Journal of Evolutionary Biology

O_LIDispersal is favoured in highly heterogenous and ephemeral environments where it can increase fitness. However, whilst we generally assume that unfavourable local conditions cue dispersal to elsewhere, little is known about how heterogeneity in local conditions influences dispersal expression. This is an important element of dispersal, since dispersal characteristics rely on such local cues to signal their development. C_LIO_LIWe assessed the role of local environmental heterogeneity on the expression of a plastic dispersal phenotype in experimental populations of the bulb mite Rhizoglyphus robini through a multigenerational, 18-week experiment. Populations were kept under either heterogenous, homogenous, or intermediate food distribution treatments, and at low or high population density, via culling. This allowed us to test for the effect of local environmental heterogeneity and competition on dispersal expression, but also on population size and growth. C_LIO_LIWe found that heterogenous environments induce greater expression of dispersal in high density environments, but not in low-density ones. Additionally, whilst culled treatments have on average larger population sizes than non-culled treatments, non-culled and heterogenous environments have similar sizes to culled treatments. Treatments did not impact population growth rate. C_LIO_LIOur findings that heterogenous treatments only induce greater dispersal expression under non-culled treatments suggests that heterogeneity may exacerbate competition around feeding patches, causing infrequent access to food. We also infer that culled populations become larger than non-culled, at least in the short term, due to rapid compensatory growth causing an overrepresentation of juveniles in the population. It is unclear why unculled heterogenous environments exhibit a similarly large population size to culled environment but could be due to food being hard to access but easy to locate. C_LIO_LIOverall, we find an ecologically significant impact of local heterogeneity on population size and dispersal expression, much of which may arise due to the influences of heterogeneity on competition around foraging spaces. Such findings highlight the need to distinguish between local and regional heterogeneity when modelling the evolution of dispersal, since these two alternate levels may perform significantly different roles in influencing the expression or adaptive value of dispersal. C_LI

Biological trade-offs present a central issue for evolutionary biology: it has been a fundamental understanding within the field that limits exist on the phenotypic traits a species is able to exhibit in part due to trade-offs. Reproduction--with its myriad forms--has been studied extensively in the context of these dynamics. And while considerable literature has explored trade-offs between seed size and number and their associated environmental conditions, none has looked at spore size trade-offs in ferns. We can hypothesise potential trade-offs in spore size: smaller spores should be able to disperse farther, but may not have sufficient provisions to survive in environments that require them to remain at the gametophyte stage for longer periods if their germination cues are mismatched. Reproductive mode (sexual vs. asexual) and ploidy may also be playing a role. In order to study trade-offs related to spore size, I focus on the Australasian fern species Cheilanthes distans (Pteridaceae), which is most often found in xeric environments, growing in crevices or on top of rocks which are haphazardly scattered across their range. Apomictic diplospores in this species are formed through first division restitution, a meiotic pathway particularly prone to mistakes in chromosome pairing and cell division (as compared to premeiotic endomitosis). Rather than being problematic, these mistakes ultimately lead to considerable additional variation in spore size, spore products (through a range of aneuploid spores), and spore ploidy. In this study, I explore trade-offs between spore size, dispersal, and germination, taking into account effects from reproductive mode and ploidy. I carried out an extensive survey of C. distans specimens to establish the prevalence of sexual vs. apomictic (asexual) specimens, and to describe in greater depth the variation in ploidy across the species. I also collected data on spore size and sporogenesis forms. With these data I then asked: is spore size correlated with range area or with germination? And does spore form correlate with either spore size or germination? Ultimately, I find that variations in sporogenesis may be leading to large variation in spore sizes--especially since spores traditionally considered abortive are in fact viable--and that this variation may provide abundant fodder for evolution to act through trade-offs between dispersal into large ranges and germination leading to establishment. Especially in light of the fact that many spores that were historically considered abortive are fully viable and likely shaping evolution in important ways, it is worth remarking on what these results illustrate more broadly: the way in which we have constructed disability ultimately affects how we perceive so-called genetic errors--both in humans and in other species--and thus limits what we allow ourselves to imagine disabled beings are capable of.

The interaction between multiple secondary sexual characters simultaneously expressed by an organism remains poorly understood. In fact, different interpretations of the role of concurrently expressed signals predict that the correlation of their elaboration across species should be positive, negative or absent --all three possibilities have been supported by studies of different avian groups. In this context, we analysed the interaction between plumage and song elaboration in the passerine family Fringillidae (finches). We used a two-scale approach and studied this association both at the family level and in more detail within the genera Crithagra and Spinus. At both scales, we determined colour elaboration (defined by the number of plumage colour patches in each species and their contrast) and established its association with three aspects of vocal elaboration: energy investment (song length and rate of syllable production), song complexity (repertoire index) and vocal performance (vocal deviation). We found an absence of association between colour elaboration and the three evaluated aspects of song elaboration at both scales, which has been the most frequent result in other avian groups. At first glance, this result appears to be partially discordant with previous findings in finches, but considering these studies together this might not necessarily be the case. We also confirmed that finches have complex songs and a remarkable vocal performance compared to other families of passerines. Considered together, our results suggest an independent evolution of colour and the notable vocal elaboration present in finches.

Sexual traits may be selected during multiple consecutive episodes of selection, occurring before, during, or after copulation. The overall strength and shape of selection acting on sexually selected traits may thus be determined by how selection (co-)varies along different episodes. However, it is challenging to measure pre- and postcopulatory phenotypic traits alongside variation in fitness components at each different episode. Here, we used a transgenic line of the transparent flatworm Macrostomum lignano expressing green fluorescent protein (GFP) in all cell types, including sperm cells, enabling in vivo sperm tracking. We exposed GFP(+) focal worms to three groups in which we assessed their mating success, sperm-transfer efficiency, and sperm fertilising efficiency. Moreover, we measured 13 morphological traits on the focal worms to study the fitness landscape in multivariate trait space. We found linear selection on sperm production rate arising from pre- and postcopulatory components, and on copulatory organ shape arising from sperm fertilising efficiency. We further found nonlinear (mostly concave) selection on combinations of copulatory organ and sperm morphology traits arising mostly from sperm-transfer efficiency and sperm fertilising efficiency. Our study shows that contrasting patterns of phenotypic selection are observed by measuring how sexual selection builds-up over consecutive episodes of selection.

Organisms alter their phenotype in response to variation in their environment by expressing phenotypic plasticity. Both sexes exhibit such plasticity in response to contrasting environmental and social cues, and this can reflect the influence of sexual conflict. However, theory predicts that plasticity expressed by both sexes may either maximise the sex-specific fitness of both, or of one sex at the expense of the other. Hence empirical tests of the predictions are sorely needed. Here we conducted novel tests of the fitness effects of interacting reproductive plasticity in Drosophila melanogaster. First, prior to mating, males were kept alone, or with same sex rivals, and females were kept alone, in same sex, or mixed sex groups. Second, we conducted matings between individuals from all these social treatments under choice and no choice scenarios. The results showed that males and females can both plastically respond to these socio-sexual environments to influence the expression of mating duration, mating latency, and fecundity. These plastic responses interacted significantly to determine mating latency and fecundity. Effects on mating latency were also observed under both choice and no-choice conditions, but in opposing directions. Variation in the outcome of interacting plasticity pivoted around the outcomes observed with focal females that had been maintained in same-sex environments prior to mating. However, not all fitness-related traits examined responded in the same way. Mating duration was determined largely by the social environment of the male. Our results show that the expression of some, but not all fitness-related reproductive traits can be determined by the outcome of interacting behavioural plasticity expressed by both sexes. This highlights the need for new predictive theory informed by these empirically-derived parameters. Overall, we conclude that variation in the expression of shared traits due to interacting plasticity represents an important and novel facet of sexual interactions. Impact SummaryAnimals and plants are able to respond to variation in their environment by expressing phenotypic plasticity. In sexual organisms, both males and females can exhibit such plasticity but the cues they respond to and the fitness consequences of these actions may be different between the sexes, and even conflicting. For example, males may respond to the presence of competitors by altering their mating behaviour or ejaculate transfer to increase their own, but not necessarily their mates reproductive output. However, females may also express phenotypic plasticity in response to their social and sexual environment to maximise their own fitness. Theory suggests that plasticity expressed by both sexes may either maximise the sex-specific fitness of both, or of one sex at the expense of the other. So far, little experimental work has been conducted to explore such interacting plasticity. Here we conducted novel tests of the fitness effects of interacting plasticity in the fruit fly Drosophila melanogaster. In doing so, we provide novel experimental evidence for interacting behavioural plasticity. We show that males and females can plastically respond to their socio-sexual environment to influence the expression of mating duration, mating latency, and fecundity. These plastic responses, while induced to increase the fitness interests of each sex, interact in the case of mating latency and fecundity and may reflect the outcome of sexual conflict. Our findings suggest that studies of reproductive behaviour should carefully consider the socio-sexual environment of both males and females and highlight the need for new predictive theory informed by empirically-derived parameters. Overall, we show that interacting plasticity between sexes represents an important and novel facet of sexual interactions.

Organisms are often coinfected by more than one parasite species. These coinfections can alter the fitness of both host and its coinfecting parasites. Changes in host fitness are usually estimated by comparing parasite virulence under coinfection to the virulence of each parasite under single infection. While many studies focus on host survival as proxies of virulence, virulence can be expressed via reduced host fecundity. Here, we studied the outcome of coinfection of water fleas (Daphnia magna) by two microparasitic species, the fungus Metschnikowia bicuspidata and the microsporidium Hamiltosporidium tvaerminnensis. We found that Metschnikowia expressed its virulence mainly via host mortality and Hamiltosporidium expressed it mainly via host reproduction. Despite their competitive interaction, indicated by their reduced spore production, both parasites were able to fully express their virulence during coinfection. Host survival was dominated by Metschnikowia while clutch-size was dominated by Hamiltosporidium. Furthermore, coinfected hosts experienced increased virulence that was manifested only by additional reduction in reproduction (less clutches were released). Our results demonstrate that focusing only on the survival component of host fitness can miss important outcomes of coinfections. Since reduced host fecundity is a common outcome of parasitism, the influence of coinfection on host reproduction deserves more attention.

Alleles with opposing effects on fitness characters are said to exhibit selectional antagonistic pleiotropy (broadly construed so that effects are not necessarily confined to the same individual). A number of theoretical investigations considered the case where a pair of alleles at a locus influences two fitness components and derived the conditions giving rise to stable polymorphism under various assumptions about the mode of trait-interaction. Strikingly, many of these analyses concluded that the potential for maintaining polymorphism is strongly constrained by the joint influence of two factors: (1) the prevalence of weak selection coefficients over coefficients of large magnitude, and (2) the absence of beneficial dominance reversals (where the deleterious effects of each allele are partially or completely masked in the heterozygous genotype). Consequently, the conclusion that selective polymorphism is unlikely to be maintained by intralocus mechanisms of antagonistic pleiotropy has achieved widespread acceptance. Here we argue that such conclusions do not apply to any of the following models of antagonism: (i) additive trait-interaction, (ii) multiplicative trait-interaction, (iii) bivoltine selection, (iv) soft selection, (v) hard selection, and (vi) sexual antagonism. We demonstrate that the parameter space giving rise to stable allelic variation is quite large throughout, and moreover, the plenitude of suitable parameters neither depends on the strength of selection nor requires dominance reversal. Dominance coefficients associated with stringent conditions for stable polymorphism are shown to be atypical as compared to all feasible parameters, and best regarded as an outcome of adherence to a special relation: dominance with a constant magnitude and direction, which includes the case of additive allelic effects at a locus. Properties of single-locus equilibria (heterozygosity, allele frequency differentiation) are investigated, as well as the contribution of dominance schemes to the genetic variance in fitness characters in populations at multilocus linkage equilibrium. Author summaryAllelic variants at a locus with opposing effects on multiple fitness components (antagonistic fitness pleiotropy) have long been appreciated as a possible source of balancing selection. The prevalence of polymorphism owing to this form of natural selection, however, has been doubted on theoretical grounds due to the fact that standard assumptions of genetic models (namely, constant magnitudes for the dominance coefficients) are hardly conducive to the maintenance of polymorphism. The major exception to this conclusion lies with schemes that exhibit dominance reversal (where the direction of dominance for antagonistic alleles flips across fitness components). Here we conduct a geometric analysis of the space of polymorphism-promoting dominance parameters and conclude that the conditions for maintaining balanced alleles is unrestrictive, with non-reversals playing an underappreciated role.

The proportion of adult males to females in the adult population, the adult sex ratio (ASR), is an important demographic parameter that has implications in sexual selection, ecology and conservation. ASR variation can be multifactorial but specific variables including sex roles - sex differences in courtship, mate competition, social pair-bonds and parental care- and sex-specific mortality have been suggested as important ASR determinants in birds. However, these relationships have not yet been comprehensively tested in specific avian groups. Here, we used phylogenetic comparative methods to study drivers of ASR variation across shorebirds (Charadrii and Scolopaci; n = 205 species), a charismatic bird group characterised by displaying extreme variations in ecology, sex roles and sexual selection traits. We found that ASR variation is associated with most sex role components but not with their sex-specific mortality. Although sex role and life history variables showed no significant sex bias, we found a trend towards reversed size dimorphism and sex role reversal across shorebirds. Sex roles components also showed correlations among each other that were surprisingly strong and in unexpected directions. Our results confirm that sex roles are important drivers of ASR variation and suggest that shorebirds may have alternative means of sex-specific mortality, possibly linked to their ecology.

Parental investment theory describes the ability of organisms to respond to an environmental challenge by increasing the fitness of future offspring. Utilizing life history changes, organisms can maximize fitness by increasing their total reproductive output or by investing more into the success of fewer offspring. In cases where parasitic infections result in castration of their host, increased reproductive effort known as fecundity compensation has been demonstrated in a variety of organisms. This response appears predictive of expected future reproductive losses. Organisms struggling with an environmental pathogen, may attempt to better prepare their offspring for the environment they are experiencing through transgenerational immune priming (TGIP). In immune priming, primary infection lowers the prevalence and intensity of a subsequent infection by the same pathogen. Transgenerational immune priming carries pathogen resistance into further generations without genotypic changes. The focus of this study was to determine whether invertebrate parental investment into offspring parasite resistance varies over the course of an infection. Utilizing the snail host Biomphalaria glabrata and its trematode parasite Schistosoma mansoni, offspring were reared from specific time intervals in the parents infection and subsequently exposed to the same pathogen when each cohort reached the same age- 12 weeks. Differences in infection prevalence and intensity were expected based on when the offspring were born during their parents infection. A trade-off was predicted between the number of offspring produced in a cohort and offspring resistance to future infections. Offspring born during the period of fecundity compensation were predicted to exhibit lower resistance due to a dilution of individual investment by parents into a larger offspring pool. While our results did not support TGIP, there were differences in offspring prevalence, as well as an indication that parent health may interact with genetics in offspring resistance. Results suggest that parental condition can influence resistance of B. glabrata offspring to S. mansoni but that TGIP may not be operating in this system.

Immunity and reproduction are two important processes that affect fitness of an organisms. Sexual activity has been previously shown to determine the degree to which a population is able to survive various infections. While many studies have demonstrated a trade-off between immune function and reproduction, many studies have found synergistic relation between the two fitness determinants. Besides it is generally hypothesised that sexes may differ in immunity due to relative cost they incur during reproduction with males losing in immunity to rather increase their reproductive success. In this study, we test the effect of immune function on the survival of mated and virgin replicates of a large outbred baseline D. melanogaster population that was infected with four different bacterial infections. We find enhanced survival in mated flies than virgin flies in response to all four bacterial infections across all replicates. While investigating sexual dimorphism in immune function, we find no difference in sexes in their ability to survive the imposed bacterial infections. Synergistic interaction between reproduction and immunity may exist if it improves Darwinian fitness of either sexes of a population under selection, and are not necessarily limited by each other due to trade-off over finite resources.

Sexual selection operates across pre- and post-copulatory episodes, driven by intra-sexual competition and inter-sexual choice for mates or gametes. In females, sexual selection often manifests as choosiness, however pre- and post-copulatory preferences can be in opposing directions. While relationships between male pre- and post-copulatory traits are well-studied, these relationships are less understood in females. Additionally, female post-copulatory environments can potentially influence post-copulatory choosiness, but this has been little investigated. Using Drosophila melanogaster, we explored whether mating latency, a proxy for inter-sexual pre-copulatory choosiness, correlates with female ejaculate ejection behaviour, a proxy for post-copulatory choosiness. We further tested whether the presence of a male in the females post-copulatory environment influences her ejection behaviour. We found no significant effect of male presence. However, males with longer mating latencies experience a smaller proportion of their sperm ejected, suggesting that males preferred as mates may produce sperm less favoured for fertilization. This finding might possible trade-offs between male investment in courtship and ejaculates. Our study highlights that female-mediated sexual selection at pre- and post-copulatory stages can shape sexual traits in complex ways. This has implications for sexual conflict, possibly providing an explanation for the maintenance of variation in sexually selected traits.

In bi-parental species, reproduction is not only a crucial life-history stage where individuals must take fitness-relevant decisions, but these decisions also need to be adjusted to the behavioural strategies of a partner. Hence, communication is required, which could be facilitated by condition-dependent signals of parental quality. Yet, these traits have (co-)evolved in multiple contexts within the family, as during reproduction different family members may coincide and interact at the site of breeding. In this study we explore whether a condition-dependent trait acts a quality signal and regulates intra-family interactions in a bird species, the blue tit (Cyanistes caeruleus). As a family is a complex network where signals could be perceived by multiple receivers, we expected that experimentally blocking the reflectance of an adults UV/yellow colouration of breast feathers may affect the behavioural strategies of all family members. We found an increase of parental investment in nests with an UV-blocked adult, as the partner compensated for the perceived lower rearing capacity. As the UV-blocked adult did not change its provisioning behaviour, as was to be expected, their partner must have responded to the (manipulated) signal but not to a behavioural change. However, offspring did not co-adjust their begging intensity to a signal of parental quality. Opposite to adults, we propose that offspring respond to the behaviour but not to the parental signal. Overall, our results show experimentally at the first time that UV/yellow colouration of blue tits acts as a quality signal revealing the rearing capacity to mates.

Dispersal is often essential for the attainment of Darwinian fitness, especially for species living on spatially structured, heterogeneous habitats. Theoretically, sex-specific resource requirement can drive the two sexes to disperse differently, resulting in sex biased dispersal (SBD). Understanding ecological factors affecting SBD is important. Using an experimental two-patch dispersal setup we measured spontaneous dispersal in laboratory adapted populations of Drosophila melanogaster under a set of common, interlinked ecological scenarios relating to - (a) dietary ecology and (b) adult density. We found deteriorating overall nutritional quality of food affects strength of SBD, and female dispersal is particularly sensitive to availability of protein. Adult density had sex specific effect on dispersal. Female dispersal was found to be density independent but males showed increased dispersal at higher density. Female tend to disperse more from male biased patch likely to avoid male harassment whereas absence of female drives male dispersal solidifying mate-finding dispersal hypothesis. These evidences of dispersal suggest that variation in dietary ecology and intraspecific competition can affect the degree and strength of existing SBD and thereby male-female interactions in a patch potentially affecting fitness components and population dynamics.

Density-dependent selection, especially together with r-K trade-offs, has been one of the most plausible suggested mechanisms for the evolution of population stability. However, experimental support for this explanation has been both meagre and mixed. One study with Drosophila melanogaster yielded no evidence for populations adapted to chronic larval crowding having also evolved greater population stability. Another study, on D. ananassae, suggested that populations adapted to larval crowding evolved both greater constancy and persistence stability, and the data also suggested an r-K trade-off in those populations, though the evidence for the latter was not conclusive. Moreover, theoretical work suggested that density-dependent selection could result in the evolution of greater population stability, even in the absence of an r-K trade-off. Here, we show that populations of D. melanogaster, selected for adaptation to larval crowding at very low food amounts per vial, evolve enhanced constancy and persistence stability. The enhanced population stability in the crowding-adapted populations seems to have evolved through the increased equlibrium size (K) and reduced sensitivity of realized population growth rates to density ().There was no clear evidence for reduced intrinsic population growth rate (r) in the more stable crowding-adapted populations. Our study adds to the growing evidence in support of the hypothesis that population stability can evolve in response to density-dependent selection through the evolution of certain life-history traits that are associated with higher K and less negative . We discuss our results in the light of previous work, and suggest that a model-free framework might be of great heuristic value in understanding the evolution of population stability through changes in the density-sensitivity of life-history traits, whether or not these changes result from density-dependent selection.

Mounting an immune response requires a considerable energy investment by the host. This makes expression of immune phenotypes susceptible to changes in availability of resources. There is ample evidence in scientific literature to suggest that hosts become more vulnerable to infection by pathogens and parasites when access to nutrition is limited. Using populations of Drosophila melanogaster experimentally evolved to better survive bacterial infections, we explore if host selection history influences host response to resource deprivation in terms of immune function. We find that when reared on a suboptimal diet (both in larval and adult stages), adult flies from evolved populations are still more immune to infections compared to flies from control populations. Furthermore, we observe a sex-dependent effect of interaction between selection history and diet on immune function. We thus conclude that immune function of hosts adapted to pathogen challenge is less affected by resource limitation compared to non-adapted hosts.

Reproductive Character Displacement (RCD) often occurs when species with mating-related polymorphism come into secondary contact, leading to divergence in reproductive traits. Ischnura elegans and Ischnura graellsii have formed two independent hybrid zones in Spain where reinforcement has strengthened a mechanical barrier, and RCD has shaped mating-related structures, although reinforcement is asymmetric only in gynochrome females. This study examines the link between asymmetric reinforcement and asymmetric RCD. Using geometric morphometrics, we analyze prothorax shape and size in both female morphs and males, and male caudal appendages, to assess morphological divergence, determine whether gynochrome females show stronger divergence, and test for morphological covariation between male traits involved in the tandem position. Our results reveal consistent patterns of size and shape variation across species and zones: in I. elegans, androchromes are larger and resemble males in size, with clear shape differentiation between female morphs that diminishes in hybrid zones. In contrast, I. graellsii shows less consistent size differences between males and morphs, and weaker shape differentiation. Our results confirm RCD in prothorax shape in I. elegans females from both hybrid zones, but reveal that RCD in prothorax size is asymmetric, occurring only in gynochrome females from the NC hybrid zone. We also detected RCD in the prothorax shape of I. elegans males from the NC hybrid zone, extending previous evidence of RCD in male caudal appendages, while morphological covariation between male cerci and the prothorax was limited to size in I. elegans. Together, these findings illustrate how hybridization may generate morph-specific patterns of reproductive divergence.

The predictability of evolution is expected to depend on the relative contribution of deterministic and stochastic processes. This ratio is modulated by effective population size. Smaller effective populations harbor less genetic diversity and stochastic processes are generally expected to play a larger role, leading to less repeatable evolutionary trajectories. Empirical insight into the relationship between effective population size and repeatability is limited and focused mostly on asexual organisms. Here, we tested whether fitness evolution was less repeatable after a population bottleneck in obligately outcrossing populations of Caenorhabditis elegans. Replicated populations founded by 500, 50, or 5 individuals (no/moderate/strong bottleneck) were exposed to a novel environment with a different bacterial prey. As a proxy for fitness, population size was measured after one week of growth before and after 15 weeks of evolution. Surprisingly, we found no significant differences among treatments in their fitness evolution. Even though the strong bottleneck reduced the relative contribution of selection to fitness variation, this did not translate to a significant reduction in the repeatability of fitness evolution. Thus, although a bottleneck reduced the contribution of deterministic processes, we conclude that the predictability of evolution may not universally depend on effective population size, especially in sexual organisms.

Interlocus sexual conflict is predicted to result in sexually antagonistic coevolution between male competitive traits, which are also female-detrimental, and mate harm resistance (MHR) in females. Little is known about connection life-history evolution and sexually antagonistic coevolution. Here, we investigated the evolution of MHR in a set of experimentally evolved populations, where mate-harming ability has been shown to have evolved in males as a correlated response to the selection for faster development and early reproduction. We measured mortality and fecundity of females of these populations and those of their matched controls, under different male exposure conditions. We observed that the evolved females were more susceptible to mate harm - suffering from significantly higher mortality under continuous exposure to control males within the twenty-day assay period. Though these evolved females are known to have shorter lifespan, such higher mortality was not observed under virgin and single-mating conditions. We used fecundity data to show that this higher mortality in evolved females is unlikely due to cost of egg production. Further analysis indicated that this decreased MHR is unlikely to be due purely to the smaller size of these females. Instead, it is more likely to be an indirect experimentally evolved response attributable to the changed breeding ecology, and/or male trait evolution. Our results underline the implications of changes in life history traits, including lifespan, to the evolution of MHR in females.

Camouflage and warning signals are contrasted prey strategies reducing predator attack, which offer an excellent opportunity to study the evolutionary forces acting on prey appearance. Edible prey are often inconspicuous and escape predation by remaining undetected. Predators learn to find the most common ones, leading to apostatic selection (advantage to rare morphs) enhancing variation in cryptic prey. By contrast, defended prey are often conspicuous and escape predation by using warning colorations identifying them as unprofitable. Predators avoid the ones they are most familiar with, leading to positive frequency-dependence and warning signal uniformity. It is less clear, however, what happens when two morphs of the same species vary strongly in conspicuousness, and how to explain the maintenance of cryptic and conspicuous morphs within populations, in the case of profitable prey. Using the white and melanic morphs of the invasive Box Tree Moth (Cydalima perspectalis) presented at three different frequencies, we investigate whether a) caterpillars and adult moths are palatable for birds, b) the less conspicuous, melanic morph experiences lower predation rates and b) whether frequency-dependence balances morph frequencies. Our results suggest that the melanic morph enjoys a survival advantage owing to a lower visibility. However, our experiments show that, unexpectedly, the two color morphs experience opposite patterns of frequency-dependent predation, despite being both fully palatable to birds. The melanic morph is under apostatic selection, whereas the conspicuous, white morph is subject to positive frequency-dependence (safety in numbers). Our experiments also show some level of unpalatability in the caterpillars. These results offer novel insight into how predation triggers contrasting evolutionary patterns in a palatable, polymorphic species within two morphs that differ markedly in conspicuousness and within two different life stages. Lay summaryUnderstanding the factors influencing character variation in natural populations is a key question in evolutionary ecology. Predation is one of the main drivers of color evolution in prey communities and prey usually mitigate predation using camouflage or warning colors. Camouflage evolves because it lowers the probability of being detected by predators. Since predators are more efficient at finding prey which they are familiar with, prey which display a rare phenotype are favoured (negative frequency-dependent selection). By contrast, aposematism is defined by conspicuous appearance in toxic or otherwise unprofitable prey, and evolves because birds identify defended prey by learning to use their appearance as a warning signal. The most common signals are usually best identified and avoided (positive-frequency dependent selection). It is not clear, however, how these two forces combine when predators are facing cryptic and conspicuous morphs of the same species, and how to explain their coexistence. Here we investigate this question in a laboratory experiment, by presenting wild birds with a melanic and a white morph of the same moth. Unexpectedly, our results show that despite being both fully palatable to birds, the two color morphs are subject to very different types of selection depending on their frequencies. The melanic morph is favored when it is rare, the conspicuous white morph as it gets common. The simultaneous action of these forces may contribute to maintain color polymorphism in natural populations. We also show that caterpillars of this species are unpalatable and chemically defended, whereas adults are not, showing opposite strategies of predator defense in different life stages of the same species.

Many simultaneous hermaphrodites use selfing for reproductive assurance only when outcrossing opportunities are limited, owing to inbreeding depression in selfed progeny. However, scenarios that enforce substantial selfing (such as during recolonisation) can rapidly select for a high selfing propensity, a shift in mating system that is expected to eliminate both inbreeding depression and the delayed reproductive onset under selfing that is typically associated with it. We tested these predictions in the flatworm Macrostomum hystrix, using a line derived from an outcrossing population that had been subjected to enforced selfing for multiple generations followed by several years of relaxed selection. As predicted, isolated individual forced to self and individuals with constant partner access (i.e. outcrossing opportunities) did not differ in reproductive onset nor in inbreeding depression estimated through offspring survival. However, a third treatment group that provided intermittent partner access (to allow outcrossing but minimise potential competition effects) showed a different pattern: no inbreeding depression in offspring but a substantially accelerated reproductive onset. Whilst our results thus support the effective purging of inbreeding depression and increased selfing propensity under enforced selfing, we suggest that cues of an unstable social or physical environment nevertheless exert a major influence on reproductive timing.