Abstract When predicting species’ responses to changing environments, one can use mathematical functions that describe how individual fitness components depend on the environment, or a single “composite” function that directly links fitness to the environmental state. The former approach is a cornerstone of process-based modelling, but the latter remains standard for developing fundamental theory and making ecological predictions. Yet, fitness is not a single instantaneous trait, but an integrated outcome of multiple underlying processes accruing throughout an organism’s life. We show that by ignoring the distinct environmental dependence of the underlying processes, predictions from composite fitness functions become inherently flawed in variable environments. We explore the magnitude of this error by leveraging empirical thermal reaction norms for four important life-history processes in an insect pest, the seed beetle Callosobruchus maculatus . We parameterize two fitness functions: one explicitly modelling the temperature-dependence of the four life-history traits independently (the “ground truth”) and one composite function, which treats fitness as a single, instantaneous outcome of the environment. By combining these two functions with hourly temperature data, we projected demographic responses under different warming scenarios across 300 sites over three beetle population origins (California, USA; Yemen; Brazil). We show that the composite function over- or underestimates fitness depending on subtle climatic differences and whether fitness is assumed to accumulate additively or multiplicatively, highlighting the problems of applying composite fitness functions to variable conditions. We conclude that explicitly modeling trait-specific processes will become increasingly important for accurate eco-evolutionary forecasting under future environmental change.

Abstract Climate change influences the physiology and population dynamics of ectothermic pests, with major repercussions for global crop production. Yet, how evolution modulates these outcomes remains unclear. We exposed the widespread beetle pest Callosobruchus maculatus to 10 years of experimental evolution at different temperatures and quantified thermal responses of life-history traits. Hot- and cold-adapted populations evolved differences in thermal sensitivity, but these were modest relative to evolved differences in trait averages. By leveraging high-resolution temperature time-series we show that the observed evolution translates into cold-adapted genotypes having highest fitness in cold climates and hot-adapted genotypes in warm climates. Hot-adapted beetles maximize fitness in warm climates by increased larval growth, resulting in larger body sizes and higher fecundity. This evolutionary strategy compounds projected crop losses under warming by increasing both intrinsic population growth and per-capita host consumption rates. By year 2100 under intermediate-to-high warming (SSP3-7.0), pest evolution is projected to increase global crop damage potential by +113% from present—twice that expected from warming alone (not accounting for evolution). In major crop-producing areas, where temperatures and the beetles’ host consumption rates are already high, warming increases average crop damage potential by +29%, but evolution amplifies this three-fold to +87%. Evolution also expands C. maculatus ’ projected colonizable range and in some regions even flips forecasted crop damage reductions into increases. These results identify climate-driven evolution of pest life-histories as an amplifier of agricultural losses and suggest that current projections may significantly understate the threat warming poses to future food security.

Abstract Transcriptional plasticity can maintain organismal homeostasis but its potential to buffer negative effects from climate change remains contentious and may rely on genetic adaptation. Theory predicts two general outcomes: genetic reinforcement, where adaptive plasticity is enhanced by genetic adaptation; or genetic compensation, where genetic adaptation reverses maladaptive plasticity. Here, we explored the prevalence, repeatability, and molecular basis of compensation and reinforcement of gene expression responses to temperature using experimental evolution in the beetle pest, Callosobruchus maculatus . We evolved lines from three genetic backgrounds under hot or cold conditions and compared them to ancestral lines after 80-135 generations. Cold adaptation was dominated by genetic compensation, revealing a highly repeatable mode of adaptation across backgrounds, however, the underlying genes and their biological functions were largely idiosyncratic. In contrast, heat adaptation showed a less consistent mode, combining compensation and reinforcement, but greater repeatability at the level of genes and their functions. Network analysis identified many heat reinforcement genes as hubs and strongly temperature dependent, suggestive of roles in orchestrated adaptive thermal plasticity, potentially explaining their high repeatability. Genomic analyses identified repeatable allele frequency changes at both hot and cold compensation and reinforcement genes during experimental evolution. However, these were not causally linked to changes in expression levels, suggesting trans-regulation. There was also no broader correspondence between the level of repeatability of sequence and expression evolution across gene categories. Our findings suggest that the predictability of evolution under climate change might critically depend on the thermal range and the level of the genotype-phenotype map that is studied.

Organismal function requires precise gene expression, as deviations reduce fitness and can cause disease. The widespread expression changes characteristic of old age therefore suggests that aging itself may partly stem from gene dysregulation. Alternatively, many of these changes may represent a plastic response to somatic decline, tuning the organism to an altered physiological state. We tested the latter hypothesis by experimentally reducing the condition of Drosophila melanogaster females independently of age and comparing the resulting expression changes with those occurring naturally during aging. Consistent with the plasticity hypothesis, we find substantial overlap between genes responding to reduced condition and old age. Downregulated genes are enriched for metabolic functions, with a consistent, albeit weaker, association with mitochondrial function and cytoplasmic translation, while upregulated genes relate to genome maintenance. Both old age and reduced condition also cause downregulation of tissue-specific and female-biased genes, as expected when energy is reallocated to core cellular processes. In line with a coordinated transcriptional response to old age, both down- and upregulated genes within enriched functional categories show reduced expression variability and experience strong purifying selection. Collectively, our results support that the aging soma elicits a plastic transcriptional program that adjusts the organism to a declining physiological condition, implying that many age-related expression changes mitigate rather than accelerate somatic aging. These findings call for a more nuanced view of the causes and consequences of age-related transcriptional change, with implications for both theoretical and applied research on aging and disease. ### Competing Interest Statement The authors have declared no competing interest. Royal Swedish Academy of Sciences, https://ror.org/00j62qv07 Royal Physiographic Society of Lund, https://ror.org/05rnj0j91 Helge Ax:son Johnsons Stiftelse, https://ror.org/0527w1g58 Lars Hierta Memorial Foundation, https://ror.org/03cpsrf24 Stiftelsen Längmanska Kulturfonden Stiftelsen Olle Engkvist Byggmästare, https://ror.org/00r9xw041 Swedish Research Council, https://ror.org/03zttf063 Carl Tryggers stiftelse för vetenskaplig forskning, https://ror.org/048p4fn82 Knut and Alice Wallenberg Foundation, KAW 835 2020.0239, KAW 2017.0003 National Bioinformatics Infrastructure Sweden

Climate change is affecting population growth rates of ectothermic pests with potentially dire consequences for agriculture and global food security. However, current projection models of pest impact typically overlook the potential for rapid genetic adaptation, making current forecasts uncertain. Here, we predict how climate change adaptation in life-history traits of insect pests affects their growth rates and impact on agricultural yields by unifying thermodynamics with classic theory on resource acquisition and allocation trade-offs between foraging, reproduction, and maintenance. Our model predicts that warming temperatures will favour resource allocation towards maintenance coupled with increased resource acquisition through larval foraging, and the evolution of this life-history strategy results in both increased population growth rates and per capita host consumption, causing a double-blow on agricultural yields. We find support for these predictions by studying thermal adaptation in life-history traits and gene expression in the wide-spread insect pest, Callosobruchus maculatus; with 5 years of evolution under experimental warming causing an almost two-fold increase in its predicted agricultural footprint. These results show that pest adaptation can offset current projections of agricultural impact and emphasize the need for integrating a mechanistic understanding of life-history evolution into forecasts of pest impact under climate change. Current statistical projections of pest impact under climate change neglect the role of rapid genetic adaptation. Here the authors show that evolutionary responses in pest life history traits in response to temperature can offset current projections and magnify agricultural loss under future climate change.

Climate warming is threatening biodiversity by increasing temperatures beyond the optima of many ectotherms. Owing to the inherent non-linear relationship between temperature and the rate of cellular processes, such shifts towards hot temperature are predicted to impose stronger selection compared with corresponding shifts towards cold temperature. This suggests that when adaptation to warming occurs, it should be relatively rapid and predictable. Here we tested this hypothesis from the level of single-nucleotide polymorphisms to life-history traits in the beetle Callosobruchus maculatus. We conducted an evolve-and-resequence experiment on three genetic backgrounds of the beetle reared at hot or cold temperature. Indeed, we find that phenotypic evolution was faster and more repeatable at hot temperature. However, at the genomic level, adaptation to heat was less repeatable when compared across genetic backgrounds. As a result, genomic predictions of phenotypic adaptation in populations exposed to hot temperature were accurate within, but not between, backgrounds. These results seem best explained by genetic redundancy and an increased importance of epistasis during adaptation to heat, and imply that the same mechanisms that exert strong selection and increase repeatability of phenotypic evolution at hot temperature reduce repeatability at the genomic level. Thus, predictions of adaptation in key phenotypes from genomic data may become increasingly difficult as climates warm.

Evolutionary change requires genetic variation, and a reigning paradigm in biology is that rates of microevolution can be predicted from estimates of available genetic variation within populations. However, the accuracy of such predictions should decay on longer evolutionary timescales, as the influence of genetic constraints diminishes. Here we show that intrinsic developmental variability and standing genetic variation in wing shape in two distantly related flies, Drosophila melanogaster and Sepsis punctum , are aligned and predict deep divergence in the dipteran phylogeny, spanning >900 taxa and 185 million years. This alignment cannot be easily explained by constraint hypotheses unless most of the quantified standing genetic variation is associated with deleterious side effects and is effectively unusable for evolution. However, phenotyping of 71 genetic lines of S. punctum revealed no covariation between wing shape and fitness, lending no support to this hypothesis. We also find little evidence for genetic constraints on the pace of wing shape evolution along the dipteran phylogeny. Instead, correlational selection related to allometric scaling, simultaneously shaping developmental variability and deep divergence in fly wings, emerges as a potential explanation for the observed alignment. This suggests that pervasive natural selection has the potential to shape developmental architectures of some morphological characters such that their intrinsic variability predicts their long-term evolution.

Climate adaptation in insects can proceed via responses in life-history traits and their thermal plasticity and through phenological shifts mediated by responses to photoperiodic cues (photoperiodism). While experimental studies demonstrate evolutionary potential for both modes of adaptation, it remains unclear how evolution will unfold in natural populations, limiting our ability to predict how insects will respond to climate change. Here, we review the literature and analyze published studies revealing that photoperiodism for diapause induction evolves predictably along latitude, with high-latitude populations entering diapause earlier. In contrast, although a few species showed clinal variation in life history and thermal plasticity, the direction of these clines was not consistent across taxa. These findings suggest that while insect life history and physiological adaptation to temperature can evolve, phenological shifts via evolution of photoperiodism are likely to be more common and predictable responses to future climate change.

Behavioural plasticity can play a key role in evolution by either facilitating or impeding genetic adaptation. The latter occurs when behaviours mitigate selection pressures that otherwise would target associated traits. Therefore, environments that facilitate adaptive behavioural plasticity could relax the strength of natural selection, but experimental evidence for this prediction remains scarce. Here, we first demonstrate that maternal care behaviour in the beetle Callosobruchus maculatus is dependent on environmental cues that allow females to reduce larval competition via learning and informed oviposition choices. We show that this facilitation of maternal care relaxes selection against deleterious alleles in offspring. We further find that mothers of low genetic quality generally provide poorer care. However, when receiving environmental cues providing accurate information about future host-quality, the increased opportunity for adaptive behavioural plasticity reduced genetic differences in maternal care, further relaxing selection against deleterious alleles. We use our data to illustrate how the identified link between adaptive behavioural plasticity in maternal care and the strength of natural selection can impact indirect genetic effects between mothers and offspring and the accumulation of cryptic genetic loads in populations inhabiting predictable environments.

Predicting if, when, and how populations can adapt to climate change constitutes one of the greatest challenges in science today. Here, we build from contributions to the special issue on evolutionary adaptation to climate change, a survey of its authors, and recent literature to explore the limits and opportunities for predicting adaptive responses to climate change. We outline what might be predictable now, in the future, and perhaps never even with our best efforts. More accurate predictions are expected for traits characterized by a well-understood mapping between genotypes and phenotypes and traits experiencing strong, direct selection due to climate change. A meta-analysis revealed an overall moderate trait heritability and evolvability in studies performed under future climate conditions but indicated no significant change between current and future climate conditions, suggesting neither more nor less genetic variation for adapting to future climates. Predicting population persistence and evolutionary rescue remains uncertain, especially for the many species without sufficient ecological data. Still, when polled, authors contributing to this special issue were relatively optimistic about our ability to predict future evolutionary responses to climate change. Predictions will improve as we expand efforts to understand diverse organisms, their ecology, and their adaptive potential. Advancements in functional genomic resources, especially their extension to non-model species and the union of evolutionary experiments and “omics,” should also enhance predictions. Although predicting evolutionary responses to climate change remains challenging, even small advances will reduce the substantial uncertainties surrounding future evolutionary responses to climate change.

An often-overlooked aspect of life-history optimization is the allocation of resources to protect the germline and secure safe transmission of genetic information. While failure to do so renders significant fitness consequences in future generations, germline maintenance comes with substantial costs. Thus, germline allocation should trade off with other life-history decisions and be optimized in accordance with an organism’s reproductive schedule. Here, we tested this hypothesis by studying germline maintenance in lines of seed beetle, selected for early (E) or late (L) reproduction for 350 and 240 generations, respectively. Female animals provide maintenance and screening of male gametes in their reproductive tract and oocytes. Here, we reveal the ability of young and aged E- and L-females to provide this form of germline maintenance by mating them to males with ejaculates with artificially elevated levels of protein and DNA damage. We find that germline maintenance in E-females peaks at young age and then declines, while the opposite is true for L-females, in accordance with the age of reproduction in the respective regime. These findings identify the central role of allocation to secure germline integrity in life-history evolution and highlight how females can play a crucial role in mitigating the effects of male germline decisions on mutation rate and offspring quality.

Exposure to extreme temperatures can negatively affect animal reproduction, by disrupting the ability of individuals to produce any offspring (fertility), or the number of offspring produced by fertile individuals (fecundity). This has important ecological consequences, because reproduction is the ultimate measure of population fitness: a reduction in reproductive output lowers the population growth rate and increases the extinction risk. Despite this importance, there have been no large-scale summaries of the evidence for effect of temperature on reproduction. We provide a systematic map of studies testing the relationship between temperature and animal reproduction. We systematically searched for published studies that statistically test for a direct link between temperature and animal reproduction, in terms of fertility, fecundity or indirect measures of reproductive potential (gamete and gonad traits). Overall, we collated a large and rich evidence base, with 1654 papers that met our inclusion criteria, encompassing 1191 species. The map revealed several important research gaps. Insects made up almost half of the dataset, but reptiles and amphibians were uncommon, as were non-arthropod invertebrates. Fecundity was the most common reproductive trait examined, and relatively few studies measured fertility. It was uncommon for experimental studies to test exposure of different life stages, exposure to short-term heat or cold shock, exposure to temperature fluctuations, or to independently assess male and female effects. Studies were most often published in journals focusing on entomology and pest control, ecology and evolution, aquaculture and fisheries science, and marine biology. Finally, while individuals were sampled from every continent, there was a strong sampling bias towards mid-latitudes in the Northern Hemisphere, such that the tropics and polar regions are less well sampled. This map reveals a rich literature of studies testing the relationship between temperature and animal reproduction, but also uncovers substantial missing treatment of taxa, traits, and thermal regimes. This database will provide a valuable resource for future quantitative meta-analyses, and direct future studies aiming to fill identified gaps. We summarise 1600+ papers testing the relationship between temperature and animal reproduction, and identify major taxonomic, geographic and methodological research gaps. This database will provide a valuable resource for future analyses, and direct future studies aiming to fill research gaps.image

Evolution should be more predictable when natural selection is strong and favors the same outcome. Climate warming is increasing temperatures beyond the optima of many ectotherms, which, due to the inherent non-linear relationship between temperature and the rate of cellular processes, is predicted to impose stronger selection compared to corresponding shifts toward cold temperatures. This suggests that adaptation to climate warming should be relatively predictable. Here, we tested this hypothesis from the level of single-nucleotide polymorphisms to life-history traits, by conducting an evolve-and-resequence experiment on three genetic backgrounds of the seed beetle, Callosobruchus maculatus . Indeed, phenotypic evolution was faster and more repeatable at hot, relative to cold, temperature. However, at the genomic level, adaptation to heat was less repeatable than to cold, especially when comparing responses between backgrounds. As a result, genomic predictions of phenotypic (mal)adaptation in populations exposed to hot temperature were highly accurate within, but inaccurate between, genetic backgrounds. These results seem best explained by an increased importance of epistasis during adaptation to heat and imply that the same biophysical mechanisms that increase the repeatability of phenotypic evolution by exerting strong selection at hot temperature, reduce repeatability at the genome level. Thus, predictions of adaptation in key phenotypes from genomic data may become increasingly difficult as climates warm. ### Competing Interest Statement The authors have declared no competing interest.

Critical thermal limits (CTLs) gauge the physiological impact of temperature on survival or critical biological function, aiding predictions of species range shifts and climatic resilience. Two recent Drosophila species studies, using similar approaches to determine temperatures that induce sterility (thermal fertility limits [TFLs]), reveal that TFLs are often lower than CTLs and that TFLs better predict both current species distributions and extinction probability. Moreover, many studies show fertility is more sensitive at less extreme temperatures than survival (thermal sensitivity of fertility [TSF]). These results present a more pessimistic outlook on the consequences of climate change. However, unlike CTLs, TFL data are limited to Drosophila, and variability in TSF methods poses challenges in predicting species responses to increasing temperature. To address these data and methodological gaps, we propose 3 standardized approaches for assessing thermal impacts on fertility. We focus on adult obligate sexual terrestrial invertebrates but also provide modifications for other animal groups and life-history stages. We first outline a "gold-standard" protocol for determining TFLs, focussing on the effects of short-term heat shocks and simulating more frequent extreme heat events predicted by climate models. As this approach may be difficult to apply to some organisms, we then provide a standardized TSF protocol. Finally, we provide a framework to quantify fertility loss in response to extreme heat events in nature, given the limitations in laboratory approaches. Applying these standardized approaches across many taxa, similar to CTLs, will allow robust tests of the impact of fertility loss on species responses to increasing temperatures.

Sexual selection and the evolution of costly mating strategies can negatively impact population viability and adaptive potential. While laboratory studies have documented outcomes stemming from these processes, recent observations suggest that the demographic impact of sexual selection is contingent on the environment and therefore may have been overestimated in simple laboratory settings. Here we find support for this claim. We exposed copies of beetle populations, previously evolved with or without sexual selection, to a 10-generation heatwave while maintaining half of them in a simple environment and the other half in a complex environment. Populations with an evolutionary history of sexual selection maintained larger sizes and more stable growth rates in complex (relative to simple) environments, an effect not seen in populations evolved without sexual selection. These results have implications for evolutionary forecasting and suggest that the negative demographic impact of sexually selected mating strategies might be low in natural populations. Sexual selection can both aid and impede evolutionary rescue. This study shows that the complexity of the environment can play a central role in how sexual selection impacts population viability and adaptive potential, and suggests that complex environments may increase the net benefit of sexual selection.image

An often-overlooked aspect of life-history optimization is the allocation of resources to protect the germline and secure safe transmission of genetic information. While failure to do so renders significant fitness consequences in future generations, germline maintenance comes with substantial costs. Thus, germline allocation should trade-off with other life history decisions and be optimized in accordance with an organism’s reproductive schedule. Here we tested this hypothesis by studying germline maintenance in lines of seed beetle, selected for early (E) or late (L) reproduction for 350 and 240 generations, respectively. Female animals provide maintenance and screening of male gametes in their reproductive tract and oocytes. Here, we revealed the ability of young and aged E and L-females to provide this form of germline maintenance by mating them to males with ejaculates with artificially elevated levels of protein and DNA damage. We find that germline maintenance in E-females peaks at young age and then declines, while the opposite is true for L-females, in accordance with the age of reproduction in respective regime. These findings identify the central role of allocation to secure germline integrity in life history evolution and highlight how females can play a crucial role in mitigating effects of male germline decisions on mutation rate and offspring quality.

Climates are changing rapidly, demanding equally rapid adaptation of natural populations. Whether sexual selection can aid such adaptation is under debate; while sexual selection should promote adaptation when individuals with high mating success are also best adapted to their local surroundings, the expression of sexually selected traits can incur costs. Here we asked what the demographic consequences of such costs may be once climates change to become harsher and the strength of natural selection increases. We first adopted a classic life history theory framework, incorporating a trade-off between reproduction and maintenance, and applied it to the male germline to generate formalized predictions for how an evolutionary history of strong postcopulatory sexual selection (sperm competition) may affect male fertility under acute adult heat stress. We then tested these predictions by assessing the thermal sensitivity of fertility (TSF) in replicated lineages of seed beetles maintained for 68 generations under three alternative mating regimes manipulating the opportunity for sexual and natural selection. In line with the theoretical predictions, we find that males evolving under strong sexual selection suffer from increased TSF. Interestingly, females from the regime under strong sexual selection, who experienced relaxed selection on their own reproductive effort, had high fertility in benign settings but suffered increased TSF, like their brothers. This implies that female fertility and TSF evolved through genetic correlation with reproductive traits sexually selected in males. Paternal but not maternal heat stress reduced offspring fertility with no evidence for adaptive transgenerational plasticity among heat-exposed offspring, indicating that the observed effects may compound over generations. Our results suggest that trade-offs between fertility and traits increasing success in postcopulatory sexual selection can be revealed in harsh environments. This can put polyandrous species under immediate risk during extreme heat waves expected under future climate change.

Evolutionary theory assumes that mutations that cause aging either have beneficial early-life effects that gradually become deleterious with advancing age (antagonistic pleiotropy [AP]) or that they only have deleterious effects at old age (mutation accumulation [MA]). Mechanistically, aging is predicted to result from damage accumulating in the soma. While this scenario is compatible with AP, it is not immediately obvious how damage would accumulate under MA. In a modified version of the MA theory, it has been suggested that mutations with weakly deleterious effects at young age can also contribute to aging, if they generate damage that gradually accumulates with age. Mutations with increasing deleterious effects have recently gained support from theoretical work and studies of large-effect mutations. Here we address if spontaneous mutations also have negative effects that increase with age. We accumulate mutations with early-life effects in Drosophila melanogaster across 27 generations and compare their relative effects on fecundity early and late in life. Our mutation accumulation lines on average have substantially lower early-life fecundity compared to controls. These effects were further maintained throughout life, but they did not increase with age. Our results suggest that most spontaneous mutations do not contribute to damage accumulation and aging.

Elevated temperatures are expected to rise beyond what the physiology of many organisms can tolerate. Behavioural responses facilitating microhabitat shifts may mitigate some of this increased thermal selection on physiology, but behaviours are themselves mediated by physiology, and any behavioural response may trade-off against other fitness-related activities. We investigated whether experimental evolution in different thermal regimes (Cold: 15 °C; Hot: 31 °C; Intergenerational fluctuation 15/31 °C; Control: 23 °C) resulted in genetic differentiation of standard locomotor activity in the dung fly Sepsis punctum. We assessed individual locomotor performance, an integral part of most behavioral repertoires, across eight warm temperatures from 24 °C to 45 °C using an automated device. We found no evidence for generalist-specialist trade-offs (i.e. changes in the breadth of the performance curve) for this trait. Instead, at the warmest assay temperatures hot-selected flies showed somewhat higher maximal performance than all other, especially cold-selected flies, overall more so in males than females. Yet, the flies' temperature optimum was not higher than that of the cold-selected flies, as expected under the 'hotter-is-better' hypothesis. Maximal locomotor performance merely weakly increased with body size. These results suggest that thermal performance curves are unlikely to evolve as an entity according to theory, and that locomotor activity is a trait of limited use in revealing thermal adaptation.

Male animals often show higher mutation rates than their female conspecifics. A hypothesis for this male bias is that competition over fertilization of female gametes leads to increased male investment into reproduction at the expense of maintenance and repair, resulting in a trade-off between male success in sperm competition and offspring quality. Here, we provide evidence for this hypothesis by harnessing the power of experimental evolution to study effects of sexual selection on the male germline in the seed beetle Callosobruchus maculatus. We first show that 50 generations of evolution under strong sexual selection, coupled with experimental removal of natural selection, resulted in males that are more successful in sperm competition. We then show that these males produce progeny of lower quality if engaging in sociosexual interactions prior to being challenged to surveil and repair experimentally induced damage in their germline and that the presence of male competitors alone can be enough to elicit this response. We identify 18 candidate genes that showed differential expression in response to the induced germline damage, with several of these previously implicated in processes associated with DNA repair and cellular maintenance. These genes also showed significant expression changes across sociosexual treatments of fathers and predicted the reduction in quality of their offspring, with expression of one gene also being strongly correlated to male sperm competition success. Sex differences in expression of the same 18 genes indicate a substantially higher female investment in germline maintenance. While more work is needed to detail the exact molecular underpinnings of our results, our findings provide rare experimental evidence for a trade-off between male success in sperm competition and germline maintenance. This suggests that sex differences in the relative strengths of sexual and natural selection are causally linked to male mutation bias. The tenet advocated here, that the allocation decisions of an individual can affect plasticity of its germline and the resulting genetic quality of subsequent generations, has several interesting implications for mate choice processes.