The Relationship between Sexual Selection and Sexual Conflict

Evolutionary conflicts of interest arise whenever genetically different individuals interact and their routes to fitness maximization differ. Sexual selection favors traits that increase an individual’s competitiveness to acquire mates and fertilizations. Sexual conflict occurs if an individual of sex A’s relative fitness would increase if it had a “tool” that could alter what an individual of sex B does (including the parental genes transferred), at a cost to B’s fitness. This definition clarifies several issues: Conflict is very common and, although it extends outside traits under sexual selection, sexual selection is a ready source of sexual conflict. Sexual conflict and sexual selection should not be presented as alternative explanations for trait evolution. Conflict is closely linked to the concept of a lag load, which is context-dependent and sex-specific. This makes it possible to ask if one sex can “win.” We expect higher population fitness if females win.Many published studies ask if sexual selection or sexual conflict drives the evolution of key reproductive traits (e.g., mate choice). Here we argue that this is an inappropriate question. By analogy, G. Evelyn Hutchinson (1965) coined the phrase “the ecological theatre and the evolutionary play” to capture how factors that influence the birth, death, and reproduction of individuals (studied by ecologists) determine which individuals reproduce, and “sets the stage” for the selective forces that drive evolutionary trajectories (studied by evolutionary biologists). The more modern concept of “eco-evolutionary feedback” (Schoener 2011) emphasizes that selection changes the character of the actors over time, altering their ecological interactions. No one would sensibly ask whether one or the other shapes the natural world, when obviously both interact to determine the outcome.So why have sexual conflict and sexual selection sometimes been elevated to alternate explanations? This approach is often associated with an assumption that sexual conflict affects traits under direct selection, favoring traits that alter the likelihood of a potential mate agreeing or refusing to mate because it affects the bearer’s immediate reproductive output, whereas “traditional” sexual selection is assumed to favor traits that are under indirect selection because they increase offspring fitness. These “traditional” models are sometimes described as “mutualistic” (e.g., Pizzari and Snook 2003; Rice et al. 2006), although this term appears to be used only when contrasting them with sexual conflict models. The investigators of the original models never describe them as “mutualistic,” which is hardly surprising given that some males are rejected by females.In this review, we first define sexual conflict and sexual selection. We then describe how the notion of a “lag load” can reveal which sex currently has greater “power” in a sexual conflict over a specific resource. Next, we discuss why sexual conflict and sexual selection are sometimes implicitly (or explicitly) presented as alternative explanations for sexual traits (usually female mate choice/resistance). To illustrate the problems with the assumptions made to take this stance, we present a “toy model” of snake mating behavior based on a study by Shine et al. (2005). We show that empirical predictions about the mating behavior that will be observed if females seek to minimize direct cost of mating or to obtain indirect genetic benefits were overly simplistic. This allows us to make the wider point that whom a female is willing to mate with and how often she mates are often related questions. Finally, we discuss the effect of sexual conflict on population fitness.     

The Sexual Cascade and the Rise of Pre-Ejaculatory (Darwinian) Sexual Selection,Sex Roles,and Sexual Conflict

After brief historic overviews of sexual selection and sexual conflict, I argue that pre-ejaculatory sexual selection (the form of sexual selection discussed by Darwin) arose at a late stage in an inevitable succession of transitions flowing from the early evolution of syngamy to the evolution of copulation and sex roles. If certain conditions were met, this “sexual cascade” progressed inevitably, if not, sexual strategy remained fixed at a given stage. Prolonged evolutionary history of intense sperm competition/selection under external fertilization preceded the rise of advanced mobility, which generated pre-ejaculatory sexual selection, followed on land by internal fertilization and reduced sperm competition in the form of postcopulatory sexual selection. I develop a prospective model of the early evolution of mobility, which, as Darwin realized, was the catalyst for pre-ejaculatory sexual selection. Stages in the cascade should be regarded as consequential rather than separate phenomena and, as such, invalidate much current opposition to Darwin–Bateman sex roles. Potential for sexual conflict occurs throughout, greatly increasing later in the cascade, reaching its peak under precopulatory sexual selection when sex roles become highly differentiated.Sexual selection and sexual conflict are vast fields in evolutionary biology; when possible, here, I refer to reviews. I begin with brief general historic overviews of sexual selection and sexual conflict; more detail can be found in Andersson (1994), Simmons (2001), Chapman et al. (2003), and Arnqvist and Rowe (2005). Much of the current state of the field of sexual conflict is covered in this collection.My principal aim, however, is to outline how sexual selection and sexual conflict have changed through evolutionary time, from mostly gamete competition in early unicellular eukaryotes, intense sperm competition in ancestral sessile and relatively immobile organisms, to both pre-ejaculatory (Darwinian) and postejaculatory sexual selection. These transitions in the evolution of sexual strategy arise as logical consequences whenever certain successive conditions are met, and together form what may be termed the “sexual cascade.”     

Speciation and Sexual Conflict

We review mathematical models that explicitly consider the dynamics of evolutionary change driven by sexual conflict over mating rate when males are selected for increasing mating success whereas females are selected to restrict mating rate. These models focus on a pair of traits each of which is controlled by a separate set of genes expressed in one sex only. The traits control the probability of mating and/or fertilization. Overall, there are at least six different dynamic regimes observed in models of sexual conflict: (1) continuous coevolutionary chase between the sexes (which can result in allopatric speciation as a byproduct), (2) evolution towards an equilibrium, (3) cyclic evolution, (4) evolution towards a line of equilibria with subsequent random drift along this line, (5) Buridan’s Ass regime involving extensive diversification in female alleles without comparable diversification in male alleles, and (6) extensive diversification in both male and female alleles (which can result in sympatric speciation). Mathematical models also show that different dynamic regimes can be observed with the same set of parameter values but under different initial conditions. It is also possible that the same population switches from one regime to another as a result of stochastic perturbations due to, say, random genetic drift. Moreover, different sets of loci controlling mating and fertilization in the same species can follow different dynamic regimes. We attempt to make some generalizations and identify important directions for theoretical and empirical work.     

Mechanisms and Evidence of Genital Coevolution: The Roles of Natural Selection,Mate Choice,and Sexual Conflict

Genital coevolution between the sexes is expected to be common because of the direct interaction between male and female genitalia during copulation. Here we review the diverse mechanisms of genital coevolution that include natural selection, female mate choice, male–male competition, and how their interactions generate sexual conflict that can lead to sexually antagonistic coevolution. Natural selection on genital morphology will result in size coevolution to allow for copulation to be mechanically possible, even as other features of genitalia may reflect the action of other mechanisms of selection. Genital coevolution is explicitly predicted by at least three mechanisms of genital evolution: lock and key to prevent hybridization, female choice, and sexual conflict. Although some good examples exist in support of each of these mechanisms, more data on quantitative female genital variation and studies of functional morphology during copulation are needed to understand more general patterns. A combination of different approaches is required to continue to advance our understanding of genital coevolution. Knowledge of the ecology and behavior of the studied species combined with functional morphology, quantitative morphological tools, experimental manipulation, and experimental evolution have been provided in the best-studied species, all of which are invertebrates. Therefore, attention to vertebrates in any of these areas is badly needed.Of all the evolutionary interactions between the sexes, the mechanical interaction of genitalia during copulation in species with internal fertilization is perhaps the most direct. For this reason alone, coevolution between genital morphologies of males and females is expected. Morphological and genetic components of male and female genitalia have been shown to covary in many taxa (Sota and Kubota 1998; Ilango and Lane 2000; Arnqvist and Rowe 2002; Brennan et al. 2007; Rönn et al. 2007; Kuntner et al. 2009; Tatarnic and Cassis 2010; Cayetano et al. 2011; Evans et al. 2011, 2013; Simmons and García-González 2011; Yassin and Orgogozo 2013; and see examples in

Sexual Cannibalism as a Manifestation of Sexual Conflict

Sexual cannibalism is a well-known example for sexual conflict and has many facets that determine the costs and benefits for the cannibal and the victim. Here, I focus on species in which sexual cannibalism is a general component of a mating system in which males invest maximally in mating with a single (monogyny) or two (bigyny) females. Sexual cannibalism can be a male strategy to maximize paternity and a female strategy to prevent paternity monopolization by any or a particular male. Considerable variation exists between species (1) in the potential of males to monopolize females, and (2) in the success of females in preventing monopolization by males. This opens up exciting future possibilities to investigate sexually antagonistic coevolution in a largely unstudied mating system.Sexual cannibalism, the killing and consumption of potential or actual mating partners in a mating context, has been termed a “pinnacle of sexual conflict” because of the dramatic ending of the act for one mating partner, mostly the male (Elgar and Schneider 2004). This contradiction of traditional sex roles may be one reason why the phenomenon of sexual cannibalism has intrigued naturalists for a long time. In the context of sexual conflict, sexually cannibalistic behavior of females is a harmful trait, and antagonistic traits are expected to evolve in males, which can be considered the reverse of most other examples in which females respond to male harm (see Perry and Rowe 2014). I will discuss potential antagonistic traits to sexual cannibalism in males but will also show that the above view is too simplistic when it comes to spider mating systems characterized by very low male mating rates.It is important to note that there are different kinds of sexual cannibalism based on very different evolutionary scenarios (Elgar and Schneider 2004; Prenter et al. 2006; Wilder et al. 2009). The most extreme divide exists between cannibalism before sperm transfer, which can only benefit the cannibal, and sexual cannibalism during or after sperm transfer (from here on termed postinsemination sexual cannibalism), which can benefit the cannibal and the victim (Elgar and Schneider 2004). Despite a longer history of research on preinsemination sexual cannibalism, the evolutionary causes and consequences of postinsemination sexual cannibalism are generally less debated.There are reports (often anecdotal) on the occurrence of sexual cannibalism from diverse invertebrate taxa (Elgar 1992) and it may well occur in all predatory invertebrates that are potentially cannibalistic (Polis 1981). It is beyond the scope of this brief review to list and evaluate all reported occurrences. Rather, I will start with a brief account of the generally discussed causes and consequences of sexual cannibalism and will then concentrate on the conflicting interests of the sexes regarding postinsemination sexual cannibalism in mating systems that are characterized by very low male mating rates.Studies that investigate sexual cannibalism experimentally are mostly concerned with (1) nutritional aspects, (2) the importance of sexual size dimorphism and sexual selection, and, increasingly, (3) behavioral syndromes. The aggressive spillover hypothesis suggests that preinsemination sexual cannibalism is part of a behavioral syndrome in which aggression against mating partners spills over from a foraging context (Arnqvist and Henriksson 1997). There is mixed support for this idea in the few species that have been looked at. In several spider species, females consistently differ in their aggressiveness and these differences affect sexual cannibalism (for a recent debate about the evidence for this hypothesis, see Johnson 2013; Kralj-Fišer et al. 2013b; Pruitt and Keiser 2013).A majority of studies have taken a unilateral view and have been concerned with the “motivation” of the cannibal; because sexual cannibalism generally occurs in predators, hunger is a well-supported motivation (Wilder et al. 2009). Many predators are food-limited, and, assuming a trade-off between foraging and mating, the balance may tilt toward foraging under particular circumstances (modeled by Newman and Elgar 1991). Food and mate availability will influence the costs and benefits of sexual cannibalism for females and have been one focus of a recent review on sexual cannibalism (Wilder et al. 2009).In all predatory and cannibalistic animals, mating partners impose selection on each other’s abilities to avoid or resist aggression. This selection pressure is asymmetrical if one sex is physically dominant. Indeed, the differences in size between females and males often determine the frequency of sexual cannibalism, perhaps because the potential to resist a cannibalistic attack is size-dependent (Elgar 1992; Wilder and Rypstra 2008). Usually, males are the victims and females are the cannibals. Yet, reversed sexual cannibalism has also been reported and appears to be associated with the reversed pattern in sexual size dimorphism. Examples are the water spider, Arygoneta aquatica (Schutz and Taborsky 2005, 2011) and role-reversed wolf spiders (Aisenberg et al. 2011). In the gnaphosid spider, Micaria sociabilis, large, young males cannibalize old and relatively smaller females (Sentenska and Pekar 2013). These examples further support the notion that the relative size differences of a mating pair play a part in determining the likelihood of sexual cannibalism. Patterns can be found both on a between-species comparative scale and on a within-species scale (Wilder and Rypstra 2008; Wilder et al. 2009), and they are also reported as an underlying pattern in cannibalism outside a mating context (Bleakley et al. 2013). Furthermore, there is anecdotal evidence for the same pattern in hermaphrodites (e.g., Goto and Yoshida 1985; Michiels et al. 2003), which may constitute a particularly interesting case to study, as the power asymmetries are less obviously related to the male or female role.In asymmetric encounters, the costs and risks of aggressive behavior toward potential mating partners are low for the dominant partner. Toward smaller males, females could use aggressiveness as a means of partner choice. Indeed, many studies suggest that sexual selection in addition to gaining a meal may be the adaptive value of sexual cannibalism (Prenter et al. 2006). From the female perspective, aggressive behavior directed toward males may serve as a general screening of partner quality, a mechanism often described as indirect mate choice (Elgar and Nash 1988; Prenter et al. 2006; Kralj-Fišer et al. 2012). A screening method implies that females attack every male, and suitors that cannot withstand and persist an attack will be killed and consumed; alternatively, females may differentiate between males and attack and consume only those males that do not meet certain quality criteria (reviewed in Prenter et al. 2006). The latter has been found in wolf spiders (Wilgers and Hebets 2012). The latter mechanism of direct choice is more complex than the indirect one as it requires perception and assessment of quality cues, and large enough benefits of choosiness are expected to match the costs. Mate rejection via sexual cannibalism is considered a particularly extreme case of sexual conflict mostly because rejection can lead to death. Although this may be true for the individual male that loses all future reproductive success, frequencies of preinsemination sexual cannibalism might be rather low (Kralj-Fišer et al. 2013b). Please note that in almost every species, a certain proportion of individuals will be excluded from the mating market and will have no mating success. The claim that prevention of mating success via sexual cannibalism results in more intense sexual conflict than exclusion from mating with less drastic measures has, to my knowledge, never been tested. Because of the scarcity of data on natural frequencies of preinsemination cannibalism, a meta-analysis would not reveal a realistic picture at this stage. Hence, to date, it is not feasible to compare the relative strength of selection imposed by a cannibalistic mate choice strategy against a strategy with less drastic consequences of mate rejection. More studies are needed to unravel the exact nature of sexual selection under the threat of ending as a meal. Below, I will briefly sketch possible responses to selection imposed by sexually cannibalistic females before or during insemination.     

The Starling Mating System as an Outcome of the Sexual Conflict

Many bird species demonstrate a variable mating system, with some males being monogamously mated and other males able to attract more than one mate. This variation in avian mating systems is often explained in terms of potential costs of sharing breeding partners and compensation for such costs. However, whenever there is a difference in the optimal mating system for males and females, a sexual conflict over the number of partners is expected. This paper contains a verbal model of how a conflict between male and female European starlings (Sturnus vulgaris),resulting from the fitness consequences of different mating systems for males and females differing over time, determines the mating system. We demonstrate that males and females have contrasting fitness interests regarding mating system, such that males gain from attracting additional mates whereas already mated females pay a cost in terms of reduced reproductive success if males are successful in attracting more mates. We demonstrate how this can be traced to the rules by which males allocate non-sharable care between different broods. Furthermore, we demonstrate that there exist male and female conflict behaviours with the potential to affect the mating system. For example, aggression from already mated females towards prospecting females can limit male mating success and males can circumvent this by spacing the nest-sites they defend. The realised mating system will emerge as a consequence of both the fitness value of the different mating systems for males and females, and the costs for males and females of intersexual competition. We discuss how this model can be developed and critically evaluated in the future.     

The Selection Limit Due to the Conflict between Truncation and Stabilizing Selection with Mutation

Long-term selection response could slow down from a decline in genetic variance or in selection differential or both. A model of conflict between truncation and stabilizing selection in infinite population size is analysed in terms of the reduction in selection differential. Under the assumption of a normal phenotypic distribution, the limit to selection is found to be a function of kappa, the intensity of truncation selection, omega 2, a measure of the intensity of stabilizing selection, and sigma 2, the phenotypic variance of the character. The maintenance of genetic variation at this limit is also analyzed in terms of mutation-selection balance by the use of the "House-of-cards" approximation. It is found that truncation selection can substantially reduce the equilibrium genetic variance below that when only stabilizing selection is acting, and the proportional reduction in variance is greatest when the selection is very weak. When truncation selection is strong, any further increase in the strength of selection has little further influence on the variance. It appears that this mutation-selection balance is insufficient to account for the high levels of genetic variation observed in many long-term selection experiments.     

Toward a New Sexual Selection Paradigm: Polyandry, Conflict and Incompatibility (Invited Article)

Darwin's recognition that male–male competition and female choice could favor the evolution of exaggerated male traits detrimental to survival set the stage for more than a century of theoretical and empirical work on sexual selection. While this Darwinian paradigm represents one of the most profound insights in biology, its preoccupation with sexual selection as a directional evolutionary force acting on males has diverted attention away from the selective processes acting on females. Our understanding of female reproduction has been further confounded by discreet female mating tactics that have perpetuated the illusion of the monogamous female and masked the potential for conflict between the sexes. With advances in molecular techniques leading to the discovery that polyandry is a pervasive mating strategy, recognition of these shortcomings has brought the study of sexual selection to its current state of flux. In this paper, we suggest that progress in two key areas is critical to formulation of a more inclusive, sexual selection paradigm that adequately incorporates selection from the female perspective. First, we need to develop a better understanding of male × female and maternal × paternal genome interactions and the role that polyandry plays in providing females with non‐additive genetic benefits such as incompatibility avoidance. Consideration of these interaction effects influencing natural selection on females is important because they can complicate and even undermine directional sexual selection on males. Secondly, because antagonistic coevolution maintains a balance between opposing sides that obscures the conflict itself, many more experimental evolution studies and interventionist investigations (e.g. gene knockouts) are needed to tease apart male manipulative adaptations and female counter‐adaptations. It seems evident that the divisiveness and controversy that has plagued sexual selection theory since Darwin first proposed the idea has often stalled progress in this important field of evolutionary biology. What is now needed is a more pluralistic and integrative approach that considers natural as well as sexual selection acting on females, incorporates multiple sexual selection mechanisms, and exploits advances in physiology and molecular biology to understand the mechanisms through which males and females achieve reproductive success.     

Expanding Sexual Selection Gradients; A Synthetic Refinement of Sexual Selection Theory

Sexual selection is usually modeled as fitness differences that are mediated through variation in the number of mates obtained (variance in mating success, V_MS). Nevertheless, empirical studies increasingly posit sexual selection even when V_MS is low or does not contribute to variance in fitness, as is the case in females of many species. In these contexts, evolution by sexual selection is only plausible if it is mediated through variation in mate quality (V_MQ) rather than exclusively through variation in the number of mates (V_MS). However, we lack a formal theoretical foundation for sexual selection in these cases. Here, I argue the need for an explicit, formal treatment of how V_MQ may result in sexual selection. Building upon the conceptually powerful framework of sexual selection gradients, I propose a graphical heuristic model that aims to serve as a foundation for future formal models. I close by discussing the implications of this perspective for sexual selection research in general, with particular attention to predictions that the model generates for the action of sexual selection on female traits.     

Sexual Conflict and Seminal Fluid Proteins: A Dynamic Landscape of Sexual Interactions

Sexual reproduction requires coordinated contributions from both sexes to proceed efficiently. However, the reproductive strategies that the sexes adopt often have the potential to give rise to sexual conflict because they can result in divergent, sex-specific costs and benefits. These conflicts can occur at many levels, from molecular to behavioral. Here, we consider sexual conflict mediated through the actions of seminal fluid proteins. These proteins provide many excellent examples in which to trace the operation of sexual conflict from molecules through to behavior. Seminal fluid proteins are made by males and provided to females during mating. As agents that can modulate egg production at several steps, as well as reproductive behavior, sperm “management,” and female feeding, activity, and longevity, the actions of seminal proteins are prime targets for sexual conflict. We review these actions in the context of sexual conflict. We discuss genomic signatures in seminal protein (and related) genes that are consistent with current or previous sexual conflict. Finally, we note promising areas for future study and highlight real-world practical situations that will benefit from understanding the nature of sexual conflicts mediated by seminal proteins.Both sexes benefit from successful reproduction, but the different reproductive strategies adopted by males and females may result in differential costs and benefits. This can result in sexual conflict before, during, and after mating. Conflict in the more familiar form of competition can also occur between females and between males, with the latter situation including interejaculate competition. Of the many “weapons” in these conflicts and competitions, this article focuses on the seminal fluid proteins (SFPs) that are made by males and transferred to females during mating. These proteins represent a crucial interface of functional activity between male and female. Transfer of SFPs can affect physiology and, in some animals, the behavior and life span of mated females (reviewed in Chapman 2001; Gillott 2003; Poiani 2006; Avila et al. 2011; Rodríguez-Martínez et al. 2011). Because SFPs have important effects on the most intimate of interactions between the sexes, they are prime candidates to become subject to sexually antagonistic selection (Arnqvist and Rowe 2005). With increasing knowledge of the functions of SFPs, their roles in inter- and intrasexual conflict and their evolutionary responses to conflict are becoming ever more apparent. Here, we explore the roles, evolution, and significance of these male-derived players in sexual conflict. We refer the reader to previous reviews for much of the detailed functional information on SFPs (e.g., Chapman 2001; Gillott 2003; Kubli 2003; Arnqvist and Rowe 2005; Poiani 2006; Sirot et al. 2009; Avila et al. 2011; Rodríguez-Martínez et al. 2011) and focus here instead on selected examples, drawn largely from the study of insects.     

Theoretical Reproductive Outcomes of the Sexual Conflict in Humans

Doklady Biological Sciences - This paper presents an updated mathematical model of the evolutionary dynamics of the sexual conflict (“battle of the sexes”) and its simulated...     

Sexual Selection and Mate Choice

After a long period of dormancy, Darwin's theory of sexual selection in general, and mate choice in particular, now represents one of the most active fields in evolutionary research. After a brief overview of the history of ideas and a short introduction into the main mechanisms of sexual selection, I discuss some recent theoretical developments and empirical findings in the study of mate choice and review the various current models of mate choice, which can be grossly divided into adaptive models and nonadaptive models. I also examine whether available primate evidence supports various hypotheses concerning mate choice. Although primatologists were long aware that nonhuman primates have preferences for certain mating partners, until recently the functions and evolutionary consequences of their preferences remained obscure. Now there is growing evidence that mate choice decisions provide primates with important direct or indirect benefits. For example, several observations are consistent with the hypothesis that by direct or indirect mate choice female primates lower the risk of infanticide or enhance the chance of producing viable offspring. Nevertheless, there are also significant holes in our knowledge. How the male mandrill, one of Darwin's famous examples, got his brightly colored face, is still unknown.     

Deception Divergence and Sexual Selection

The hypothesis that blacknose dace Rhinichthys atratulus discriminate between piscivorous creek chub Semotilus atromaculatus and nonpiscivorous common suckers Catostomus commersoni was tested in a seminatural, artificial stream. Dace avoided the nonpredatory suckers less than they did the predatory chub, which they avoided during both day and night. However, dace significantly avoided the nonpredator at night. Further, experienced and inexperienced dace avoided the nonpredator similarly, suggesting that they had not learned to discriminate predator from nonpredator in their natural habitat. These results are consistent with two hypotheses (1) that individuals varied in ability to discriminate predatory from nonpredatory fishes, and (2) that the predator was more active than the nonpredator, thereby “frightening” the dace more. At temperatures of < 6°C the nonpredator was inactive and dace no longer avoided it at night, but continued to avoid the active chub, suggesting that movement constituted part of the avoidance-releasing stimulus. Because dace avoided nonpredators, they may waste time in natural streams avoiding harmless fish. It is suggested that the dace cannot afford the risk of a mistaken identity, which may be high in the stream environment, and selection favors those that avoid all large, moving fish.     

Conflict Among Individual Mitochondrial Proteins in Resolving the Phylogeny of Eutherian Orders

The phylogenetic relationship among primates, ferungulates (artiodactyls + cetaceans + perissodactyls + carnivores), and rodents was examined using proteins encoded by the H strand of mtDNA, with marsupials and monotremes as the outgroup. Trees estimated from individual proteins were compared in detail with the tree estimated from all 12 proteins (either concatenated or summing up log-likelihood scores for each gene). Although the overall evidence strongly suggests ((primates, ferungulates), rodents), the ND1 data clearly support another tree, ((primates, rodents), ferungulates). To clarify whether this contradiction is due to (1) a stochastic (sampling) error; (2) minor model-based errors (e.g., ignoring site rate variability), or (3) convergent and parallel evolution (specifically between either primates and rodents or ferungulates and the outgroup), the ND1 genes from many additional species of primates, rodents, other eutherian orders, and the outgroup (marsupials + monotremes) were sequenced. The phylogenetic analyses were extensive and aimed to eliminate the following artifacts as possible causes of the aberrant result: base composition biases, unequal site substitution rates, or the cumulative effects of both. Neither more sophisticated evolutionary analyses nor the addition of species changed the previous conclusion. That is, the statistical support for grouping rodents and primates to the exclusion of all other taxa fluctuates upward or downward in quite a tight range centered near 95% confidence. These results and a site-by-site examination of the sequences clearly suggest that convergent or parallel evolution has occurred in ND1 between primates and rodents and/or between ferungulates and the outgroup. While the primate/rodent grouping is strange, ND1 also throws some interesting light on the relationships of some eutherian orders, marsupials, and montremes. In these parts of the tree, ND1 shows no apparent tendency for unexplained convergences. Received: 5 December 1997 / Accepted: 24 February 1998     

The Impact of the Law of the Sea Convention on Conflict and Conflict Management in the South China Sea

This article examines the impact of the UN Law of the Sea Convention on conflict behavior and management in the South China Sea during four periods: during its negotiation (1973–1982); from its signing to the entry into force (1982–1994); from then until the China-ASEAN Declaration on the Conduct of Parties in the South China Sea (1995–2002); and from the setting of a timeline for outer limits of continental shelf submissions to the events following the 2009 submissions (2003–2013). Ambiguous effects were found. On the one hand, the Convention has generated or exacerbated conflict by raising the stakes, failing to resolve key legal issues, and encouraging overlapping zone claims. On the other hand, it has provided obligations, language, and techniques for conflict management and resolution. The conflict-enhancing impact was found to have been more substantial than the peace-promoting effects. Nevertheless, the balance has shifted toward more emphasis on conflict management and also some utilization of the Convention's peacemaking potential. If this long-term trend continues and the Convention is more rigorously respected and applied, the Convention may in the end be found to have contributed to regional peace.     

Sexual Conflict,Life Span,and Aging

The potential for sexual conflict to influence the evolution of life span and aging has been recognized for more than a decade, and recent work also suggests that variation in life span and aging can influence sexually antagonistic coevolution. However, empirical exploration of these ideas is only beginning. Here, we provide an overview of the ideas and evidence linking inter- and intralocus sexual conflicts with life span and aging. We aim to clarify the conceptual basis of this research program, examine the current state of knowledge, and suggest key questions for further investigation.Sexual conflict arises because the sexes maximize their fitness via different, and often mutually incompatible, strategies, and its signature has been detected across a wide range of morphological, physiological, behavioral, and life-history traits in many species. A number of investigators have suggested that sexual conflict could play an important role in the evolution of two particularly interesting life-history traits: life span and aging (Svensson and Sheldon 1998; Promislow 2003; Bonduriansky et al. 2008; Maklakov and Lummaa 2013). Sexual conflict can affect life span and aging rate at both proximate (within-generation) and ultimate (evolutionary) scales. Sexually antagonistic behavioral or physiological interactions that increase mortality rate in one or both sexes (interlocus sexual conflict) could drive the evolution of faster life histories. Moreover, sex-specific optimization of reproductive strategies may often result in sex differences in life span and aging rates, and sexually antagonistic selection on shared genetic architecture can displace one or both sexes from their sex-specific optima for these traits (intralocus sexual conflict). Conversely, a change in life histories because of environmental fluctuations could affect the degree of sexual conflict in a population and influence sexual coevolution. Although evidence for sexual conflict is rapidly accumulating, our understanding of its relationship to life span and aging remains rudimentary. In this review, we provide a critical review of recent literature and highlight areas that require further investigation.     

The Role of Sexual Selection and Conflict in Mediating Among-Population Variation in Mating Strategies and Sexually Dimorphic Traits in Sepsis punctum

The black scavenger fly Sepsis punctum exhibits striking among-population variation in the direction and magnitude of sexual size dimorphism, modification to the male forelimb and pre-copulatory behaviour. In some populations, male-biased sexual size dimorphism is observed; in other, less dimorphic, populations males court prior to mating. Such variation in reproductive traits is of interest to evolutionary biologists because it has the potential to limit gene flow among populations, contributing to speciation. Here, we investigate whether large male body size and modified forefemur are associated with higher male mating success within populations, whether these traits are associated with higher mating success among populations, and if these traits carry viability costs that could constrain their response to sexual selection. Flies from five distinct populations were reared at high or low food, generating high and low quality males. The expression of body size, forelimb morphology and courtship rate were each greater at high food, but high food males experienced higher mating success or reduced latency to first copulation in only one of the populations. Among populations, overall mating success increased with the degree of male-bias in overall body size and forelimb modification, suggesting that these traits have evolved as a means of increasing male mating rate. The increased mating success observed in large-male populations raises the question of why variation in magnitude of dimorphism persists among populations. One reason may be that costs of producing a large size constrain the evolution of ever-larger males. We found no evidence that juvenile mortality under food stress was greater for large-male populations, but development time was considerably longer and may represent an important constraint in an ephemeral and competitive growth environment.     

Sexual Selection and the Psychophysics of Female Choice

Woodland birds of 20 species were captured in mist-nets in winter (September to February) and summer (April—August) and scored for whether or not they made distress calls while being handled. Birds of only two species showed significant tendencies to be consistent in calling or failing to call when recaptured, but most species had weak trends in. the same direction. Some species changed seasonally in the tendency to call, but there was no general pattern. In winter, but not summer, the incidence of calling was correlated across species with an index of the number of conspecifics likely to hear the call, but there was no comparable correlation with variations within species. In the long-tailed tit, calling was associated with struggling, and with eyelid colour, which reflects a bird's mood. This implies that calling may be directed at a captor as well as at flock companions.