Journal of Evolutionary Biology

Reinforcing natural selection against maladaptive hybrids can favor the strengthening of premating reproductive isolation driving a pattern of Reproductive Character Displacement (RCD). In a recent study conducted in North-West (NW) (older) Spanish hybrid zone, was detected an asymmetric reinforcement of the mechanical isolation in the reciprocal cross direction between I. graellsii males and I. elegans females. Furthermore, in the North-Central and Mediterranean (NCM) (younger) hybrid zone was also detected a similar strengthening of the mechanical isolation, consistent with a pattern of asymmetric reinforcement in this hybrid zone as well. In this study, we did geometric morphometrics analyses, shape, and Centroid Size (CS), on male and female secondary sexual traits to investigate whether reinforcement has generated a pattern of RCD of these traits in both hybrid zones. We detected, in the NW hybrid zone, unidirectional RCD (CS) of the male caudal appendages of I. graellsii, and bidirectional RCD (shape) of the female prothorax. Consistently with the prediction that the signal of reinforcement may diminish rapidly once reinforcement ceases to operate, a stronger signal of RCD was detected in the NCM than in the NW hybrid region. In this region, was detected unidirectional RCD (CS) of the male caudal appendages which was consistent with the lock-and-key mechanism of genital coevolution, as well as RCD (shape) of the female prothorax of I. elegans. Interestingly, our study highlights the importance of using geometric morphometrics to deal with the complexity of reproductive structures and controlling for environmental and geographic factors to investigate RCD.

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.

In many species that fight over resources, individuals use specialized structures to gain mechanical advantage over their rivals during contests (i.e., weapons). Although weapons are widespread across animals, how they affect the probability of winning contests is still debated. According to theory, understanding the weapons function in contests depends on identifying differences in how weapons are measured (e.g., weapon length versus shape), and in how weapons are used during fights. Here, we developed a meta-analysis spanning 1,138 studies, from which were drawn 52 species and 107 effect sizes to identify: (1) what aspects of animal weapons are measured in the literature, and how these measures bias our knowledge; (2) how animals use their weapons during fights - i.e., weapon function; and (3) if weapon function correlates to the magnitude of how weapons influence contest resolution. First, we found that most of the literature focuses on linear measures of weapons, such as length. The few reports on weapon performance (e.g., biting force) were found only for Crustacea and Squamata. This bias highlights that measuring performance of weapons such as horns and spines might increase the breadth of our knowledge on weapons. Furthermore, we also found that linear measures showed stronger effects on contest success than performance measures. Second, we divided weapon function into displays and fighting style (i.e., how the weapon is used during fights). Regarding displays, most species displayed their weapons before contests (59.61%), rather than the body (34.61%). A minority (three species, 5.76%) engaged in fights without any type of display. Thus, species that bear weapons almost always perform displays before engaging in physical contact, a common hypothesis in contest theory that was never tested across taxa until now. Regarding fighting style, we found that most weapons were used for more than one behaviour during fights (e.g., squeezing and pushing). Further, pushing seems to be the most common behaviour among species, but it is usually accompanied by another behaviour, such as lifting or squeezing. Thus, oversimplifying fighting style can bias results because some styles might impose contrasting biomechanical pressures (e.g., pushing vs squeezing). Third, we found that display type did not influence the importance of weapon size on contests. Fighting style, on the other hand, influenced the effect of weapon size on contest outcome significantly. Species that used their weapons to impact, pierce or squeeze showed smaller differences between winners and losers when compared to pushing or lifting (and multifunctional weapons). Thus, pushing and lifting seem important for selecting larger weapons - even though some of them might also be used for squeezing, piercing or impacting. Overall, our results show that we have a biased understanding of animal weapons, built mostly on weapon size alone. Further, our analyses show that the importance of weapon size differs depending on the fighting style. If we lessen those biases, we will have a better and broader understanding of how weapons evolve and diversify.

In polyandrous species, competition between males for offspring paternity goes on after copulation through the competition of their ejaculates for the fertilisation of females oocytes. Given that males allocating more spermatozoa are favored, different models of sperm competition predict adaptive plasticity in male sperm production and allocation. These predictions were tested experimentally in the black soldier fly (BSF) Hermetia illucens. In this farmed insect, adult biology is little known despite the economic interest of larvae for bioconversion and as an animal feedstuff. Two sets of experiments were carried out to modify the risk of sperm competition perceived by males. The first consisted of placing adult males alone or in groups of 10 - modifying mean risk of sperm competition - and then measuring their sperm production. The second took place at the beginning of copulation; pairs with males from the two mean risk of sperm competition treatments were transferred to different contexts of immediate risk of sperm competition (empty cages, cages containing 10 males, or cages containing 10 females) and the number of spermatozoa stored by the females was counted. Males reared in groups of 10 showed more spermatozoa in their seminal vesicles than males reared alone. Regarding sperm allocation, females that mated in the presence of conspecifics - either 10 males or 10 females - stored more spermatozoa than those that mated alone. This study shows that sperm production and allocation are dependent on sperm competition risk in BSF, revealing a plasticity of reproduction under socio-sexual situations.

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.

We ask if three decades and over 1 500 generations of divergent life history selection on age at reproduction has resulted in the evolution of reproductive isolation (RI) between laboratory populations of Drosophila melanogaster. We tested for premating, postmating-prezygotic and postzygotic reproductive isolation between 3 replicate population pairs. Large evolved differences in body size between selection treatments suggested the potential for prezygotic barriers driven by sexual selection or physical incompatibilities between the sexes. Although a simple prediction would be preference for larger size, creating directional isolation, our results from individual mate choice trials indicate that populations from both selection treatments show a marked bias towards homotypic mate choice; indicative of prezygotic RI driven by sexual selection or sexual conflict. Hybridization between the focal populations resulted in the production of viable adult flies with intermediate size and developmental traits. We observed a suggestive but statistically non-significant trend of fitness decline in the F2 generation of hybrids, but no significant evidence suggesting the evolution of postmating-prezygotic or postzygotic RI. Our findings are in accord with extant literature that posits that premating RI evolves before postmating forms of RI.

Males and females evolved distinct life-history strategies, reflected in diverse inter-linked life-history traits. The sex that allocates more resources towards offspring relies on an increased life span, and long life requires an efficient immune system. The other sex needs to attract mates and thus allocates its resources towards ornamentation, which may negatively correlate with investment into the immune defense. Such sex-specific resource allocation trade-offs are not always strictly female or male-specific but may depend on the overall resources allocated towards life-history traits. Informed by experimental data, we designed a theoretical framework that combines multiple life-history traits. We disentangled specific life-history strategies from particular sex, allowing us to include species with reversed sex-roles and male parental investment. We computed the lifetime reproductive success (combining fitness components from diverse sex-specific life-history traits) observing a strong bias in adult sex ratio depending on sex-specific resource allocation towards life-history traits. Overall, our work provides a generalized method to combine various life-history traits with sex-specific differences to calculate lifetime reproductive success. The results explain specific population-level empirical observations as a consequence of sexual dimorphism in life-history traits.

D. melanogaster laboratory populations subjected to selection for rapid development and early reproduction have been found to have evolved reduced adult body size and lower levels of inter-locus sexual conflict compared to their ancestral controls. We tested the contribution of a smaller body to the evolution of reduced sexual conflict in these populations, since body size differences are known to affect sexual conflict levels in this species. We cultured larvae from the control populations at high density to obtain flies as small as those from the selected populations. The effect of body size reduction on sexual conflict was asymmetric, with smaller body size resulting in reduced male manipulative ability but not female resistance to mating-induced harm. These results were not due to differences in behavioural patterns of smaller flies, such as differences in overall mating exposure of females to different types of males. We hypothesize that evolution for rapid development and the correlated reduction in body size has resulted in lower male manipulative ability, and sexually antagonistic co-evolution has lowered female resistance to such manipulations. These populations have also evolved incipient reproductive isolation from their controls, likely through sexual conflict (reported earlier), and our results support the view that this is an outcome of strong, directional selection for rapid development.

The evolution of alternative male reproductive strategies represents an intriguing evolutionary phenomenon. Divergent strategies are persistently at risk of local extinction or invasion, depending on the suites of traits expressed within and between morphs; hence, understanding the correlational selection that aligns reproductive strategies with behaviour, morphology and physiology is key to understanding the origin and maintenance of genetic polymorphisms. In the polychromatic painted dragon, Ctenophorus pictus, yellow, orange and red morphs are well characterised, but the blue morph has been historically absent from studied populations. Here we document the local distribution, morphology and male-contest interactions in a population where blue males are relatively common. We find that blue males express head colouration after a reaching a threshold body size, and that small blue males can reside in close proximity to other males; patterns consistent with a novel size-dependent conditional tactic within the suite of genetic strategies seen in this species. Condition-dependent, positively allometric throat bibs were non-randomly distributed among male morphs, implicating variation in correlational selection and the genetic architecture of the polymorphism. We were unable to definitively assign a morph that was superior in male competition but found that within morphs, male size was the determinant of competitive success, whilst between morphs it was not. Furthermore, contests between morphs were resolved with less aggression than contests within morphs, supporting the idea that badges resolve conflict, and that the invasion of new colour morphs may be facilitated by negative frequency dependent benefits to novel colour variants. These findings highlight the divergent phenotypic, genetic and selective environments that lead to the diversity of colour morphs.

One of the most pressing questions we face as biologists is to understand how climate change will affect the evolutionary dynamics of natural populations and how these dynamics will in turn affect population recovery. Increasing evidence shows that sexual selection favours population viability and local adaptation. However, sexual selection can also foster sexual conflict and drive the evolution of male harm to females. Male harm is extraordinarily widespread and has the potential to suppress female fitness and compromise population growth, yet we currently ignore its net effects across taxa, or its effects on local adaptation and evolutionary rescue. We conducted a comparative meta-analysis to quantify the impact of male harm on female fitness and found an overall negative effect of male harm on female fitness. Negative effects seem to depend on proxies of sexual selection, increasing in species with larger sexual size dimorphism and strong sperm competition. We then developed theoretical models to explore how male harm affects adaptation and evolutionary rescue. We show that, when sexual conflict depends on local adaptation, population decline is reduced, but at the cost of slowing down genetic adaptation. This trade-off suggests that eco-evolutionary feedbacks on sexual conflict can act like a double-edge sword, reducing extinction risk by buffering the demographic costs of climate change, but delaying genetic adaptation. However, variation in the mating system and male harm type can mitigate this trade-off. Our work shows that male harm has widespread negative effects on female fitness and productivity, identifies potential mechanistic factors underlying variability in such costs across taxa, and underscores the importance of male harm on the demographic and evolutionary processes that impact how species adapt to environmental change. Impact summaryFor species to persist in the face of climate change, adaptation needs to be fast enough to prevent extinction. If population decline is too abrupt, adaptation will be less likely to promote recovery, leading to extinction. Therefore, numerous studies have sought to determine how species can adapt and escape extinction. Sexual selection can promote genetic adaptation, but often has a by-product, sexual conflict, that promotes adaptations beneficial for one sex and detrimental to the other. Such is the case of male adaptations that increase male reproduction by harming females (male harm). Male harm is widespread and has been shown to decrease female and population productivity in some species, facilitating extinction. Furthermore, there is increasing evidence that the degree of male harm to females depends on environmental changes and how well males are adapted to them. However, we ignore how strong the effects of sexual conflict across taxa are, or how ecological feedback on sexual conflict may affect the rate of adaptation and population recovery. Here, we first conducted a meta-analysis to quantify the effect of male harm on female fitness and show, across taxa, that there is an overall negative effect that seems to be dependent on proxies of sexual selection. Then, we used a series of theoretical models to show that, although eco-evolutionary feedback on sexual conflict can limit population decline, this comes at the cost of slowing down the rate of adaptation and population recovery. Our study suggests that understanding how quick environmental changes affect sexual conflict can increase our understanding of how populations adapt and recover in the face of climate change.

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.

The canonical model of sex-chromosome evolution assigns a key role to sexually antagonistic (SA) genes on the arrest of recombination and ensuing degeneration of Y chromosomes. This assumption cannot be tested in organisms with highly differentiated sex chromosomes, such as mammals or birds, owing to the lack of polymorphism. Fixation of SA alleles, furthermore, might be the consequence rather than the cause of recombination arrest. Here we focus on a population of common frogs (Rana temporaria) where XY males with genetically differentiated Y chromosomes (non-recombinant Y haplotypes) coexist with both XY{degrees} males with proto-Y chromosomes (only differentiated from X chromosomes in the immediate vicinity of the candidate sex-determining locus Dmrt1) and XX males with undifferentiated sex chromosomes (genetically identical to XX females). Our study shows no effect of sex-chromosome differentiation on male phenotype, mating success or fathering success. Our conclusions rejoin genomic studies that found no differences in gene expression between XY, XY{degrees} and XX males. Sexual dimorphism in common frogs seems to result from the differential expression of autosomal genes rather than sex-linked SA genes. Among-male variance in sex-chromosome differentiation is better explained by a polymorphism in the penetrance of alleles at the sex locus, resulting in variable levels of sex reversal (and thus of X-Y recombination in XY females), independent of sex-linked SA genes.\n\nImpact Summary\n\nHumans, like other mammals, present highly differentiated sex chromosomes, with a large, gene-rich X chromosome contrasting with a small, gene-poor Y chromosome. This differentiation results from a process that started approximately 160 Mya, when the Y first stopped recombining with the X. How and why this happened, however, remain controversial. According to the canonical model, the process was initiated by sexually antagonistic selection; namely, selection on the proto-Y chromosome for alleles that were beneficial to males but detrimental to females. The arrest of XY recombination then allowed such alleles to be only transmitted to sons, not to daughters. Although appealing and elegant, this model can no longer be tested in mammals, as it requires a sex-chromosome system at an incipient stage of evolution. Here we focus on a frog that displays within-population polymorphism is sex-chromosome differentiation, where XY males with differentiated chromosomes coexist with XX males lacking Y chromosomes. We find no effect of sex-chromosome differentiation on male phenotype or mating success, opposing expectations from the standard model. Sex linked genes do not seem to have a disproportionate effect on sexual dimorphism. From our results, sexually antagonistic genes show no association with sex-chromosome differentiation in frogs, which calls for alternative models of sex-chromosome evolution.

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.

Parental care in birds consists of many elaborate forms, including nest building, incubation, and offspring provision, but we still do not know how much each parent contributes to the different forms. Furthermore, the variations, relationships, and potential drivers of sex differentiation in providing care across different care stages remain largely unknown. Here, we surveyed species in birds and uncovered remarkable differences in the sex role patterns across different care forms. This result implied that parental care should not be treated as a unitary trait but as a composite of integrated features with great variations. Further analyses revealed moderate correlations of the sex roles between care forms, indicating the existence of shared intrinsic drivers. We tested the effects of sexual selection, certainty of paternity, predation risk, and offsprings life history traits in driving sex role variations. Results showed that species with strong sexual selection on males or uncertainty of paternity tend to have female-biased care.

Relatedness underlies the evolution of reproductive altruism, yet eusocial insect colonies occasionally accept unrelated reproductive queens. To better understand this seemingly paradox, we investigated whether acceptance of unrelated queens by workers is an incidental phenomenon resulting from failure to recognize non-nestmate queens, or whether it is an adaptive behavior favored in specific contexts where cooperation is preferable to rejection. Our study system is the socially polymorphic Alpine silver ant, Formica selysi. Within populations some colonies have a single queen (monogynous), and others have multiple, sometimes unrelated, breeding queens (polygynous). Social organization is determined by a supergene with two haplotypes. In a first experiment we investigated whether workers from polygynous colonies were inherently more prone to accepting unrelated queens than workers from the alternate, monogynous social form. We found that workers rejected all alien queens, independently of their social origin and of the number of queens heading their colony. We then investigated whether queen acceptance was favored in specific conditions. We found that workers from polygynous colonies accepted alien queens when these queens were accompanied by workers. These results show that workers flexibly adjust their acceptance of alien queens according to the situation. We discuss how conditional acceptance of unrelated queens may be adaptive by providing benefits through increased colony size and/or genetic diversity, and by avoiding the rejection costs resulting from fighting.

Sex differences in behavioral traits are common, but we know little about the role of sexual selection in shaping these traits. Estimating sex-specific genetic effects and cross-sex genetic correlations can provide insights into sex-specific selection and on whether evolution can shape independent expression of behavioral traits across the sexes. We conducted a quantitative genetic study in a sexually-size-dimorphic spider, Larinioides sclopetarius, which exhibits sex differences in adult life-styles. We observed pedigreed spiders for aggression, activity, exploration and boldness, and used animal models to disentangle genetic and environmental influences on these behaviors. We detected higher additive genetic variances in activity and aggression in males compared to females, but no sex differences in quantitative genetic estimates for exploration and boldness. The estimated mean cross-sex genetic correlation in all traits were close to zero suggesting these traits could have flexibility for sex-independent evolution. We note however, that the 95% credible intervals of cross-sex genetic correlation are large, and thus estimates uncertain. Our results imply that individual variation in aggression and activity might stem from sex-specific selection acting on these traits. The estimates of sex-specific additive genetic variation and cross-sex genetic correlation suggests their further sex-independent evolution. Taken together, our results support the notion that sexual selection can play an important role shaping behavioral traits.

Mullerian mimicry is the convergent evolution of warning signals among sympatric prey driven by predator learning. Theory therefore predicts signal homogeneity both within communities and within species that participate in Mullerian mimicry. Though rare, sexual dimorphism in Mullerian species does occur, but the underlying eco-evolutionary mechanisms are still relatively unexplored. Basing on the biology of aposematic butterflies, this study uses a modeling approach to test the hypothesis that sexual difference in defense can lead to the evolution of imperfect Mullerian mimicry in the less defended females as the consequence of opposing demands to minimize the cost of automimicry while maximizing reproductive output. Additionally, both the occurrence and degree of sexual dimorphism would decrease when the less defended sex becomes more valuable for reproduction, for example when offspring sex ratio is male biased or when females can mate only once in their lifetime. Findings from this study could help explain the evolution of extreme sexual dimorphism in some Mullerian systems, in which each sex mimics different models. Moreover, through understanding this intriguing exception to the rule, we will be able to gain a more complete picture of how a multitude of selective forces might shape the diversity in prey phenotypes.

The Drosophila wing is a structure shared by males and females with the main function of flight. However, in males, wings are also used to produce songs, or visual displays during courtship. Thus, observed changes in wing phenotype depend on the interaction between sex-specific selective pressures and the genetic and ontogenetic restrictions imposed by a common genetic architecture. Here, we investigate these issues by studying how the wing has evolved in twelve populations of Drosophila buzzatii raised in common-garden conditions and using an isofemale line design. The between-population divergence shows that sexual dimorphism is greater when sex evolves in different directions. Multivariate Qst-Fst analyses confirm that male wing shape is the target for multiple selective pressures, leading males wings to diverge more than females wings. While the wing blade and the wing base appear to be valid modules at the genetic (G matrix) and among-population (D matrix) levels, the reconstruction of between-population adaptive landscapes ({Omega} matrix) shows selection as an integrative force. Also, cross-sex covariances reduced the predicted response to selection in the direction of the extant sexual dimorphism, suggesting that selection had to be intensified in order to circumvent the limitations imposed by G. However, such intensity of selection was not able to break the modularity pattern of the wing. The results obtained here show that the evolution of D. buzzatii wing shape is the product of a complex interplay between ontogenetic constraints and conflicting sexual and natural selections.

Sexual reproduction can facilitate adaptation by reshuffling genetic variation. However, sexual reproduction can also bear costs. Such costs come in two forms: direct costs and evolutionary costs. Direct costs are associated with the cost of producing males (twofold cost of sex), the cost of meiosis, and the typically slower cell division during sexual reproduction of single-celled organisms. Evolutionary costs occur when too frequent sexual reproduction would hinder adaptation, by breaking apart adaptive allele combinations. Whereas the direct costs of sexual reproduction have been studied repeatedly in theoretical studies, the evolutionary costs of sex remain less well understood. We investigate here how the frequency of sexual reproduction affects adaptation to a non-stressful and a stressful environment in populations of the green alga Chlamydomonas reinhardtii, while minimizing the direct costs of sexual reproduction. Contrary to several previous studies, we found that an increasing frequency of sexual reproduction hindered adaptation of populations. In populations experiencing the highest frequency of sexual reproduction, adaptation was entirely prevented. These findings suggest that there were strong evolutionary costs associated with too frequent sexual reproduction in our populations. This observation may help to explain why in many facultative sexual species, there is a low frequency of sexual reproduction.

Cyclical parthenogenesis is a widespread reproductive strategy in which organisms go through one or multiple rounds of clonal reproduction before sexual reproduction. Because sexual reproduction is typically less common than parthenogenesis in populations of the planktonic cladoceran Daphnia magna, it is not frequently studied. Here we examine the sexual process of D. magna and its relation to sexual selection in Daphnia rockpool populations by observing natural mating in these shallow habitats where sex generally occurs throughout the summer. Although microsatellite markers were found to reveal no evidence of disassortative mating or, thus, of inbreeding avoidance, body length and infection status did reveal assortative mating, suggesting sexual selection to act. When two males mated with a single female, the larger male was observed to remain longer, possibly giving it an advantage in sperm competition. Indirect evidence points at the brood pouch as the likely site of fertilization and thus, sperm competition. Sperm length was as variable within ejaculates as it was among males from different populations. Our data provide firm evidence that sexual selection is present in this species, most likely manifesting itself through a combination of female choice and male-male competition.