts in body shape and thermal tolerance breadth. This provides strong evidence that the climate-driven species pump can generate ecological novelty, though it is argued that spatial scale may be a key determinant of broader patterns of macroevolution in alpine communities.

Quantification and decomposition of environment-selection relationships

In nature, selection varies across time in most environments, but we lack an understanding of how specific ecological changes drive this variation. Ecological factors can alter phenotypic selection coefficients through changes in trait distributions or individual mean fitness, even when the trait-absolute fitness relationship remains constant. We apply and extend a regression-based approach in a population of Soay sheep (Ovis aries) and suggest metrics of environment-selection relationships that can be compared across studies. We then introduce a novel method that constructs an environmentally structured fitness function. This allows calculation of full (as in existing approaches) and partial (acting separately through the absolute fitness function slope, mean fitness, and phenotype distribution) sensitivities of selection to an ecological variable. Both approaches show positive overall effects of density on viability selection of lamb mass. However, the second approach demonstrates that this relationship is largely driven by effects of density on mean fitness, rather than on the trait-fitness relationship slope. If such mechanisms of environmental dependence of selection are common, this could have important implications regarding the frequency of fluctuating selection, and how previous selection inferences relate to longer term evolutionary dynamics.

Linking local adaptation with the evolution of sex differences

Many conspicuous forms of evolutionary diversity occur within species. Two prominent examples include evolutionary divergence between populations differentially adapted to their local environments (local adaptation), and divergence between females and males in response to sex differences in selection (sexual dimorphism sensu lato). These two forms of diversity have inspired vibrant research programmes, yet these fields have largely developed in isolation from one another. Nevertheless, conceptual parallels between these research traditions are striking. Opportunities for local adaptation strike a balance between local selection, which promotes divergence, and gene flow-via dispersal and interbreeding between populations-which constrains it. Sex differences are similarly constrained by fundamental features of inheritance that mimic gene flow. Offspring of each sex inherit genes from same-sex and opposite-sex parents, leading to gene flow between each differentially selected half of the population, and raising the question of how sex differences arise and are maintained. This special issue synthesizes and extends emerging research at the interface between the research traditions of local adaptation and sex differences. Each field can promote understanding of the other, and interactions between local adaptation and sex differences can generate new empirical predictions about the evolutionary consequences of selection that varies across space, time, and between the sexes.This article is part of the theme issue 'Linking local adaptation with the evolution of sex differences'.

The evolution of phenotypic plasticity when environments fluctuate in time and space

Most theoretical studies have explored the evolution of plasticity when the environment, and therefore the optimal trait value, varies in time or space. When the environment varies in time and space, we show that genetic adaptation to Markovian temporal fluctuations depends on the between-generation autocorrelation in the environment in exactly the same way that genetic adaptation to spatial fluctuations depends on the probability of philopatry. This is because both measure the correlation in parent-offspring environments and therefore the effectiveness of a genetic response to selection. If the capacity to genetically respond to selection is stronger in one dimension (e.g., space), then plasticity mainly evolves in response to fluctuations in the other dimension (e.g., time). If the relationships between the environments of development and selection are the same in time and space, the evolved plastic response to temporal fluctuations is useful in a spatial context and genetic differentiation in space is reduced. However, if the relationships between the environments of development and selection are different, the optimal level of plasticity is different in the two dimensions. In this case, the plastic response that evolves to cope with temporal fluctuations may actually be maladaptive in space, resulting in the evolution of hyperplasticity or negative plasticity. These effects can be mitigated by spatial genetic differentiation that acts in opposition to plasticity resulting in counter-gradient variation. These results highlight the difficulty of making space-for-time substitutions in empirical work but identify the key parameters that need to be measured in order to test whether space-for-time substitutions are likely to be valid.

Positive correlation between dispersal and body size in Green Frogs (Rana clamitans) naturally colonizing an experimental landscape

Dispersers are often assumed to have the mean phenotype observed across the entire metapopulation, despite growing evidence of dispersal–phenotype correlations. We examined three dispersal–phenotype correlations in Green Frogs (Rana clamitans Latreille, 1801 = Lithobates clamitans (Latreille, 1801)). Two were in traits that have been previously tied to fitness (body size and body condition), while a third (relative hindlimb length) has been linked to movement performance. We constructed a spatially dispersed array of experimental ponds in close proximity to source ponds known to support Green Frog breeding populations. Over the course of two breeding seasons (four sampling periods), we measured phenotypes of all Green Frogs that had colonized the experimental ponds and a sample of individuals from the source ponds. After only 1 month, a positive correlation was detected between dispersal and body size within the population of dispersers occupying the experimental ponds. After a 2nd month, this positive dispersal – body size correlation was also present when comparing the population of dispersers to the population of nondispersers remaining at the source ponds. Even if generated solely by plasticity, a positive correlation between dispersal and body size (a trait tightly linked to fitness) has the ability to alter metapopulation capacity and thus the probability of regional species persistence.

Phenological synchrony between a butterfly and its host plants: Experimental test of effects of spring temperature

Climate-driven changes in the relative phenologies of interacting species may potentially alter the outcome of species interactions. Phenotypic plasticity is expected to be important for short-term response to new climate conditions, and differences between species in plasticity are likely to influence their temporal overlap and interaction patterns. As reaction norms of interacting species may be locally adapted, any such climate-induced change in interaction patterns may vary among localities. However, consequences of spatial variation in plastic responses for species interactions are understudied. We experimentally explored how temperature affected synchrony between spring emergence of a butterfly, Anthocharis cardamines, and onset of flowering of five of its host plant species across a latitudinal gradient. We also studied potential effects on synchrony if climate-driven northward expansions would be faster in the butterflies than in host plants. Lastly, to assess how changes in synchrony influence host use we carried out an experiment to examine the importance of the developmental stage of plant reproductive structures for butterfly oviposition preference. In southern locations, the butterflies were well-synchronized with the majority of their local host plant species across temperatures, suggesting that thermal plasticity in butterfly development matches oviposition to host plant development and that thermal reaction norms of insects and plants result in similar advancement of spring phenology in response to warming. In the most northern region, however, relative phenology between the butterfly and two of its host plant species changed with increased temperature. We also show that the developmental stage of plants was important for egg-laying, and conclude that temperature-induced changes in synchrony in the northernmost region are likely to lead to shifts in host use in A. cardamines if spring temperatures become warmer. Northern expansion of butterfly populations might possibly have a positive effect on keeping up with host plant phenology with more northern host plant populations. Considering that the majority of insect herbivores exploit multiple plant species differing in their phenological response to spring temperatures, temperature-induced changes in synchrony might lead to shifts in host use and changes in species interactions in many temperate communities.

Grow with the flow: a latitudinal cline in physiology is associated with more variable precipitation in Erythranthe cardinalis

Local adaptation is commonly observed in nature: organisms perform well in their natal environment, but poorly outside it. Correlations between traits and latitude, or latitudinal clines, are among the most common pieces of evidence for local adaptation, but identifying the traits under selection and the selective agents is challenging. Here, we investigated a latitudinal cline in growth and photosynthesis across 16 populations of the perennial herb Erythranthe cardinalis (Phrymaceae). Using machine learning methods, we identify interannual variation in precipitation as a likely selective agent: southern populations from more variable environments had higher photosynthetic rates and grew faster. We hypothesize that selection may favour a more annualized life history - grow now rather than save for next year - in environments where severe droughts occur more often. Thus, our study provides insight into how species may adapt if Mediterranean climates become more variable due to climate change.

Estimating the ability of plants to plastically track temperature-mediated shifts in the spring phenological optimum

One consequence of rising spring temperatures is that the optimum timing of key life-history events may advance. Where this is the case, a population's fate may depend on the degree to which it is able to track a change in the optimum timing either via plasticity or via adaptation. Estimating the effect that temperature change will have on optimum timing using standard approaches is logistically challenging, with the result that very few estimates of this important parameter exist. Here we adopt an alternative statistical method that substitutes space for time to estimate the temperature sensitivity of the optimum timing of 22 plant species based on >200 000 spatiotemporal phenological observations from across the United Kingdom. We find that first leafing and flowering dates are sensitive to forcing (spring) temperatures, with optimum timing advancing by an average of 3 days °C^-1 and plastic responses to forcing between -3 and -8 days °C^-1 . Chilling (autumn/winter) temperatures and photoperiod tend to be important cues for species with early and late phenology, respectively. For most species, we find that plasticity is adaptive, and for seven species, plasticity is sufficient to track geographic variation in the optimum phenology. For four species, we find that plasticity is significantly steeper than the optimum slope that we estimate between forcing temperature and phenology, and we examine possible explanations for this countergradient pattern, including local adaptation.

Social carry-over effects underpin trans-seasonally linked structure in a wild bird population

Spatial structure underpins numerous population processes by determining the environment individuals' experience and which other individuals they encounter. Yet, how the social landscape influences individuals' spatial decisions remains largely unexplored. Wild great tits (Parus major) form freely moving winter flocks, but choose a single location to establish a breeding territory over the spring. We demonstrate that individuals' winter social associations carry‐over into their subsequent spatial decisions, as individuals breed nearer to those they were most associated with during winter. Further, they also form territory boundaries with their closest winter associates, irrespective of breeding distance. These findings were consistent across years, and among all demographic classes, suggesting that such social carry‐over effects may be general. Thus, prior social structure can shape the spatial proximity, and fine‐scale arrangement, of breeding individuals. In this way, social networks can influence a wide range of processes linked to individuals' breeding locations, including other social interactions themselves.

Morphological and Genetic Variation along a North-to-South Transect in Stipa purpurea, a Dominant Grass on the Qinghai-Tibetan Plateau: Implications for Response to Climate Change

Estimating the potential of species to cope with rapid environmental climatic modifications is of vital importance for determining their future viability and conservation. The variation between existing populations along a climatic gradient may predict how a species will respond to future climate change. Stipa purpurea is a dominant grass species in the alpine steppe and meadow of the Qinghai-Tibetan Plateau (QTP). Ecological niche modelling was applied to S. purpurea, and its distribution was found to be most strongly correlated with the annual precipitation and the mean temperature of the warmest quarter. We established a north-to-south transect over 2000 km long on the QTP reflecting the gradients of temperature and precipitation, and then we estimated the morphological by sampling fruited tussocks and genetic divergence by using 11 microsatellite markers between 20 populations along the transect. Reproductive traits (the number of seeds and reproductive shoots), the reproductive-vegetative growth ratio and the length of roots in the S. purpurea populations varied significantly with climate variables. S. purpurea has high genetic diversity (He = 0.585), a large effective population size (Ne >1,000), and a considerable level of gene flow between populations. The S. purpurea populations have a mosaic genetic structure: some distant populations (over 1000 km apart) clustered genetically, whereas closer populations (< 100 km apart) had diverged significantly, suggesting local adaptation. Asymmetrical long-distance inter-population gene flow occurs along the sampling transect and might be mediated by seed dispersal via migratory herbivores, such as the chiru (Pantholops hodgsonii). These findings suggest that population performance variation and gene flow both facilitate the response of S. purpurea to climate change.