Larval plastic responses to warming and desiccation delay gonadal maturation in postmetamorphic spadefoot toads

Developmental plasticity allows organisms to adjust life-history traits to varying environmental conditions, which can have concomitant effects across life stages. Many amphibians are suitable model systems to study plasticity because their larvae can adjust growth and differentiation under fluctuating environments. It is unknown, however, whether somatic and gonadal differentiation are equally affected by environmentally induced plasticity or whether their decoupling alters gonadal maturation postmetamorphosis, which may affect fitness. We tested if developmental acceleration in response to warming and desiccation risk results in shifts in gonadal maturation during metamorphosis and postmetamorphic growth in western spadefoot toads (Pelobates cultripes). We found additive effects of increased temperature and desiccation risk on development and growth at metamorphosis, which largely constrained gonadal maturation in metamorphic and postmetamorphic individuals of both sexes. Furthermore, the conditions experienced by larvae incurred sex-specific carryover effects on the gonadal maturation of juveniles 5 months after metamorphosis. In females, high temperature delayed ovarian maturation regardless of the water level. In males, exposure to high temperature and high water levels slightly delayed the testes' maturation. These results highlight the relevance of larval plasticity in the gonadal maturation of species undergoing metamorphosis, which may have implications for population demographics and the evolution of life histories.

Reproductive biology of Gazella arabica: Predictors of offspring weight and short- and long-term offspring survival

Reproductive traits are central to organismal fitness, and so the factors influencing patterns of reproduction and offspring survival are at the heart of biology. Making use of breeding data collected over 16 years at the King Khalid Wildlife Research Centre in Saudi Arabia, we investigated the reproductive biology of Arabian gazelles Gazella arabica. Offspring survival was mainly a function of birth weight, with heavier offspring having higher survival rates than lighter offspring. However, while sons were heavier than daughters, daughters had higher survival rates. We could not find evidence that giving birth to sons negatively impacts offspring weight in the following year. We uncovered large narrow-sense heritability (h2) in offspring weight at birth, while maternal effects (m2) on birth weight were of lesser importance. However, maternal effects on offspring survival were strong until weaning age, while paternal effects dominated survival to sexual maturity and first reproduction. We propose that variation in maternal postnatal care might overshadow the effects of maternal inheritance of birth weights, while the overall strong heritability of weight at birth and the paternal effects on survival illustrates strong variance in sire fitness based on genetic quality, suggesting a role for sexual selection by female mate choice in wild populations.

The adaptive significance of large size at birth in marine snakes

Evolutionary shifts from one habitat type to another can clarify selective forces that affect life-history attributes. Four lineages of snakes (acrochordids and three clades within the Elapidae) have invaded marine habitats, and all have larger offspring than do terrestrial snakes. Predation by fishes on small neonates offers a plausible selective mechanism for that shift, because ascending to breathe at the ocean surface exposes a marine snake to midwater predation whereas juvenile snakes in terrestrial habitats can remain hidden. Consistent with this hypothesis, snake-shaped models moving through a coral-reef habitat in New Caledonia attracted high rates of attack by predatory fishes, and small models (the size of neonatal terrestrial snakes) were attacked more frequently than were large models (the size of neonatal sea snakes). Vulnerability to predatory fishes may have imposed strong selection for increased offspring size in marine snakes.

Phenotypic evolution of SARS-CoV-2: a statistical inference approach

Since its emergence in late 2019, the SARS-CoV-2 virus has spread globally, causing the ongoing COVID-19 pandemic. In the fall of 2020, the Alpha variant (lineage B.1.1.7) was detected in England and spread rapidly, outcompeting the previous lineage. Yet, very little is known about the underlying modifications of the infection process that can explain this selective advantage. Here, we try to quantify how the Alpha variant differed from its predecessor on two phenotypic traits: The transmission rate and the duration of infectiousness. To this end, we analyzed the joint epidemiological and evolutionary dynamics as a function of the Stringency Index, a measure of the amount of Non-Pharmaceutical Interventions. Assuming that these control measures reduce contact rates and transmission, we developed a two-step approach based on ${{SEIR}}$ models and the analysis of a combination of epidemiological and evolutionary information. First, we quantify the link between the Stringency Index and the reduction in viral transmission. Second, based on a novel theoretical derivation of the selection gradient in an ${{SEIR}}$ model, we infer the phenotype of the Alpha variant from its frequency changes. We show that its selective advantage is more likely to result from a higher transmission than from a longer infectious period. Our work illustrates how the analysis of the joint epidemiological and evolutionary dynamics of infectious diseases can help understand the phenotypic evolution driving pathogen adaptation.

Ecological and life-history drivers of avian skull evolution

One of the most famous examples of adaptive radiation is that of the Galápagos finches, where skull morphology, particularly the beak, varies with feeding ecology. Yet increasingly studies are questioning the strength of this correlation between feeding ecology and morphology in relation to the entire neornithine radiation, suggesting that other factors also significantly affect skull evolution. Here, we broaden this debate to assess the influence of a range of ecological and life-history factors, specifically habitat density, migration, and developmental mode, in shaping avian skull evolution. Using 3D geometric morphometric data to robustly quantify skull shape for 354 extant species spanning avian diversity, we fitted flexible phylogenetic regressions and estimated evolutionary rates for each of these factors across the full data set. The results support a highly significant relationship between skull shape and both habitat density and migration, but not developmental mode. We further found heterogenous rates of evolution between different character states within habitat density, migration, and developmental mode, with rapid skull evolution in species that occupy dense habitats, are migratory, or are precocial. These patterns demonstrate that diverse factors affect the tempo and mode of avian phenotypic evolution and that skull evolution in birds is not simply a reflection of feeding ecology.

Temporal dynamics of mother-offspring relationships in Bigg's killer whales: opportunities for kin-directed help by post-reproductive females

Age-related changes in the patterns of local relatedness (kinship dynamics) can be a significant selective force shaping the evolution of life history and social behaviour. In humans and some species of toothed whales, average female relatedness increases with age, which can select for a prolonged post-reproductive lifespan in older females due to both costs of reproductive conflict and benefits of late-life helping of kin. Killer whales (Orcinus orca) provide a valuable system for exploring social dynamics related to such costs and benefits in a mammal with an extended post-reproductive female lifespan. We use more than 40 years of demographic and association data on the mammal-eating Bigg's killer whale to quantify how mother-offspring social relationships change with offspring age and identify opportunities for late-life helping and the potential for an intergenerational reproductive conflict. Our results suggest a high degree of male philopatry and female-biased budding dispersal in Bigg's killer whales, with some variability in the dispersal rate for both sexes. These patterns of dispersal provide opportunities for late-life helping particularly between mothers and their adult sons, while partly mitigating the costs of mother-daughter reproductive conflict. Our results provide an important step towards understanding why and how menopause has evolved in Bigg's killer whales.

Seed size variation impacts local adaptation and life-history strategies in a perennial grass

Seed mass is an ecologically important trait that often differs considerably among ecotypes. Yet, because few studies examine the impacts of seed mass on adult life-history traits, its role in local adaptation is unclear. In this study, using accessions of Panicum hallii that spanned the two major ecotypes, we examined whether covariation between seed mass, seedling and reproductive traits impacts ecotypic divergence and local adaptation. The perennial grass P. hallii has two distinct ecotypes-a large-seeded upland ecotype adapted to xeric environments and a small-seeded lowland ecotype adapted to mesic environments. In the greenhouse, seed mass varied greatly across P. hallii genotypes in a manner consistent with ecotypic divergence. Seed mass covaried significantly with several seedling and reproductive traits. At field sites representing the habitats of the two ecotypes, seed mass had different impacts on seedling and adult recruitment: selection favoured large seeds in upland habitat and small seeds in lowland habitat, which was consistent with local adaptation. By demonstrating the central role of seed mass in ecotypic differences in P. hallii and its importance to seedling and adult recruitment under field conditions, these studies show that early life-history traits can promote local adaptation and potentially explain ecotype formation.

Internal cues for optimizing reproduction in a varying environment

In varying environments, it is beneficial for organisms to utilize available cues to infer the conditions they may encounter and express potentially favourable traits. However, external cues can be unreliable or too costly to use. We consider an alternative strategy where organisms exploit internal sources of information. Even without sensing environmental cues, their internal states may become correlated with the environment as a result of selection, which then form a memory that helps predict future conditions. To demonstrate the adaptive value of such internal cues in varying environments, we revisit the classic example of seed dormancy in annual plants. Previous studies have considered the germination fraction of seeds and its dependence on environmental cues. In contrast, we consider a model of germination fraction that depends on the seed age, which is an internal state that can serve as a memory. We show that, if the environmental variation has temporal structure, then age-dependent germination fractions will allow the population to have an increased long-term growth rate. The more the organisms can remember through their internal states, the higher the growth rate a population can potentially achieve. Our results suggest experimental ways to infer internal memory and its benefit for adaptation in varying environments.

Adaption, neutrality and life-course diversity

Heterogeneity among individuals in fitness components is what selection acts upon. Evolutionary theories predict that selection in constant environments acts against such heterogeneity. But observations reveal substantial non-genetic and also non-environmental variability in phenotypes. Here, we examine whether there is a relationship between selection pressure and phenotypic variability by analysing structured population models based on data from a large and diverse set of species. Our findings suggest that non-genetic, non-environmental variation is in general neither truly neutral, selected for, nor selected against. We find much variations among species and populations within species, with mean patterns suggesting nearly neutral evolution of life-course variability. Populations that show greater diversity of life courses do not show, in general, increased or decreased population growth rates. Our analysis suggests we are only at the beginning of understanding the evolution and maintenance of non-genetic non-environmental variation.

Patterns, sources, and consequences of variation in age-specific vital rates: Insights from a long-term study of Weddell seals

Variations in the reproductive and survival abilities of individuals within a population are ubiquitous in nature, key to individual fitness, and affect population dynamics, which leads to strong interest in understanding causes and consequences of vital-rate variation. For long-lived species, long-term studies of large samples of known-age individuals are ideal for evaluating vital-rate variation. A population of Weddell seals in Erebus Bay, Antarctica, has been studied each Austral spring since the 1960s. Since 1982, all newborns have been tagged each year and multiple capture-mark-recapture (CMR) surveys have been conducted annually. Over the past 20 years, a series of analyses have built on results of earlier research by taking advantage of steady improvements in the project's long-term CMR data and available analytical methods. Here, I summarize progress made on four major topics related to variation in age-specific vital rates for females: early-life survival and age at first reproduction, costs of reproduction, demographic buffering, and demographic senescence. Multistate modelling found that age at first reproduction varies widely (4-14 years of age) and identified contrasting influences of maternal age on survival and recruitment rates of offspring. Subsequent analyses of data for females after recruitment revealed costs of reproduction to both survival and future reproduction and provided strong evidence of demographic buffering. Recent results indicated that important levels of among-individual variation exist in vital rates and revealed contrasting patterns for senescence in reproduction and survival. Sources of variation in vital rates include age, reproductive state, year, and individual. The combination of luck and individual quality results in strong variation in individual fitness outcomes: ~80% of females born in the population produce no offspring, and the remaining 20% vary strongly in lifetime reproductive output (range: 1-23 pups). Further research is needed to identify the specific environmental conditions that lead to annual variation in vital rates and to better understand the origins of individual heterogeneity. Work is also needed to better quantify the relative roles of luck, maternal effects, and environmental conditions on variation in vital rates and to learn the importance of such variation to demographic performance of offspring and on overall population dynamics.

Experimentally elevated testosterone shortens telomeres across years in a free-living songbird

Reproductive investment often comes at a cost to longevity, but the mechanisms that underlie these long-term effects are not well understood. In male vertebrates, elevated testosterone has been shown to increase reproductive success, but simultaneously to decrease survival. One factor that may contribute to or serve as a biomarker of these long-term effects of testosterone on longevity is telomeres, which are often positively related to lifespan and have been shown to shorten in response to reproduction. In this longitudinal study, we measured the effects of experimentally elevated testosterone on telomere shortening in free-living, male dark-eyed juncos (Junco hyemalis carolinensis), a system in which the experimental elevation of testosterone has previously been shown to increase reproductive success and reduce survival. We found a small, significant effect of testosterone treatment on telomeres, with testosterone-treated males exhibiting significantly greater telomere shortening with age than controls. These results are consistent with the hypothesis that increased telomere shortening may be a long-term cost of elevated testosterone exposure. As both testosterone and telomeres are conserved physiological mechanisms, our results suggest that their interaction may apply broadly to the long-term costs of reproduction in male vertebrates.

Multigenerational downregulation of insulin/IGF-1 signaling in adulthood improves lineage survival, reproduction, and fitness in Caenorhabditis elegans supporting the developmental theory of ageing

Adulthood-only downregulation of insulin/IGF-1 signaling (IIS), an evolutionarily conserved pathway regulating resource allocation between somatic maintenance and reproduction, increases life span without fecundity cost in the nematode, Caenorhabditis elegans. However, long-term multigenerational effects of reduced IIS remain unexplored and are proposed to carry costs for offspring quality. To test this hypothesis, we ran a mutation accumulation (MA) experiment and downregulated IIS in half of the 400 MA lines by silencing daf-2 gene expression using RNA interference (RNAi) across 40 generations. Contrary to the prediction, adulthood-only daf-2 RNAi reduced extinction of MA lines both under UV-induced and spontaneous MA. Fitness of the surviving UV-induced MA lines was higher under daf-2 RNAi. Reduced IIS increased intergenerational F1 offspring fitness under UV stress but had no quantifiable transgenerational effects. Functional hrde-1 was required for the benefits of multigenerational daf-2 RNAi. Overall, we found net benefit to fitness from multigenerational reduction of IIS and the benefits became more apparent under stress. Because reduced daf-2 expression during development carries fitness costs, we suggest that our findings are best explained by the developmental theory of ageing, which maintains that the decline in the force of selection with age results in poorly regulated gene expression in adulthood.

Alternative developmental and transcriptomic responses to host plant water limitation in a butterfly metapopulation

Predicting how climate change affects biotic interactions poses a challenge. Plant-insect herbivore interactions are particularly sensitive to climate change, as climate-induced changes in plant quality cascade into the performance of insect herbivores. Whereas the immediate survival of herbivore individuals depends on plastic responses to climate change-induced nutritional stress, long-term population persistence via evolutionary adaptation requires genetic variation for these responses. To assess the prospects for population persistence under climate change, it is therefore crucial to characterize response mechanisms to climate change-induced stressors, and quantify their variability in natural populations. Here, we test developmental and transcriptomic responses to water limitation-induced host plant quality change in a Glanville fritillary butterfly (Melitaea cinxia) metapopulation. We combine nuclear magnetic resonance spectroscopy on the plant metabolome, larval developmental assays and an RNA sequencing analysis of the larval transcriptome. We observed that responses to feeding on water-limited plants, in which amino acids and aromatic compounds are enriched, showed marked variation within the metapopulation, with individuals of some families performing better on control and others on water-limited plants. The transcriptomic responses were concordant with the developmental responses: families exhibiting opposite developmental responses also produced opposite transcriptomic responses (e.g. in growth-associated transcripts). The divergent responses in both larval development and transcriptome are associated with differences between families in amino acid catabolism and storage protein production. The results reveal intrapopulation variability in plasticity, suggesting that the Finnish M. cinxia metapopulation harbours potential for buffering against drought-induced changes in host plant quality.

Latitudinal but not elevational variation in blood glucose level is linked to life history across passerine birds

Macrophysiological research is vital to our understanding of mechanisms underpinning global life history variation and adaptation to diverse environments. Here, we examined latitudinal and elevational variation in a key substrate of energy metabolism and an emerging physiological component of pace-of-life syndromes, blood glucose concentration. Our data, collected from 61 European temperate and 99 Afrotropical passerine species, revealed that baseline blood glucose increases with both latitude and elevation, whereas blood glucose stress response shows divergent directions, being stronger at low latitudes and high elevations. Low baseline glucose in tropical birds, compared to their temperate counterparts, was mainly explained by their low fecundity, consistent with the slow pace-of-life syndrome in the tropics. In contrast, elevational variation in this trait was decoupled from fecundity, implying a unique montane pace-of-life syndrome combining slow-paced life histories with fast-paced physiology. The observed patterns suggest that pace-of-life syndromes do not evolve along the single fast-slow axis.

Trade-offs in defence to pathogen species revealed in expanding nematode populations

Many host organisms live in polymicrobial environments and must respond to a diversity of pathogens. The degree to which host defences towards one pathogen species affect susceptibility to others is unclear. We used a panel of Caenorhabditis elegans nematode isolates to test for natural genetic variation in fitness costs of immune upregulation and pathogen damage, as well as for trade-offs in defence against two pathogen species, Staphylococcus aureus and Pseudomonas aeruginosa. We examined the fitness impacts of transient pathogen exposure (pathogen damage and immune upregulation) or exposure to heat-killed culture (immune upregulation only) by measuring host population sizes, which allowed us to simultaneously capture changes in reproductive output, developmental time and survival. We found significant decreases in population sizes for hosts exposed to live versus heat-killed S. aureus and found increased reproductive output after live P. aeruginosa exposure, compared with the corresponding heat-killed challenge. Nematode isolates with relatively higher population sizes after live P. aeruginosa infection produced fewer offspring after live S. aureus challenge. These findings reveal that wild C. elegans genotypes display a trade-off in defences against two distinct pathogen species that are evident in subsequent generations.

Experimental sexual selection affects the evolution of physiological and life-history traits

Sexual selection and sexual conflict are expected to affect all aspects of the phenotype, not only traits that are directly involved in reproduction. Here, we show coordinated evolution of multiple physiological and life-history traits in response to long-term experimental manipulation of the mating system in populations of Drosophila pseudoobscura. Development time was extended under polyandry relative to monogamy in both sexes, potentially due to higher investment in traits linked to sexual selection and sexual conflict. Individuals (especially males) evolving under polyandry had higher metabolic rates and locomotor activity than those evolving under monogamy. Polyandry individuals also invested more in metabolites associated with increased endurance capacity and efficient energy metabolism and regulation, namely lipids and glycogen. Finally, polyandry males were less desiccation- and starvation resistant than monogamy males, suggesting trade-offs between resistance and sexually selected traits. Our results provide experimental evidence that mating systems can impose selection that influences the evolution of non-sexual phenotypes such as development, activity, metabolism and nutrient homeostasis.

From microbes to mammals: The experimental evolution of aging and longevity across species

Senescence, the functional deterioration of cells or organisms associated with increased age, is pervasive across the tree of life. Yet our understanding of the genetic and physiological basis underlying age-related declines in health and reproduction remains limited. Experimental evolution allows empirical examination of the question of why aging occurs; imposing selection for age-specific fitness traits shifts patterns of aging in experimental populations, enabling investigations of the variation underlying senescence and the mechanisms governing it. Whole-genome sequencing of experimentally evolved populations may reveal candidate genomic variants underlying particular aging patterns; unfortunately, most study systems suffer from limitations that weaken associations between genotypes and phenotypes. In this review, we provide a survey of experimental evolution studies that have altered population-level patterns of reproductive timing and senescence in a variety of species. We discuss the specific selection conditions that have increased longevity, the phenotypic responses and trade-offs that accompany these increases, and examine genomic data collected from these experiments. Additionally, we consider how selected field studies complement laboratory experiments on life-history evolution. Finally, we address the strengths and weaknesses of existing study systems, and evaluate which model organisms appear most promising for future genomic investigations of the evolutionary biology of aging.

Environmental patterns of adaptation after range expansion in Leontodon longirostris: The effect of phenological events on fitness-related traits

PREMISE

Because of expected range shifts associated with climate change, there is a renewed interest in the evolutionary factors constraining adaptation, among which are genetic bottlenecks, drift, and increased mutational load after range expansion. Here we study adaptation in the short-lived species Leontodon longirostris showing reduced genetic diversity and increased genetic load along an expansion route.

METHODS

We assessed the phenological patterns of variation, and their effect on fitness-related traits, on 42 L. longirostris populations and six populations of the sister taxa L. saxatilis in a common garden located within the current range of both species. The comparison among L. longirostris populations allowed us to test for genetic clines consistent with local adaptation, whereas the comparison between taxa provided evidence for common adaptive features at the species level.

RESULTS

We found significant within-species variability for most traits, as well as differences with its close relative L. saxatilis. In general, seeds from drier, warmer, and unpredictable habitats showed overall lower and more restricted conditions for germination, seedlings emerged later and plants flowered earlier. Consequently, genotypes from arid and unpredictable environments attained smaller reproductive sizes and allocated more biomass to reproduction. Flowering time had the strongest direct effect on total plant size, but seedling emergence also showed an important indirect effect.

CONCLUSIONS

Our results show the crucial role of phenological patterns in shaping adaptive clines for major life-history stage transitions. Furthermore, the genetic load observed in L. longirostris does not seem to preclude adaptation to the climatic variability encountered along the expansion route.

The Importance of Noise Colour in Simulations of Evolutionary Systems

Simulations of evolutionary dynamics often employ white noise as a model of stochastic environmental variation. Whilst white noise has the advantages of being simply generated and analytically tractable, empirical analyses demonstrate that most real environmental time series have power spectral densities consistent with pink or red noise, in which lower frequencies contribute proportionally greater amplitudes than higher frequencies. Simulated white noise environments may therefore fail to capture key components of real environmental time series, leading to erroneous results. To explore the effects of different noise colours on evolving populations, a simple evolutionary model of the interaction between life-history and the specialism-generalism axis was developed. Simulations were conducted using a range of noise colours as the environments to which agents adapted. Results demonstrate complex interactions between noise colour, reproductive rate, and the degree of evolved generalism; importantly, contradictory conclusions arise from simulations using white as opposed to red noise, suggesting that noise colour plays a fundamental role in generating adaptive responses. These results are discussed in the context of previous research on evolutionary responses to fluctuating environments, and it is suggested that Artificial Life as a field should embrace a wider spectrum of coloured noise models to ensure that results are truly representative of environmental and evolutionary dynamics.

Dental cementum virtual histology of Neanderthal teeth from Krapina (Croatia, 130-120 kyr): an informed estimate of age, sex and adult stressors

The evolution of modern human reproductive scheduling is an aspect of our life history that remains vastly uncomprehended. The present work aims to address this gap by validating a non-destructive cutting-edge methodology to infer adult life-history events on modern teeth with known life history and then applying it to fossil specimens. We use phase-contrast synchrotron X-ray microtomography to visualize the dental cementum of 21 specimens: nine contemporary humans; 10 Neanderthals from Krapina (Croatia, 130-120 kyr); one Neolithic Homo sapiens from Ajmana (Serbia); and one Mesolithic H. sapiens from Vlasac (Serbia). We were able to correctly detect and time (root mean square error = 2.1 years; R2 = 0.98) all reproductive (menarche, parturition, menopause) and other physiologically impactful events in the modern sample. Nonetheless, we could not distinguish between the causes of the events detected. For the fossil specimens, we estimated age at death and age at occurrence of biologically significant events. Finally, we performed an exploratory analysis regarding possible sexual dimorphism in dental cementum microstructure, which allowed us to correctly infer the sex of the Neolithic specimen, for which the true value was known via DNA analysis.

Life histories as mosaics: plastic and genetic components differ among traits that underpin life-history strategies

Life‐history phenotypes emerge from clusters of traits that are the product of genes and phenotypic plasticity. If the impact of the environment differs substantially between traits, then life histories might not evolve as a cohesive whole. We quantified the sensitivity of components of the life history to food availability, a key environmental difference in the habitat occupied by contrasting ecotypes, for 36 traits in fast‐ and slow‐reproducing Trinidadian guppies. Our dataset included six putatively independent origins of the slow‐reproducing, derived ecotype. Traits varied substantially in plastic and genetic control. Twelve traits were influenced only by food availability (body lengths, body weights), five only by genetic differentiation (interbirth intervals, offspring sizes), 10 by both (litter sizes, reproductive timing), and nine by neither (fat contents, reproductive allotment). Ecotype‐by‐food interactions were negligible. The response to low food was aligned with the genetic difference between high‐ and low‐food environments, suggesting that plasticity was adaptive. The heterogeneity among traits in environmental sensitivity and genetic differentiation reveals that the components of the life history may not evolve in concert. Ecotypes may instead represent mosaics of trait groups that differ in their rate of evolution.

Genetic coupling of life-history and aerobic performance in Atlantic salmon

A better understanding of the genetic and phenotypic architecture underlying life-history variation is a longstanding aim in biology. Theories suggest energy metabolism determines life-history variation by modulating resource acquisition and allocation trade-offs, but the genetic underpinnings of the relationship and its dependence on ecological conditions have rarely been demonstrated. The strong genetic determination of age-at-maturity by two unlinked genomic regions (vgll3 and six6) makes Atlantic salmon (Salmo salar) an ideal model to address these questions. Using more than 250 juveniles in common garden conditions, we quantified the covariation between metabolic phenotypes-standard and maximum metabolic rates (SMR and MMR), and aerobic scope (AS)-and the life-history genomic regions, and tested if food availability modulates the relationships. We found that the early maturation genotype in vgll3 was associated with higher MMR and consequently AS. Additionally, MMR exhibited physiological epistasis; it was decreased when late maturation genotypes co-occurred in both genomic regions. Contrary to our expectation, the life-history genotypes had no effects on SMR. Furthermore, food availability had no effect on the genetic covariation, suggesting a lack of genotype-by-environment interactions. Our results provide insights on the key organismal processes that link energy use at the juvenile stage to age-at-maturity, indicating potential mechanisms by which metabolism and life-history can coevolve.

A comment on The adaptive value of gluttony: predators mediate the life history trade-offs of satiation threshold by Pruitt & Krauel (2010)

Inspection of the data that accompany Pruitt and Krauel's study of individual variation in satiation threshold and a comparison of these data with the Materials and Methods and Results sections of the paper have revealed a number of issues that cast doubts on the reliability of the data and any results based on these data. In particular, we show that, following our analyses, the data are unlikely to have been obtained using the study design outlined in the publication and that statistical analyses of these data provide results that differ in important ways from those reported. These findings illustrate the importance of making raw data and analysis code available for the rigour and reproducibility of the scientific literature.

Reproductive trade-offs and phenotypic selection change with body condition, but not with predation regime, across island lizard populations

Trade-offs between reproduction and survival are central to life-history theory and are expected to shape patterns of phenotypic selection, but the ecological factors structuring these trade-offs and resultant patterns of selection are generally unknown. We manipulated reproductive investment and predation regime in island populations of brown anole lizards (Anolis sagrei) to test (1) whether previously documented increases in the survival of experimentally non-reproductive females (OVX = ovariectomy) reflect the greater susceptibility of reproductive females (SHAM = control) to predation and (2) whether phenotypic selection differs as a function of reproductive investment and predation regime. OVX females exceeded SHAM controls in growth, mass gain and body condition, indicating pronounced energetic costs of reproduction. Although mortality was greatest in the presence of bird and snake predators, differences in survival between OVX and SHAM were unrelated to predation regime, as were patterns of natural selection on body size. Instead, we found that body condition at the conclusion of the experiment differed significantly across populations, suggesting that local environments varied in their ability to support mass gain and positive energy balance. As mean body condition improved across populations, the magnitude of the survival cost of reproduction increased, linear selection on body size shifted from positive to negative, and quadratic selection shifted from stabilizing to weakly disruptive. Our results suggest that reproductive trade-offs and patterns of phenotypic selection in female brown anoles are more sensitive to inferred variation in environmental quality than to experimentally induced variation in predation.

Multiple paternity in garter snakes with evolutionarily divergent life-histories

Many animal species exhibit multiple paternity, defined as multiple males genetically contributing to a single female reproductive event, such as a clutch or litter. Although this phenomenon is well documented across a broad range of taxa, the underlying causes and consequences remain poorly understood. For example, it is unclear how multiple paternity correlates with life-history strategies. Furthermore, males and females may differ in mating strategies and these patterns may shift with ecological context and life-history variation. Here, we take advantage of natural life-history variation in garter snakes (Thamnophis elegans) to address these questions in a robust field setting where populations have diverged along a slow-to-fast life-history continuum. We determine both female (observed) and male (using molecular markers) reproductive success in replicate populations of 2 life-history strategies. We find that despite dramatic differences in annual female reproductive output: 1) females of both life-history ecotypes average 1.5 sires per litter and equivalent proportions of multiply-sired litters, whereas 2) males from the slow-living ecotype experience greater reproductive skew and greater variance in reproductive success relative to males from the fast-living ecotype males despite having equivalent average reproductive success. Together, these results indicate strong intrasexual competition among males, particularly in the fast-paced life-history ecotype. We discuss these results in the context of competing hypotheses for multiple paternity related to population density, resource variability, and life-history strategy.

Three sex phenotypes in a haploid algal species give insights into the evolutionary transition to a self-compatible mating system

Mating systems of haploid species such as fungi, algae, and bryophytes are either heterothallic (self‐incompatible) with two sex phenotypes (male and female, or mating type minus and plus in isogamous species) or homothallic (self‐compatible) with only a bisexual phenotype producing zygotes within a clone. The anisogamous volvocine green alga Pleodorina starrii is a haploid species previously reported to have a heterothallic mating system. Here, we found that two additional culture strains originating from the same water system of P. starrii were taxonomically identified as P. starrii and produced male and female gametes and zygotes within a clone (bisexual). Sequences of rapidly evolving plastid genome regions were identical between the bisexual and unisexual (male or female) P. starrii strains. Intercrossings between the bisexual and unisexual strains demonstrated normal thick‐walled zygotes and high survivability of F1 strains. Thus, these strains belong to the same biological species. Pleodorina starrii has a new haploid mating system that is unique in having three sex phenotypes, namely, male, female, and bisexual. Genetic analyses suggested the existence of autosomal “bisexual factor” locus independent of volvocine male and female determining regions. The present findings increase our understanding of the initial evolutionary step of transition from heterothallism to homothallism.

Tails of reproduction: Regeneration leads to increased reproductive investment

Trade-offs between life-history traits are due to limited resources or constraints in the regulation of genetic and physiological networks. Tail autotomy, with subsequent regeneration, is a common anti-predation mechanism in lizards and is predicted to trade-off with life-history traits, such as reproduction. We utilize the brown anole lizard with its unusual reproductive pattern of single-egg clutches every 7-10 days to test for a trade-off in reproductive investment over 8 weeks of tail regeneration on a limited diet. In contrast to predictions, we found that investing in tissue regeneration had a positive effect on reproduction in terms of egg size (11.7% relative to controls) and hatchling size (11.5% relative to controls), and no effect on egg number or survival, with the increase in reproduction starting at peak regeneration. We discuss mechanistic hypotheses that the process of regeneration may cause increased energetic efficiency or utilized shared physiological pathways with reproductive investment.

Queen loss increases worker survival in leaf-cutting ants under paraquat-induced oxidative stress

Longevity is traded off with fecundity in most solitary species, but the two traits are positively linked in social insects. In ants, the most fecund individuals (queens and kings) live longer than the non-reproductive individuals, the workers. In many species, workers may become fertile following queen loss, and recent evidence suggests that worker fecundity extends worker lifespan. We postulated that this effect is in part owing to improved resilience to oxidative stress, and tested this hypothesis in three Myrmicine ants: Temnothorax rugatulus, and the leaf-cutting ants Atta colombica and Acromyrmex echinatior. We removed the queen from colonies to induce worker reproduction and subjected workers to oxidative stress. Oxidative stress drastically reduced survival, but this effect was less pronounced in leaf-cutting ant workers from queenless nests. We also found that, irrespective of oxidative stress, outside workers died earlier than inside workers did, likely because they were older. Since At. colombica workers cannot produce fertile offspring, our results indicate that direct reproduction is not necessary to extend the lives of queenless workers. Our findings suggest that workers are less resilient to oxidative stress in the presence of the queen, and raise questions on the proximate and ultimate mechanisms underlying socially mediated variation in worker lifespan. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

An ecological driver for the macroevolution of morphological polymorphism within colonial invertebrates

Colonial marine invertebrates, such as corals and bryozoans, have modular growth. Individual modules within a colony are homologous to an individual solitary animal body. But in contrast to the predominately sexual origin of solitary animal bodies, modules within a colony are always produced asexually. The repetition of modules and the indeterminism of their organization gives colonies the ability to grow in ways solitary animals cannot. Colonial invertebrates consequently grow in such a way as to resemble weeds, bushes, or trees. The multitude of growth forms of colonial invertebrates arise from differences how individual colonies within a species tend to invest their energy into modular growth, persistence, asexual propagation, and sexual reproduction. Moreover, many colonial invertebrates possess several body types, morphological polymorphism among modules, where modules qualitatively differ in shape, size, and function. In this paper, I propose a mechanism that links the origin of novel body types to the evolution of life-history strategies among species. When colonies first evolve from solitary ancestors, the life-history strategy of the colony remains constrained by the life-history strategies of the individual modules within the colony until a new polymorph type evolves. The addition of novel body types within a colony introduces potential variation in life-history strategies. Colonies can then change strategies by regulating the frequencies of body types within the colony. This, along with the ability of body types to simplify their structure permits colonies to evolve more complex life-histories. Each new polymorph type that evolves permits more variation in colonial life-histories to exist.

Long-Lived Species of Bivalves Exhibit Low MT-DNA Substitution Rates

Bivalves represent valuable taxonomic group for aging studies given their wide variation in longevity (from 1–2 to >500 years). It is well known that aging is associated to the maintenance of Reactive Oxygen Species homeostasis and that mitochondria phenotype and genotype dysfunctions accumulation is a hallmark of these processes. Previous studies have shown that mitochondrial DNA mutation rates are linked to lifespan in vertebrate species, but no study has explored this in invertebrates. To this end, we performed a Bayesian Phylogenetic Covariance model of evolution analysis using 12 mitochondrial protein-coding genes of 76 bivalve species. Three life history traits (maximum longevity, generation time and mean temperature tolerance) were tested against 1) synonymous substitution rates (dS), 2) conservative amino acid replacement rates (Kc) and 3) ratios of radical over conservative amino acid replacement rates (Kr/Kc). Our results confirm the already known correlation between longevity and generation time and show, for the first time in an invertebrate class, a significant negative correlation between dS and longevity. This correlation was not as strong when generation time and mean temperature tolerance variations were also considered in our model (marginal correlation), suggesting a confounding effect of these traits on the relationship between longevity and mtDNA substitution rate. By confirming the negative correlation between dS and longevity previously documented in birds and mammals, our results provide support for a general pattern in substitution rates.

Cost-free lifespan extension via optimization of gene expression in adulthood aligns with the developmental theory of ageing

Ageing evolves because the force of selection on traits declines with age but the proximate causes of ageing are incompletely understood. The 'disposable soma' theory of ageing (DST) upholds that competitive resource allocation between reproduction and somatic maintenance underpins the evolution of ageing and lifespan. In contrast, the developmental theory of ageing (DTA) suggests that organismal senescence is caused by suboptimal gene expression in adulthood. While the DST predicts the trade-off between reproduction and lifespan, the DTA predicts that age-specific optimization of gene expression can increase lifespan without reproduction costs. Here we investigated the consequences for lifespan, reproduction, egg size and individual fitness of early-life, adulthood and post-reproductive onset of RNAi knockdown of five 'longevity' genes involved in key biological processes in Caenorhabditis elegans. Downregulation of these genes in adulthood and/or during post-reproductive period increases lifespan, while we found limited evidence for a link between impaired reproduction and extended lifespan. Our findings demonstrate that suboptimal gene expression in adulthood often contributes to reduced lifespan directly rather than through competitive resource allocation between reproduction and somatic maintenance. Therefore, age-specific optimization of gene expression in evolutionarily conserved signalling pathways that regulate organismal life histories can increase lifespan without fitness costs.

Density-Dependent Adaptive Topography in a Small Passerine Bird, the Collared Flycatcher

AbstractAdaptive topography is a central concept in evolutionary biology, describing how the mean fitness of a population changes with gene frequencies or mean phenotypes. We use expected population size as a quantity to be maximized by natural selection to show that selection on pairwise combinations of reproductive traits of collared flycatchers caused by fluctuations in population size generated an adaptive topography with distinct peaks often located at intermediate phenotypes. This occurred because r- and K-selection made phenotypes favored at small densities different from those with higher fitness at population sizes close to the carrying capacity K. Fitness decreased rapidly with a delay in the timing of egg laying, with a density-dependent effect especially occurring among early-laying females. The number of fledglings maximizing fitness was larger at small population sizes than when close to K. Finally, there was directional selection for large fledglings independent of population size. We suggest that these patterns can be explained by increased competition for some limiting resources or access to favorable nest sites at high population densities. Thus, r- and K-selection based on expected population size as an evolutionary maximization criterion may influence life-history evolution and constrain the selective responses to changes in the environment.

Reproductive senescence in a short-lived fish

Reproductive senescence is an age-associated decline in reproductive performance, which often arises as a trade-off between current and future reproduction. Given that mortality is inevitable, increased allocation into current reproduction is favoured despite costs paid later in life. This assumption is violated in organisms with post-maturity growth whose reproductive output increases long after maturity. While reproductive senescence is frequently studied in animals with determinate growth at maturity, such as insects or mammals, we have very limited understanding of reproductive senescence in organisms with an extensive post-maturity growth period. The fact that many post-maturity growers experience strong adult mortality leads to conflicting expectations for reproductive senescence. The aim of this study was to investigate how co-occurrence of rapid life history and post-maturity growth mould reproductive senescence in a short-lived killifish, Nothobranchius furzeri, using longitudinal data on laboratory and wild-type populations. We followed the individual fecundity, fertility and fertilization of 132 singly housed fish from the perspectives of chronological and biological age. At the onset of senescence, the sex-specific contribution to decrease in fertilization capacity was investigated. Allocation trade-offs were estimated through the association between reproductive parameters and life span, and between early-life and late-life fecundity. We demonstrate that female fecundity increased steadily after maturity and reproductive senescence occurred long after the growth asymptote. The prime age for fecundity coincided with 50% female survival and consequent decline in fecundity implies an association with somatic deterioration. Reproductive senescence in fertilization rate was stronger in females than in males. Females with high early fecundity experienced a long life span and high late-life fecundity, discounting the role of allocation trade-offs in reproductive senescence. The present study reports a clear case of reproductive senescence in a fish with a long post-maturation growth period, unusually rapid development and short life span. The onset of reproductive senescence was postponed compared to animals that cease growing at sexual maturity. Fish and other animals with post-maturity growth have long been considered insusceptible to ageing but this conclusion may be related to the previous lack of longitudinal data rather than to the absence of reproductive senescence in such organisms.

Strong stabilizing selection on timing of germination in a Mediterranean population of Arabidopsis thaliana

PREMISE

Timing of germination can strongly influence plant fitness by affecting seedling survival and by having cascading effects on later life-history traits. In seasonal environments, the period favorable for seedling establishment and growth is limited, and timing of germination is likely to be under stabilizing selection because of conflicting selection through survival and fecundity. Moreover, optimal germination time may vary among genotypes because of inherent differences in later life-history traits.

METHODS

To examine how germination time affects survival, fecundity, and the relative fitness of two genotypes differing in time to first flower, we conducted a field experiment in an Italian population of the winter annual Arabidopsis thaliana, in which seedling establishment occurs mainly in November. We transplanted seedlings of the local genotype and of a Swedish genotype monthly from August to December and monitored survival and fecundity.

RESULTS

Only seedlings transplanted in November and December survived until reproduction, and fitness of the November cohort was 35 times higher than that of the December cohort, indicating strong stabilizing selection on timing of germination. There was no evidence of conflicting selection: seedling survival, adult survival, and fecundity were all highest in the November cohort. Moreover, the relative fitness of the two genotypes did not differ significantly between cohorts.

CONCLUSIONS

The very narrow window of opportunity for seedling establishment was related to rapid seasonal changes in soil moisture and temperature, suggesting that rate of seasonal change is an important aspect to consider for understanding spatiotemporal variation in selection on phenological traits.

Life-History Modeling Reveals the Ecological and Evolutionary Significance of Autotomy

AbstractAutotomy, the self-amputation of body parts, serves as an antipredator defense in many taxonomic groups of animals. However, its adaptive value has seldom been quantified. Here, we propose a novel modeling approach for measuring the fitness advantage conferred by the capability for autotomy in the wild. Using a predator-prey system where a land snail autotomizes and regenerates its foot specifically in response to snake bites, we conducted a laboratory behavioral experiment and a 3-year multievent capture-mark-recapture study. Combining these empirical data, we developed a hierarchical model and estimated the basic life-history parameters of the snail. Using samples from the posterior distribution, we constructed the snail's life table as well as that of a snail variant incapable of foot autotomy. As a result of our analyses, we estimated the monthly encounter rate with snake predators at 3.3% (95% credible interval: 1.6%-4.9%), the contribution of snake predation to total mortality until maturity at 43.3% (15.0%-95.3%), and the fitness advantage conferred by foot autotomy at 6.5% (2.7%-11.5%). This study demonstrated the utility of the multimethod hierarchical-modeling approach for the quantitative understanding of the ecological and evolutionary processes of antipredator defenses in the wild.

An estimator of the Opportunity for Selection that is independent of mean fitness

Variation among individuals in number of offspring (fitness, k) sets an upper limit to the evolutionary response to selection. This constraint is quantified by Crow's Opportunity for Selection (I), which is the variance in relative fitness (I = σ² _k /(u_k )² ). Crow's I has been widely used but remains controversial because it depends on mean offspring number in a sample ( k ¯ ). Here, I used a generalized Wright-Fisher model that allows for unequal probabilities of producing offspring to evaluate behavior of Crow's I and related indices under a wide range of sampling scenarios. Analytical and numerical results are congruent and show that rescaling the sample variance (s² _k ) to its expected value at a fixed k ¯ 2 removes dependence of I on mean offspring number, but the result still depends on choice of k ¯ 2 . A new index is introduced, Δ_I = Π- E(Î_drift ) = Π- 1/ k ¯ , which makes Î independent of sample k ¯ without the need for variance rescaling. Δ_I has a straightforward interpretation as the component of variance in relative fitness that exceeds that expected under a null model of random reproductive success. Δ_I can be used to directly compare estimates of the Opportunity for Selection for samples from different studies, different sexes, and different life stages.

Condition-dependent mortality exacerbates male (but not female) reproductive senescence and the potential for sexual conflict

Disentangling the relationship between age and reproduction is central to understand life-history evolution, and recent evidence shows that considering condition-dependent mortality is a crucial piece of this puzzle. For example, nonrandom mortality of 'low-condition' individuals can lead to an increase in average lifespan. However, selective disappearance of such low-condition individuals may also affect reproductive senescence at the population level due to trade-offs between physiological functions related to survival/lifespan and the maintenance of reproductive functions. Here, we address the idea that condition-dependent extrinsic mortality (i.e. simulated predation) may increase the age-related decline in male reproductive success and with it the potential for sexual conflict, by comparing reproductive ageing in Drosophila melanogaster male/female cohorts exposed (or not) to condition-dependent simulated predation across time. Although female reproductive senescence was not affected by predation, male reproductive senescence was considerably higher under predation, due mainly to an accelerated decline in offspring viability of 'surviving' males with age. This sex-specific effect suggests that condition-dependent extrinsic mortality can exacerbate survival-reproduction trade-offs in males, which are typically under stronger condition-dependent selection than females. Interestingly, condition-dependent extrinsic mortality did not affect mating success, hinting that accelerated reproductive senescence is due to a decrease in male post-copulatory fitness components. Our results support the recent proposal that male ageing can be an important source of sexual conflict, further suggesting this effect could be exacerbated under more natural conditions.

The optimal timing of teaching and learning across the life course

The evolutionary biologist W. D. Hamilton (Hamilton 1966 J. Theor. Biol.12, 12-45. (doi:10.1016/0022-5193(66)90184-6)) famously showed that the force of natural selection declines with age, and reaches zero by the age of reproductive cessation. However, in social species, the transfer of fitness-enhancing resources by postreproductive adults increases the value of survival to late ages. While most research has focused on intergenerational food transfers in social animals, here we consider the potential fitness benefits of information transfer, and investigate the ecological contexts where pedagogy is likely to occur. Although the evolution of teaching is an important topic in behavioural biology and in studies of human cultural evolution, few formal models of teaching exist. Here, we present a modelling framework for predicting the timing of both information transfer and learning across the life course, and find that under a broad range of conditions, optimal patterns of information transfer in a skills-intensive ecology often involve postreproductive aged teachers. We explore several implications among human subsistence populations, evaluating the cost of hunting pedagogy and the relationship between activity skill complexity and the timing of pedagogy for several subsistence activities. Long lifespan and extended juvenility that characterize the human life history likely evolved in the context of a skills-intensive ecological niche with multi-stage pedagogy and multigenerational cooperation. This article is part of the theme issue 'Life history and learning: how childhood, caregiving and old age shape cognition and culture in humans and other animals'.

Habitat deterioration promotes the evolution of direct development in metamorphosing species

Although metamorphosis is widespread in the animal kingdom, several species have evolved life‐cycle modifications to avoid complete metamorphosis. Some species, for example, many salamanders and newts, have deleted the adult stage via a process called paedomorphosis. Others, for example, some frog species and marine invertebrates, no longer have a distinct larval stage and reach maturation via direct development. Here we study which ecological conditions can lead to the loss of metamorphosis via the evolution of direct development. To do so, we use size‐structured consumer‐resource models in conjunction with the adaptive‐dynamics approach. In case the larval habitat deteriorates, individuals will produce larger offspring and in concert accelerate metamorphosis. Although this leads to the evolutionary transition from metamorphosis to direct development when the adult habitat is highly favorable, the population will go extinct in case the adult habitat does not provide sufficient food to escape metamorphosis. With a phylogenetic approach we furthermore show that among amphibians the transition of metamorphosis to direct development is indeed, in line with model predictions, conditional on and preceded by the evolution of larger egg sizes.

Long-term research and hierarchical models reveal consistent fitness costs of being the last egg in a clutch

Maintenance of phenotypic heterogeneity in the face of strong selection is an important component of evolutionary ecology, as are the consequences of such heterogeneity. Organisms may experience diminishing returns of increased reproductive allocation as clutch or litter size increases, affecting current and residual reproductive success. Given existing uncertainty regarding trade‐offs between the quantity and quality of offspring, we sought to examine the potential for diminishing returns on increased reproductive allocation in a long‐lived species of goose, with a particular emphasis on the effect of position in the laying sequence on offspring quality.

To better understand the effects of maternal allocation on offspring survival and growth, we estimated the effects of egg size, timing of breeding, inter‐ and intra‐annual variation, and position in the laying sequence on gosling survival and growth rates of black brant Branta bernicla nigricans breeding in western Alaska from 1987 to 2007.

We found that gosling growth rates and survival decreased with position in the laying sequence, regardless of clutch size. Mean egg volume of the clutch a gosling originated from had a positive effect on gosling survival (β = 0.095, 95% CRI: 0.024, 0.165) and gosling growth rates (β = 0.626, 95% CRI: 0.469, 0.738). Gosling survival (β = −0.146, 95% CRI: −0.214, −0.079) and growth rates (β = −1.286, 95% CRI: −1.435, −1.132) were negatively related to hatching date.

These findings indicate substantial heterogeneity in offspring quality associated with their position in the laying sequence. They also potentially suggest a trade‐off mechanism for females whose total reproductive investment is governed by pre‐breeding state.

Cyclical environments drive variation in life-history strategies: a general theory of cyclical phenology

Cycles, such as seasons or tides, characterize many systems in nature. Overwhelming evidence shows that climate change-driven alterations to environmental cycles-such as longer seasons-are associated with phenological shifts around the world, suggesting a deep link between environmental cycles and life cycles. However, general mechanisms of life-history evolution in cyclical environments are still not well understood. Here, I build a demographic framework and ask how life-history strategies optimize fitness when the environment perturbs a structured population cyclically and how strategies should change as cyclicality changes. I show that cycle periodicity alters optimality predictions of classic life-history theory because repeated cycles have rippling selective consequences over time and generations. Notably, fitness landscapes that relate environmental cyclicality and life-history optimality vary dramatically depending on which trade-offs govern a given species. The model tuned with known life-history trade-offs in a marine intertidal copepod Tigriopus californicus successfully predicted the shape of life-history variation across natural populations spanning a gradient of tidal periodicities. This framework shows how environmental cycles can drive life-history variation-without complex assumptions of individual responses to cues such as temperature-thus expanding the range of life-history diversity explained by theory and providing a basis for adaptive phenology.

Life-History Evolution and the Genetics of Fitness Components in Drosophila melanogaster

Life-history traits or "fitness components"-such as age and size at maturity, fecundity and fertility, age-specific rates of survival, and life span-are the major phenotypic determinants of Darwinian fitness. Analyzing the evolution and genetics of these phenotypic targets of selection is central to our understanding of adaptation. Due to its simple and rapid life cycle, cosmopolitan distribution, ease of maintenance in the laboratory, well-understood evolutionary genetics, and its versatile genetic toolbox, the "vinegar fly" Drosophila melanogaster is one of the most powerful, experimentally tractable model systems for studying "life-history evolution." Here, I review what has been learned about the evolution and genetics of life-history variation in D. melanogaster by drawing on numerous sources spanning population and quantitative genetics, genomics, experimental evolution, evolutionary ecology, and physiology. This body of work has contributed greatly to our knowledge of several fundamental problems in evolutionary biology, including the amount and maintenance of genetic variation, the evolution of body size, clines and climate adaptation, the evolution of senescence, phenotypic plasticity, the nature of life-history trade-offs, and so forth. While major progress has been made, important facets of these and other questions remain open, and the D. melanogaster system will undoubtedly continue to deliver key insights into central issues of life-history evolution and the genetics of adaptation.

Density-dependent offspring interactions do not explain macroevolutionary scaling of adult size and offspring size

Most life forms exhibit a correlated evolution of adult size (AS) and size at independence (SI), giving rise to AS-SI scaling relationships. Theory suggests that scaling arises because relatively large adults have relatively high reproductive output, resulting in strong density-dependent competition in early life, where large size at independence provides a competitive advantage to juveniles. The primary goal of our study is to test this density hypothesis, using large datasets that span the vertebrate tree of life (fishes, amphibians, reptiles, birds, and mammals). Our secondary goal is to motivate new hypotheses for AS-SI scaling by exploring how subtle variation in life-histories among closely related species is associated with variation in scaling. Our phylogenetically informed comparisons do not support the density hypothesis. Instead, exploration of AS-SI scaling among life-history variants suggests that steeper AS-SI scaling slopes are associated with evolutionary increases in size at independence. We suggest that a positive association between size at independence and juvenile growth rate may represent an important mechanism underlying AS-SI scaling, a mechanism that has been underappreciated by theorists. If faster juvenile growth is a consequence of evolutionary increases in size at independence, this may help offset the cost of delayed maturation, leading to steeper AS-SI scaling slopes.

Co-inheritance of sea age at maturity and iteroparity in the Atlantic salmon vgll3 genomic region

Co-inheritance in life-history traits may result in unpredictable evolutionary trajectories if not accounted for in life-history models. Iteroparity (the reproductive strategy of reproducing more than once) in Atlantic salmon (Salmo salar) is a fitness trait with substantial variation within and among populations. In the Teno River in northern Europe, iteroparous individuals constitute an important component of many populations and have experienced a sharp increase in abundance in the last 20 years, partly overlapping with a general decrease in age structure. The physiological basis of iteroparity bears similarities to that of age at first maturity, another life-history trait with substantial fitness effects in salmon. Sea age at maturity in Atlantic salmon is controlled by a major locus around the vgll3 gene, and we used this opportunity demonstrate that these two traits are co-inherited around this genome region. The odds ratio of survival until second reproduction was up to 2.4 (1.8-3.5 90% CI) times higher for fish with the early-maturing vgll3 genotype (EE) compared to fish with the late-maturing genotype (LL). The L allele was dominant in individuals remaining only one year at sea before maturation, but the dominance was reversed, with the E allele being dominant in individuals maturing after two or more years at sea. Post hoc analysis indicated that iteroparous fish with the EE genotype had accelerated growth prior to first reproduction compared to first-time spawners, across all age groups, whereas this effect was not detected in fish with the LL genotype. These results broaden the functional link around the vgll3 genome region and help us understand constraints in the evolution of life-history variation in salmon. Our results further highlight the need to account for genetic correlations between fitness traits when predicting demographic changes in changing environments.

Adaptation to developmental diet influences the response to selection on age at reproduction in the fruit fly

Experimental evolution (EE) is a powerful tool for addressing how environmental factors influence life-history evolution. While in nature different selection pressures experienced across the lifespan shape life histories, EE studies typically apply selection pressures one at a time. Here, we assess the consequences of adaptation to three different developmental diets in combination with classical selection for early or late reproduction in the fruit fly Drosophila melanogaster. We find that the response to each selection pressure is similar to that observed when they are applied independently, but the overall magnitude of the response depends on the selection regime experienced in the other life stage. For example, adaptation to increased age at reproduction increased lifespan across all diets; however, the extent of the increase was dependent on the dietary selection regime. Similarly, adaptation to a lower calorie developmental diet led to faster development and decreased adult weight, but the magnitude of the response was dependent on the age-at-reproduction selection regime. Given that multiple selection pressures are prevalent in nature, our findings suggest that trade-offs should be considered not only among traits within an organism, but also among adaptive responses to different-sometimes conflicting-selection pressures, including across life stages.

Phenological mismatch drives selection on elevation, but not on slope, of breeding time plasticity in a wild songbird

Phenotypic plasticity is an important mechanism for populations to respond to fluctuating environments, yet may be insufficient to adapt to a directionally changing environment. To study whether plasticity can evolve under current climate change, we quantified selection and genetic variation in both the elevation (RNE ) and slope (RNS ) of the breeding time reaction norm in a long-term (1973-2016) study population of great tits (Parus major). The optimal RNE (the caterpillar biomass peak date regressed against the temperature used as cue by great tits) changed over time, whereas the optimal RNS did not. Concordantly, we found strong directional selection on RNE , but not RNS , of egg-laying date in the second third of the study period; this selection subsequently waned, potentially due to increased between-year variability in optimal laying dates. We found individual and additive genetic variation in RNE but, contrary to previous studies on our population, not in RNS . The predicted and observed evolutionary change in RNE was, however, marginal, due to low heritability and the sex limitation of laying date. We conclude that adaptation to climate change can only occur via micro-evolution of RNE, but this will necessarily be slow and potentially hampered by increased variability in phenotypic optima.

All's well that ends well: why large species have short telomeres

Among mammal species, almost all life-history traits are strongly size dependent. This size dependence even occurs at a molecular level. For example, both telomere length and telomerase expression show a size-dependent threshold. With some exceptions, species smaller than approximately 2 kg express telomerase, while species larger than that do not. Among species greater than approximately 5 kg, telomeres tend to be short-less than 25 kb-while among smaller species, some species have short and some have long telomeres. Here, we present a model to explore the role of body size-dependent trade-offs in shaping this threshold. We assume that selection favours short telomeres as a mechanism to protect against cancer. At the same time, selection favours long telomeres as a protective mechanism against DNA damage and replicative senescence. The relative importance of these two selective forces will depend on underlying intrinsic mortality and risk of cancer, both of which are size-dependent. Results from this model suggest that a cost-benefit model for the evolution of telomere length could explain phylogenetic patterns observed within the Class Mammalia. In addition, the model suggests a general conceptual framework to think about the role that body size plays in the evolution of tumour suppressor mechanisms.This article is part of the theme issue 'Understanding diversity in telomere dynamics'.

Fitness and Genomic Consequences of Chronic Exposure to Low Levels of Copper and Nickel in Daphnia pulex Mutation Accumulation Lines

In at least some unicellular organisms, mutation rates are temporarily raised upon exposure to environmental stress, potentially contributing to the evolutionary response to stress. Whether this is true for multicellular organisms, however, has received little attention. This study investigated the effects of chronic mild stress, in the form of low-level copper and nickel exposure, on mutational processes in Daphnia pulex using a combination of mutation accumulation, whole genome sequencing and life-history assays. After over 100 generations of mutation accumulation, we found no effects of metal exposure on the rates of single nucleotide mutations and of loss of heterozygosity events, the two mutation classes that occurred in sufficient numbers to allow statistical analysis. Similarly, rates of decline in fitness, as measured by intrinsic rate of population increase and of body size at first reproduction, were negligibly affected by metal exposure. We can reject the possibility that Daphnia were insufficiently stressed to invoke genetic responses as we have previously shown rates of large-scale deletions and duplications are elevated under metal exposure in this experiment. Overall, the mutation accumulation lines did not significantly depart from initial values for phenotypic traits measured, indicating the lineage used was broadly mutationally robust. Taken together, these results indicate that the mutagenic effects of chronic low-level exposure to these metals are restricted to certain mutation classes and that fitness consequences are likely minor and therefore unlikely to be relevant in determining the evolutionary responses of populations exposed to these stressors.