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12 - Sexual Conflict Theory
- from Part II - Middle-Level Theories
- Edited by Todd K. Shackelford, Oakland University, Michigan
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- Book:
- The Cambridge Handbook of Evolutionary Perspectives on Sexual Psychology
- Published online:
- 30 June 2022
- Print publication:
- 21 July 2022, pp 264-279
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Summary
Sexual conflict arises from differences in the fitness interests of males and females. A trait that is beneficial for the reproductive success of one sex reduces the fitness of the other sex, resulting in opposing selection pressures on the two sexes. The two sexes need each other for reproduction, but their dependence is asymmetric. Males benefit from a higher mating rate than females, as their reproductive success is usually constrained by the number of receptive mates, while female reproductive success is limited by egg production. Sexual conflict can occur at any stage of reproductive interactions – before or during copulation, or after insemination – and over almost any aspect of reproduction, from the decision to mate to the investment into parental care. The conflict results in a perpetual tug of war between the sexes. Each sex attempts to maximize its fitness at a cost to the other sex, which results in sexually antagonistic selection. This can cause the rapid evolution of sexual traits, and ultimately results in the diversification of traits, and possibly even in speciation. Sexual conflict can manifest in two ways, intra- and interlocus sexual conflict. Intralocus sexual conflict occurs when a trait expressed in both sexes (determined by alleles in the same locus in the two sexes) has opposite effects on male and female fitness. Interlocus sexual conflict occurs when the conflicting traits are determined by alleles in different loci in the two sexes and the optimal outcome of male–female interactions differs between the two sexes. Intralocus sexual conflict generates a genetic tug of war between the sexes over the optimal trait expression, while interlocus sexual conflict can lead to open-ended cycles of sexually antagonistic coevolution. Sexual conflict before mating has resulted in a diversity of tactics and strategies that males use to overcome female unwillingness to mate, from forced copulations and sneaky fertilization to the emission of love darts. Sexual conflict after mating has favored the evolution of male traits that increase success in sperm competition, such as postcopulatory mate guarding, toxic seminal substances that destroy the sperm of other males, mating plugs that prevent other males from mating with the female, and morphological and physiological traits that harm females, such as spines on the intromittent organ that pierce the reproductive tract of the female. Females have evolved cryptic choice of sperm to influence which males fertilize her ova. Whether the sexual conflict between males and females can be resolved depends on the type of conflict. Several mechanisms may reduce the strength of intralocus conflict, such as sex-limited expression of traits, but the interests of males and females are unlikely to become aligned when it comes to interlocus conflict. The relative influence of sexual conflict on the fitness of organisms, and the degree to which it can be resolved, are open questions.
Human-induced eutrophication maintains high parasite prevalence in breeding threespine stickleback populations
- ALEXANDRE BUDRIA, ULRIKA CANDOLIN
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- Journal:
- Parasitology / Volume 142 / Issue 5 / April 2015
- Published online by Cambridge University Press:
- 12 December 2014, pp. 719-727
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Anthropogenic activities are having profound impacts on species interactions, with further consequences for populations and communities. We investigated the influence that anthropogenic eutrophication has on the prevalence of the parasitic tapeworm Schistocephalus solidus in threespine stickleback Gasterosteus aculeatus populations. We caught stickleback from four areas along the coast of Finland, and within each area from one undisturbed and one eutrophied habitat. We found the prevalence of the parasite to be lower in the eutrophied habitats at the start of the breeding season, probably because of fewer piscivorous birds that transmit the parasite. However, while the prevalence of the parasite declined across the season in the undisturbed habitat, it did less so in eutrophied habitats. We discuss different processes that could be behind the differences, such as lower predation rate on infected fish, higher food availability and less dispersal in eutrophied habitats. We found no effect of eutrophication on the proportion of infected stickleback that entered reproductive condition. Together with earlier findings, this suggests that eutrophication increases the proportion of infected stickleback that reproduce. This could promote the evolution of less parasite resistant populations, with potential consequences for the viability of the interacting parties of the host–parasite system.
How does human-induced environmental change influence host-parasite interactions?
- ALEXANDRE BUDRIA, ULRIKA CANDOLIN
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- Journal:
- Parasitology / Volume 141 / Issue 4 / April 2014
- Published online by Cambridge University Press:
- 05 December 2013, pp. 462-474
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Host-parasite interactions are an integral part of ecosystems that influence both ecological and evolutionary processes. Humans are currently altering environments the world over, often with drastic consequences for host-parasite interactions and the prevalence of parasites. The mechanisms behind the changes are, however, poorly known. Here, we explain how host-parasite interactions depend on two crucial steps – encounter rate and host-parasite compatibility – and how human activities are altering them and thereby host-parasite interactions. By drawing on examples from the literature, we show that changes in the two steps depend on the influence of human activities on a range of factors, such as the density and diversity of hosts and parasites, the search strategy of the parasite, and the avoidance strategy of the host. Thus, to unravel the mechanisms behind human-induced changes in host-parasite interactions, we have to consider the characteristics of all three parts of the interaction: the host, the parasite and the environment. More attention should now be directed to unfold these mechanisms, focusing on effects of environmental change on the factors that determine encounter rate and compatibility. We end with identifying several areas in urgent need of more investigations.
How is female mate choice affected by male competition?
- Bob B. M. Wong, Ulrika Candolin
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- Journal:
- Biological Reviews / Volume 80 / Issue 4 / November 2005
- Published online by Cambridge University Press:
- 16 June 2005, pp. 559-571
- Print publication:
- November 2005
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The plethora of studies devoted to the topics of male competition and female mate choice belie the fact that their interaction remains poorly understood. Indeed, on the question of whether competition should help or hinder the choice process, opinions scattered throughout the sexual selection literature seem unnecessarily polarised. We argue, in the light of recent theoretical and empirical advances, that the effect of competition on mate choice depends on whether it results in the choosy sex attaining high breeding value for total fitness, considering both direct and indirect fitness benefits. Specifically, trade-offs may occur between different fitness benefits if some are correlated with male competitive ability whilst others are not. Moreover, the costs and benefits of mating with competitive males may vary in time and/or space. These considerations highlight the importance of injecting a life-history perspective into sexual selection studies. Within this context, we turn to the sexual selection literature to try to offer insights into the circumstances when competition might be expected to have positive or negative implications for pre-copulatory female choice. In this regard, we elaborate on three stages where competition might impact upon the choice process: (i) during mate detection, (ii) mate evaluation, and (iii) in dictating actual mating outcomes. We conclude by offering researchers several potentially rewarding avenues for future research.
The use of multiple cues in mate choice
- ULRIKA CANDOLIN
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- Journal:
- Biological Reviews / Volume 78 / Issue 4 / November 2003
- Published online by Cambridge University Press:
- 11 November 2003, pp. 575-595
- Print publication:
- November 2003
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An increasing number of studies find females to base their mate choice on several cues. Why this occurs is debated and many different hypotheses have been proposed. Here I review the hypotheses and the evidence in favour of them. At the same time I provide a new categorisation based on the adaptiveness of the preferences and the information content of the cues. A few comparative and empirical studies suggest that most multiple cues are Fisherian attractiveness cues or uninformative cues that occur alongside a viability indicator and facilitate detection, improve signal reception, or are remnants from past selection pressures. However, much evidence exists for multiple cues providing additional information and serving as multiple messages that either indicate general mate quality or enable females that differ in mate preferences to choose the most suitable male. Less evidence exists for multiple cues serving as back-up signals. The importance of receiver psychology, multiple sensory environments and signal interaction in the evolution of multiple cues and preferences has received surprisingly little attention but may be of crucial importance. Similarly, sexual conflict has been proposed to result in maladaptive preferences for manipulative cues, and in neutral preferences for threshold cues, but no reliable evidence exists so far. An important factor in the evolution of multiple preferences is the cost of using additional cues. Most theoretical work assumes that the cost of choice increases with the number of cues used, which restricts the conditions under which preferences for multiple cues are expected to evolve. I suggest that in contrast to this expectation, the use of multiple cues can reduce mate choice costs by decreasing the number of mates inspected more closely or the time and energy spent inspecting a set of mates. This may be one explanation for why multiple cues are more common than usually expected. Finally I discuss the consequences that the use of multiple cues may have for the process of sexual selection, the maintenance of genetic variation, and speciation.