Temperature and sex shape reproductive barriers in a climate change hotspot

Climate change is altering species ranges and reproductive interactions in existing ranges, offering species new scope to mate and hybridize. The outcomes will depend on how environmental factors shape reproductive barriers across life stages, yet this is rarely assessed across the environments that species encounter in nature. We assess prezygotic and postzygotic barriers, and their dependence on temperature and parental sex, in species of a reef-building tubeworm (Galeolaria) from a fast-warming biodiversity hotspot in southern Australia. By replicating pure and reciprocal hybrid crosses across 5 temperatures spanning species' thermal ranges, we estimate thermal tolerance curves (defining niches) for crosses and reproductive isolation at each temperature. By also replicating crosses at 3 life stages, we partition the contributions of prezygotic barriers at fertilization, postzygotic barriers at embryogenesis, and postzygotic barriers at larval development to reproductive isolation. We show that barriers are weaker at fertilization and embryogenesis, but stronger and more temperature sensitive at larval development, as species diverge in thermal niche. Asymmetry of barriers between parental sexes, moreover, suggests a complex interplay between niche differentiation and maternal inheritance. Our findings point to a key role for temperature in reproductive isolation, but also challenges for predicting the fate of isolation in future climates.

Temperature and nutrition do not interact to shape the evolution of metabolic rate

Metabolic cold adaptation, or Krogh's rule, is the controversial hypothesis that predicts a monotonically negative relationship between metabolic rate and environmental temperature for ectotherms living along thermal clines measured at a common temperature. Macrophysiological patterns consistent with Krogh's rule are not always evident in nature, and experimentally evolved responses to temperature have failed to replicate such patterns. Hence, temperature may not be the sole driver of observed variation in metabolic rate. We tested the hypothesis that temperature, as a driver of energy demand, interacts with nutrition, a driver of energy supply, to shape the evolution of metabolic rate to produce a pattern resembling Krogh's rule. To do this, we evolved replicate lines of Drosophila melanogaster at 18, 25 or 28°C on control, low-calorie or low-protein diets. Contrary to our prediction, we observed no effect of nutrition, alone or interacting with temperature, on adult female and male metabolic rates. Moreover, support for Krogh's rule was only in females at lower temperatures. We, therefore, hypothesize that observed variation in metabolic rate along environmental clines arises from the metabolic consequences of environment-specific life-history optimization, rather than because of the direct effect of temperature on metabolic rate. This article is part of the theme issue 'The evolutionary significance of variation in metabolic rates'.

Exome-informed formulations of food proteins enhance body growth and feed conversion efficiency in ad libitum-fed mice

Meeting requirements for dietary proteins, especially of essential amino acids (EAAs), is critical for the life-long health of living organisms. However, defining EAA targets for preparing biologically-matched nutrition that satisfies metabolic requirements for protein remains challenging. Previous research has shown the advantages of 'exome matching' in representing the specific requirement of dietary AAs, where the target dietary AA profile was derived from in silico translation of the genome of an organism, specifically responsible for protein expression (the 'exome'). However, past studies have assessed these effects in only one sex, for few parameters (body mass and composition), and have used purified diets in which protein is supplied as a mixture of individual AAs. Here, for the first time, we utilise a computational method to guide the formulation of custom protein blends and test if exome matching can be achieved at the intact protein level, through blending standard protein ingredients, ultimately leading to optimal growth, longevity and reproductive function. Mice were provided ad libitum (ad lib) access to one of the four iso-energetic protein-limited diets, two matched and two mis-matched to the mouse exome target, and fed at a fixed protein energy level of 6.2%. During or following 13-weeks of feeding, the food intake, body growth, composition and reproductive functions were measured. Compared to the two mis-matched diets, male and female animals on the exome-matched diet with protein digestibility correction applied, exhibited significantly improved growth rates and final body mass. The feed conversion efficiency in the same diet was also increased by 62% and 40% over the worst diets for males and females, respectively. Male, not female, exhibited higher accretion of lean body mass with the matched, digestibility-corrected diet. All reproductive function measures in both sexes were comparable among diets, with the exception of testicular daily sperm production in males, which was higher in the two matched diets versus the mis-matched diets. The results collectively demonstrate the pronounced advantages of exome-matching in supporting body growth and improving feed conversion efficiency in both sexes. However, the potential impact of this approach in enhancing fertility needs further investigation.

Climate adaptation and vulnerability of foundation species in a global change hotspot

Climate change is altering species ranges, and relative abundances within ranges, as populations become differentially adapted and vulnerable to the climates they face. Understanding present species ranges, whether species harbour and exchange adaptive variants, and how variants are distributed across landscapes undergoing rapid change, is therefore crucial to predicting responses to future climates and informing conservation strategies. Such insights are nonetheless lacking for most species of conservation concern. We assess genomic patterns of neutral variation, climate adaptation and climate vulnerability (offsets in predicted distributions of putatively adaptive variants across present and future landscapes) for sister foundation species, the marine tubeworms Galeolaria caespitosa and Galeolaria gemineoa, in a sentinel region for climate change impacts. We find that species are genetically isolated despite uncovering sympatry in their ranges, show parallel and nonparallel signals of thermal adaptation on spatial scales smaller than gene flow across their ranges, and are predicted to face different risks of maladaptation under future temperatures across their ranges. Our findings have implications for understanding local adaptation in the face of gene flow, and generate spatially explicit predictions for climatic disruption of adaptation and species distributions in coastal ecosystems that could guide experimental validation and conservation planning.

Within-population variation in body size plasticity in response to combined nutritional and thermal stress is partially independent from variation in development time

Ongoing climate change has forced animals to face changing thermal and nutritional environments. Animals can adjust to such combinations of stressors via plasticity. Body size is a key trait influencing organismal fitness, and plasticity in this trait in response to nutritional and thermal conditions varies among genetically diverse, locally adapted populations. The standing genetic variation within a population can also influence the extent of body size plasticity. We generated near-isogenic lines from a newly collected population of Drosophila melanogaster at the mid-point of east coast Australia and assayed body size for all lines in combinations of thermal and nutritional stress. We found that isogenic lines showed distinct underlying patterns of body size plasticity in response to temperature and nutrition that were often different from the overall population response. We then tested whether plasticity in development time could explain, and therefore regulate, variation in body size to these combinations of environmental conditions. We selected five genotypes that showed the greatest variation in response to combined thermal and nutritional stress and assessed the correlation between response of developmental time and body size. While we found significant genetic variation in development time plasticity, it was a poor predictor of body size among genotypes. Our results therefore suggest that multiple developmental pathways could generate genetic variation in body size plasticity. Our study emphasizes the need to better understand genetic variation in plasticity within a population, which will help determine the potential for populations to adapt to ongoing environmental change.

Thermal Performance Curves Are Shaped by Prior Thermal Environment in Early Life

Understanding links between thermal performance and environmental variation is necessary to predict organismal responses to climate change, and remains an ongoing challenge for ectotherms with complex life cycles. Distinct life stages can differ in thermal sensitivity, experience different environmental conditions as development unfolds, and, because stages are by nature interdependent, environmental effects can carry over from one stage to affect performance at others. Thermal performance may therefore respond to carryover effects of prior thermal environments, yet detailed insights into the nature, strength, and direction of those responses are still lacking. Here, in an aquatic ectotherm whose early planktonic stages (gametes, embryos, and larvae) govern adult abundances and dynamics, we explore the effects of prior thermal environments at fertilization and embryogenesis on thermal performance curves at the end of planktonic development. We factorially manipulate temperatures at fertilization and embryogenesis, then, for each combination of prior temperatures, measure thermal performance curves for survival of planktonic development (end of the larval stage) throughout the performance range. By combining generalized linear mixed modeling with parametric bootstrapping, we formally estimate and compare curve descriptors (thermal optima, limits, and breadth) among prior environments, and reveal carryover effects of temperature at embryogenesis, but not fertilization, on thermal optima at completion of development. Specifically, thermal optima shifted to track temperature during embryogenesis, while thermal limits and breadth remained unchanged. Our results argue that key aspects of thermal performance are shaped by prior thermal environment in early life, warranting further investigation of the possible mechanisms underpinning that response, and closer consideration of thermal carryover effects when predicting organismal responses to climate change.

Multilevel Selection on Offspring Size and the Maintenance of Variation

AbstractMultilevel selection on offspring size occurs when offspring fitness depends on both absolute size (hard selection) and size relative to neighbors (soft selection). We examined multilevel selection on egg size at two biological scales-within clutches and among clutches from different females-using an external fertilizing tube worm. We exposed clutches of eggs to two sperm environments (limiting and saturating) and measured their fertilization success. We then modeled environmental (sperm-dependent) differences in hard and soft selection on individual eggs as well as selection on clutch-level traits (means and variances). Hard and soft selection differed in strength and form depending on sperm availability-hard selection was consistently stabilizing; soft selection was directional and favored eggs relatively larger (sperm limitation) or smaller (sperm saturation) than the clutch mean. At the clutch level, selection on mean egg size was largely concave, while selection on within-clutch variance was weak but generally negative-although some correlational selection occurred between these two traits. Importantly, we found that the optimal clutch mean egg size differed for mothers and offspring, suggesting some antagonism between the levels of selection. We thus identify several pathways that may maintain offspring size variation: environmentally (sperm-) dependent soft selection, antagonistic multilevel selection, and correlational selection on clutch means and variances. Multilevel approaches are powerful but seldom-used tools for studies of offspring size, and we encourage their future use.

The thermal environment at fertilization mediates adaptive potential in the sea

Additive genetic variation for fitness at vulnerable life stages governs the adaptive potential of populations facing stressful conditions under climate change, and can depend on current conditions as well as those experienced by past stages or generations. For sexual populations, fertilization is the key stage that links one generation to the next, yet the effects of fertilization environment on the adaptive potential at the vulnerable stages that then unfold during development are rarely considered, despite climatic stress posing risks for gamete function and fertility in many taxa and external fertilizers especially. Here, we develop a simple fitness landscape model exploring the effects of environmental stress at fertilization and development on the adaptive potential in early life. We then test our model with a quantitative genetic breeding design exposing family groups of a marine external fertilizer, the tubeworm Galeolaria caespitosa, to a factorial manipulation of current and projected temperatures at fertilization and development. We find that adaptive potential in early life is substantially reduced, to the point of being no longer detectable, by genotype‐specific carryover effects of fertilization under projected warming. We interpret these results in light of our fitness landscape model, and argue that the thermal environment at fertilization deserves more attention than it currently receives when forecasting the adaptive potential of populations confronting climate change.

Thermal performance curves reveal shifts in optima, limits and breadth in early life

Understanding thermal performance at life stages that limit persistence is necessary to predict responses to climate change, especially for ectotherms whose fitness (survival and reproduction) depends on environmental temperature. Ectotherms often undergo stage-specific changes in size, complexity and duration that are predicted to modify thermal performance. Yet performance is mostly explored for adults, while performance at earlier stages that typically limit persistence remains poorly understood. Here, we experimentally isolate thermal performance curves at fertilization, embryo development and larval development stages in an aquatic ectotherm whose early planktonic stages (gametes, embryos and larvae) govern adult abundances and dynamics. Unlike previous studies based on short-term exposures, responses with unclear links to fitness or proxies in lieu of explicit curve descriptors (thermal optima, limits and breadth), we measured performance as successful completion of each stage after exposure throughout, and at temperatures that explicitly capture curve descriptors at all stages. Formal comparisons of descriptors using a combination of generalized linear mixed modelling and parametric bootstrapping reveal important differences among life stages. Thermal performance differs significantly from fertilization to embryo development (with thermal optimum declining by ∼2°C, thermal limits shifting inwards by ∼8-10°C and thermal breadth narrowing by ∼10°C), while performance declines independently of temperature thereafter. Our comparisons show that thermal performance at one life stage can misrepresent performance at others, and point to gains in complexity during embryogenesis, rather than subsequent gains in size or duration of exposure, as a key driver of thermal sensitivity in early life.

A note on simulating null distributions for G matrix comparisons

Genetic variances and covariances, summarized in G matrices, are key determinants of the course of adaptive evolution. Consequently, understanding how G matrices vary among populations is critical to answering a variety of questions in evolutionary biology. A method has recently been proposed for generating null distributions of statistics pertaining to differences in G matrices among populations. The general approach facilitated by this method is likely to prove to be very important in studies of the evolution of G. We have identified an issue in the method that will cause it to create null distributions of differences in G matrices that are likely to be far too narrow. The issue arises from the fact that the method as currently used generates null distributions of statistics pertaining to differences in G matrices across populations by simulating breeding value vectors based on G matrices estimated from data, randomizing these vectors across populations, and then calculating null values of statistics from G matrices that are calculated directly from the variances and covariances among randomized vectors. This calculation treats breeding values as quantities that are directly measurable, instead of predicted from G matrices that are themselves estimated from patterns of covariance among kin. The existing method thus neglects a major source of uncertainty in G matrices, which renders it anti-conservative. We first suggest a correction to the method. We then apply the original and modified methods to a very simple instructive scenario. Finally, we demonstrate the use of both methods in the analysis of a real data set.

Legs of male fiddler crabs evolved to compensate for claw exaggeration and enhance claw functionality during waving displays

Many exaggerated morphological traits evolve under sexual selection. However, the optimal level of exaggeration is dictated by a trade-off between natural and sexual selection, representing a balance between its benefits and associated costs. Male fiddler crabs wave an enlarged major claw during behavioural displays that eliminates the need for direct combat, and determines courtship outcomes. The outcomes of these displays often depend on claw size, exposing males to selection for larger claws to improve mating and combat success. Applying phylogenetic comparative methods to 27 fiddler crab species, we examined the evolution of major claw morphologies, leg morphologies, and waving displays to determine whether these traits coevolved to optimise functioning of the exaggerated claw, or to mitigate potential metabolic or locomotor costs. We found legs to be sexually dimorphic, with males having longer legs than females. Legs were also longer in species that waved laterally rather than vertically, in species with larger major claws, and in species whose major claws were relatively elongate. These results suggest that leg morphology has coevolved with claw enlargement to enhance functionality of the major claw during waving displays, in addition to compensating for any negative effects of claw size.

How does parental environment influence the potential for adaptation to global change?

Parental environments are regularly shown to alter the mean fitness of offspring, but their impacts on the genetic variation for fitness, which predicts adaptive capacity and is also measured on offspring, are unclear. Consequently, how parental environments mediate adaptation to environmental stressors, like those accompanying global change, is largely unknown. Here, using an ecologically important marine tubeworm in a quantitative-genetic breeding design, we tested how parental exposure to projected ocean warming alters the mean survival, and genetic variation for survival, of offspring during their most vulnerable life stage under current and projected temperatures. Offspring survival was higher when parent and offspring temperatures matched. Across offspring temperatures, parental exposure to warming altered the distribution of additive genetic variance for survival, making it covary across current and projected temperatures in a way that may aid adaptation to future warming. Parental exposure to warming also amplified nonadditive genetic variance for survival, suggesting that compatibilities between parental genomes may grow increasingly important under future warming. Our study shows that parental environments potentially have broader-ranging effects on adaptive capacity than currently appreciated, not only mitigating the negative impacts of global change but also reshaping the raw fuel for evolutionary responses to it.

Biochemical evolution in response to intensive harvesting in algae: Evolution of quality and quantity

Evolutionary responses to indirect selection pressures imposed by intensive harvesting are increasingly common. While artificial selection has shown that biochemical components can show rapid and dramatic evolution, it remains unclear as to whether intensive harvesting can inadvertently induce changes in the biochemistry of harvested populations. For applications such as algal culture, many of the desirable bioproducts could evolve in response to harvesting, reducing cost-effectiveness, but experimental tests are lacking. We used an experimental evolution approach where we imposed heavy and light harvesting regimes on multiple lines of an alga of commercial interest for twelve cycles of harvesting and then placed all lines in a common garden regime for four cycles. We have previously shown that lines in a heavy harvesting regime evolve a "live fast" phenotype with higher growth rates relative to light harvesting regimes. Here, we show that algal biochemistry also shows evolutionary responses, although they were temporarily masked by differences in density under the different harvesting regimes. Heavy harvesting regimes, relative to light harvesting regimes, had reduced productivity of desirable bioproducts, particularly fatty acids. We suggest that commercial operators wishing to maximize productivity of desirable bioproducts should maintain mother cultures, kept at higher densities (which tend to select for desirable phenotypes), and periodically restart their intensively harvested cultures to minimize the negative consequences of biochemical evolution. Our study shows that the burgeoning algal culture industry should pay careful attention to the role of evolution in intensively harvested crops as these effects are nontrivial if subtle.

Resources mediate selection on module longevity in the field

The life histories of modular organisms are complicated, where selection and optimization can occur at both organismal and modular levels. At a modular level, growth, reproduction and death can occur in one module, independently of others. Across modular groups, there are no formal investigations of selection on module longevity. We used two field experiments to test whether selection acts on module longevity in a sessile marine invertebrate and whether selection varies across successional gradients and resource regimes. We found that selection does act on module longevity and that the strength of selection varies with environmental conditions. In environments where interspecific competition is high, selection favours colonies with longer zooid (module) longevity for colonies that initially received high levels of maternal investment. In environments where food availability is high and flow rate is low, selection also favours colonies with longer zooid longevity. These patterns of selection provide partial support for module longevity theory developed for plants. Nevertheless, that selection on module longevity is so context-dependent suggests that variation in module longevity is likely to be maintained in this system.

Unravelling anisogamy: egg size and ejaculate size mediate selection on morphology in free-swimming sperm

Gamete dimorphism (anisogamy) defines the sexes in most multicellular organisms. Theoretical explanations for its maintenance usually emphasize the size-related selection pressures of sperm competition and zygote survival, assuming that fertilization of all eggs precludes selection for phenotypes that enhance fertility. In external fertilizers, however, fertilization is often incomplete due to sperm limitation, and the risk of polyspermy weakens the advantage of high sperm numbers that is predicted to limit sperm size, allowing alternative selection pressures to target free-swimming sperm. We asked whether egg size and ejaculate size mediate selection on the free-swimming sperm of Galeolaria caespitosa, a marine tubeworm with external fertilization, by comparing relationships between sperm morphology and male fertility across manipulations of egg size and sperm density. Our results suggest that selection pressures exerted by these factors may aid the maintenance of anisogamy in external fertilizers by limiting the adaptive value of larger sperm in the absence of competition. In doing so, our study offers a more complete explanation for the stability of anisogamy across the range of sperm environments typical of this mating system and identifies new potential for the sexes to coevolve via mutual selection pressures exerted by gametes at fertilization.

Should mothers provision their offspring equally? A manipulative field test

Within-brood variation in offspring size is universal, but its causes are unclear. Theoretical explanations for within-brood variation commonly invoke bet-hedging, although alternatives consider the role of sibling competition. Despite abundant theory, empirical manipulations of within-brood variation in offspring size are rare. Using a field experiment, we investigate the consequences of unequal maternal provisioning for both maternal and offspring fitness in a marine invertebrate. We create experimental broods of siblings with identical mean, but different variance, in offspring size, and different sibling densities. Overall, more-variable broods had higher mean performance than less-variable broods, suggesting benefits of unequal provisioning that arise independently of bet-hedging. Complementarity effects drove these benefits, apparently because offspring-size variation promotes resource partitioning. We suggest that when siblings compete for the same resources, and offspring size affects niche usage, the production of more-variable broods can provide greater fitness returns given the same maternal investment; a process unanticipated by the current theory.

Why do larger mothers produce larger offspring? A test of classic theory

Across a wide range of taxa, larger mothers produce larger offspring. Theory assumes that larger, more fecund mothers create higher local densities of siblings, and so larger mothers produce larger offspring to offset sibling competition. This assumption has been debated for over 30 yr, but direct empirical tests are surprisingly rare. Here, we test two key assumptions of classic theories that predict sibling competition drives maternal-size-offspring-size (MSOS) correlations: (1) independent effects of offspring size and sibling density on offspring performance or (2) as a product of an interaction between these two factors. To simultaneously test these alternative assumptions, we manipulate offspring size and sibling density in the marine invertebrate, Bugula neritina, and monitor offspring performance in the field. We found that, depending on the fitness metric being considered, offspring size and sibling density can either independently or interactively affect offspring performance. Yet sibling density did not affect offspring performance in the ways that classic theories assume. Given our results, it is unlikely that sibling competition drives the positive MSOS correlation observed in this species. Empirical support for these classic theories remains lacking, suggesting alternative explanations are necessary.

Relaxation of herbivore-mediated selection drives the evolution of genetic covariances between plant competitive and defense traits

Insect herbivores are important mediators of selection on traits that impact plant defense against herbivory and competitive ability. Although recent experiments demonstrate a central role for herbivory in driving rapid evolution of defense and competition-mediating traits, whether and how herbivory shapes heritable variation in these traits remains poorly understood. Here, we evaluate the structure and evolutionary stability of the G matrix for plant metabolites that are involved in defense and allelopathy in the tall goldenrod, Solidago altissima. We show that G has evolutionarily diverged between experimentally replicated populations that evolved in the presence versus the absence of ambient herbivory, providing direct evidence for the evolution of G by natural selection. Specifically, evolution in an herbivore-free habitat altered the orientation of G, revealing a negative genetic covariation between defense- and competition-related metabolites that is typically masked in herbivore-exposed populations. Our results may be explained by predictions of classical quantitative genetic theory, as well as the theory of acquisition-allocation trade-offs. The study provides compelling evidence that herbivory drives the evolution of plant genetic architecture.

Field manipulations of resources mediate the transition from intraspecific competition to facilitation

Population density affects individual performance, though its effects are often mixed. For sessile species, increases in population density typically reduce performance. Still, cases of positive density-dependence do occur in sessile systems and demand explanation. The stress gradient hypothesis (SGH) predicts that under stressful conditions, positive effects of facilitation may outweigh the negative effects of competition. While some elements of the SGH are well studied, its potential to explain intraspecific facilitation has received little attention. Further, there have been questions regarding whether the SGH holds if the stressor is a resource. Most studies of interactions between the environment and intraspecific facilitation have relied on natural environmental gradients; manipulative studies are much rarer. To test the effects of intraspecific density and resources, we manipulated resource availability over natural population densities for the marine bryozoan Watersipora subtorquata. We found negative effects of density on colony performance in low resource environments, but mainly positive density-dependence in high resource environments. By adding resources, competition effects were reduced and the positive effects of facilitation were revealed. Our results suggest that resource availability mediates the relative strength of competition and facilitation in our system. We also suggest that intraspecific facilitation is more common than may be appreciated and that environmental variation may mediate the balance between negative and positive density-dependence.

The other 96%: Can neglected sources of fitness variation offer new insights into adaptation to global change?

Mounting research considers whether populations may adapt to global change based on additive genetic variance in fitness. Yet selection acts on phenotypes, not additive genetic variance alone, meaning that persistence and evolutionary potential in the near term, at least, may be influenced by other sources of fitness variation, including nonadditive genetic and maternal environmental effects. The fitness consequences of these effects, and their environmental sensitivity, are largely unknown. Here, applying a quantitative genetic breeding design to an ecologically important marine tubeworm, we examined nonadditive genetic and maternal environmental effects on fitness (larval survival) across three thermal environments. We found that these effects are nontrivial and environment dependent, explaining at least 44% of all parentally derived effects on survival at any temperature and 96% of parental effects at the most stressful temperature. Unlike maternal environmental effects, which manifested at the latter temperature only, nonadditive genetic effects were consistently significant and covaried positively across temperatures (i.e., parental combinations that enhanced survival at one temperature also enhanced survival at elevated temperatures). Thus, while nonadditive genetic and maternal environmental effects have long been neglected because their evolutionary consequences are complex, unpredictable, or seen as transient, we argue that they warrant further attention in a rapidly warming world.

Environment-dependent variation in selection on life history across small spatial scales

Variation in life-history traits is ubiquitous, even though genetic variation is thought to be depleted by selection. One potential mechanism for the maintenance of trait variation is spatially variable selection. We explored spatial variation in selection in the field for a colonial marine invertebrate that shows phenotypic differences across a depth gradient of only 3 m. Our analysis included life-history traits relating to module size, colony growth, and phenology. Directional selection on colony growth varied in strength across depths, while module size was under directional selection at one depth but not the other. Differences in selection may explain some of the observed phenotypic differentiation among depths for one trait but not another: instead, selection should actually erode the differences observed for this trait. Our results suggest selection is not acting alone to maintain trait variation within and across environments in this system.

Revealing hidden evolutionary capacity to cope with global change

The extent to which global change will impact the long-term persistence of species depends on their evolutionary potential to adapt to future conditions. While the number of studies that estimate the standing levels of adaptive genetic variation in populations under predicted global change scenarios is growing all the time, few studies have considered multiple environments simultaneously and even fewer have considered evolutionary potential in multivariate context. Because conditions will not be constant, adaptation to climate change is fundamentally a multivariate process so viewing genetic variances and covariances over multivariate space will always be more informative than relying on bivariate genetic correlations between traits. A multivariate approach to understanding the evolutionary capacity to cope with global change is necessary to avoid misestimating adaptive genetic variation in the dimensions in which selection will act. We assessed the evolutionary capacity of the larval stage of the marine polychaete Galeolaria caespitosa to adapt to warmer water temperatures. Galeolaria is an important habitat-forming species in Australia, and its earlier life-history stages tend to be more susceptible to stress. We used a powerful quantitative genetics design that assessed the impacts of three temperatures on subsequent survival across over 30 000 embryos across 204 unique families. We found adaptive genetic variation in the two cooler temperatures in our study, but none in the warmest temperature. Based on these results, we would have concluded that this species has very little capacity to evolve to the warmest temperature. However, when we explored genetic variation in multivariate space, we found evidence that larval survival has the potential to evolve even in the warmest temperatures via correlated responses to selection across thermal environments. Future studies should take a multivariate approach to estimating evolutionary capacity to cope with global change lest they misestimate a species' true adaptive potential.

Two sexes, one body: intra- and intersex covariation of gamete phenotypes in simultaneous hermaphrodites

By harboring male and female functions in the same genome and expressing them in every individual, simultaneous hermaphrodites may incur sexual conflict unless both sex functions can evolve phenotypic optima independently of each other. The first step toward understanding their capacity to do so lies in understanding whether sex functions are phenotypically correlated within individuals, but remarkably few data address this issue. We tested the potential for intra- and intersex covariation of gamete phenotypes to mediate sexual conflict in broadcast-spawning hermaphrodites (the ascidians Ciona intestinalis and Pyura praeputialis), for which sex-specific selection acts predominantly on sperm–egg interactions in the water column. In both species, gamete phenotypes covaried within and across sex functions, implying that selection may be unable to target them independently because its direct effects on male gametes translate into correlated effects on female gametes and vice versa. This alone does not preclude the evolution of a different phenotypic optimum for each sex function, but imposes the more restrictive requirement that selection – which ultimately sorts among whole individuals, not sex functions – aligns with the direction in which gamete phenotypes covary at this level.

Evolutionary constraints and the maintenance of individual specialization throughout succession

Constraints on life-history traits, with their close links to fitness, are widely invoked as limits to niche expansion at most organizational levels. Theoretically, such constraints can maintain individual specialization by preventing adaptation to all niches available, but empirical evidence of them remains elusive for natural populations. This problem may be compounded by a tendency to seek constraints involving multiple traits, neglecting their added potential to manifest in trait expression across environments (i.e., within reaction norms). By replicating genotypes of a colonial marine invertebrate across successional stages in its local community, and taking a holistic approach to the analysis of ensuing reaction norms for fitness, we show the potential for individual specialization to be maintained by genetic constraints associated with these norms, which limit the potential for fitness at one successional stage to improve without loss of fitness at others. Our study provides new insight into the evolutionary maintenance of individual specialization in natural populations and reinforces the importance of reaction norms for studying this phenomenon.

Predicting evolutionary responses to climate change in the sea

An increasing number of short-term experimental studies show significant effects of projected ocean warming and ocean acidification on the performance on marine organisms. Yet, it remains unclear if we can reliably predict the impact of climate change on marine populations and ecosystems, because we lack sufficient understanding of the capacity for marine organisms to adapt to rapid climate change. In this review, we emphasise why an evolutionary perspective is crucial to understanding climate change impacts in the sea and examine the approaches that may be useful for addressing this challenge. We first consider what the geological record and present-day analogues of future climate conditions can tell us about the potential for adaptation to climate change. We also examine evidence that phenotypic plasticity may assist marine species to persist in a rapidly changing climate. We then outline the various experimental approaches that can be used to estimate evolutionary potential, focusing on molecular tools, quantitative genetics, and experimental evolution, and we describe the benefits of combining different approaches to gain a deeper understanding of evolutionary potential. Our goal is to provide a platform for future research addressing the evolutionary potential for marine organisms to cope with climate change.

The adaptive value of functional and life-history traits across fertility treatments in an annual plant

BACKGROUND AND AIMS

Plant functional traits are assumed to be adaptive. As selection acts on individuals and not on traits, interpreting the adaptive value of a trait not may be straightforward. For example, productive leaves are associated with fertile environments. However, it is not clear if productive leaves confer an advantage in these habitats, or if they are an advantage as part of a suite of coordinated traits.

METHODS

Genotypes of Arabidopsis thaliana were grown in high and low nutrient treatments and low, neutral and high pH treatments. Nutrient availability is reduced in acidic or basic soils relative to neutral pH soils. pH treatments were used to alter the availability of resources rather than the amount of resources.

KEY RESULTS

Leaf function (specific leaf area, SLA) and life history (size at reproduction, age at reproduction) were variable across genotypes and were plastic. High nutrient availability induced higher SLA and larger size at reproduction. Genotypes that reproduced at large size in high nutrient conditions at neutral pH had the greatest fruit production. SLA was only indirectly related to fruit production through a causal relationship with rosette size; in high nutrient conditions, plants with high SLA were large at reproduction and had higher fruit production. In high nutrient and high pH treatments, plants were large at reproduction, but large size at reproduction was associated with low fecundity. This suggests that large size is adaptive under high nutrient availability.

CONCLUSIONS

Interpreting the adaptive value of functional traits will sometimes only be possible when these traits are considered as a suite of correlated and coordinated traits. Leaf functional traits may be important in defining adaptive strategies in A. thaliana but only through how they affect plant life history. Finally, manipulating soil pH can be a valuable tool in assessing adaptive plasticity on nutrient gradients.

The potential for evolutionary responses to cell-lineage selection on growth form and its plasticity in a red seaweed

Despite much theoretical discussion on the evolutionary significance of intraclonal genetic variation, particularly for modular organisms whose lack of germ-soma segregation allows for variants arising in clonal growth to contribute to evolutionary change, the potential of this variation to fuel adaptation remains surprisingly untested. Given intraclonal variation, mitotic cell lineages, rather than sexual offspring, may frequently act as units of selection. Here, we applied artificial selection to such lineages in the branching red seaweed Asparagopsis armata, targeting aspects of clonal growth form and growth-form plasticity that enhance light acquisition on patchy subtidal reefs and predicting that a genetic basis to intraclonal variation may promote significant responses that cannot accompany phenotypic variation alone. Cell-lineage selection increased variation in branch proliferation among A. armata genets and successfully altered its plasticity to light. Correlated responses in the plasticity of branch elongation, moreover, showed that cell-lineage selection may be transmitted among the plasticities of growth-form traits in A. armata via pleiotropy. By demonstrating significant responses to cell-lineage selection on growth-form plasticity in this seaweed, our study lends support to the notion that intraclonal genetic variation may potentially help clonal organisms to evolve adaptively in the absence of sex and thereby prove surprisingly resilient to environmental change.

Phenotype-environment mismatches reduce connectivity in the sea

The connectivity of marine populations is often surprisingly lower than predicted by the dispersal capabilities of propagules alone. Estimates of connectivity, moreover, do not always scale with distance and are sometimes counterintuitive. Population connectivity requires more than just the simple exchange of settlers among populations: it also requires the successful establishment and reproduction of exogenous colonizers. Marine organisms often disperse over large spatial scales, encountering very different environments and suffering extremely high levels of post-colonization mortality. Given the growing evidence that such selection pressures often vary over spatial scales that are much smaller than those of dispersal, we argue that selection will bias survival against exogenous colonizers. We call this selection against exogenous colonizers a phenotype-environment mismatch and argue that phenotype-environment mismatches represent an important barrier to connectivity in the sea. Crucially, these mismatches may operate independently of distance and thereby have the potential to explain the counterintuitive patterns of connectivity often seen in marine environments. We discuss how such mismatches might alter our understanding and management of marine populations.

Light quantity and quality induce shade-avoiding plasticity in a marine macroalga

Light-induced plasticity in plant morphology is considered adaptive in terrestrial habitats that vary in light, but remains unexplored for marine habitats. This is despite similar modes of growth, development and photosynthetic equipment in terrestrial and marine photoautotrophs and similarly dynamic light environments. We tested whether manipulations of light quantity and quality induce morphological plasticity in the marine macroalga, Asparagopsis armata. Using multivariate analyses (principal components analyses and multivariate analyses of covariance), we show that correlated morphological traits underlie a fundamental growth strategy characterized by the production of phalanx and guerrilla phenotypes in environments that mimic light and shade respectively. This foraging response is not under simple genetic or environmental control, but influenced by interactions between genotype and environment. Evidence of plasticity and genetic variation in plasticity in a marine modular organism generates additional, testable hypotheses on the ecological consequences of variation in growth form that may further explain the evolution of plasticity.

Selection in Modular Organisms: Is Intraclonal Variation in Macroalgae Evolutionarily Important?

Theoretical and practical difficulties occur when defining the units of selection in modular organisms that grow by iteration of repeated parts (modules). Modules may become physically autonomous through fragmentation and may vary because of genetic variation arising in somatic cell lineages. Since cells destined for gamete production are not sequestered in early development, heritable variation and selection among asexual progeny are possible. We used the branching red macroalgae Delisea pulchra and Asparagopsis armata to test whether modules fulfill three fundamental criteria for units of selection: that they replicate, that they display heritable variation, and that selective agents distinguish among the variants. We detected significant phenotypic variation among modules for fitness-related traits (growth, secondary metabolite concentrations, and rates of tissue loss to herbivory) in each species and significant heritability estimates for secondary metabolite production and tissue loss to herbivory in D. pulchra. Variation in growth rate among A. armata modules was largely phenotypic with small but important estimates of genetic variation. Our results indicate that selection may indeed act on phenotypic variation among modules within individuals and that this process may effect evolutionary change within asexual lineages given sufficient genetic variation in the traits examined.