SUMMARYWhy is it that some parasites cause high levels of host damage (i.e. virulence) whereas others are relatively benign? There are now numerous reviews of virulence evolution in the literature but it is nevertheless still difficult to find a comprehensive treatment of the theory and data on the subject that is easily accessible to non-specialists. Here we attempt to do so by distilling the vast theoretical literature on the topic into a set of relatively few robust predictions. We then provide a comprehensive assessment of the available empirical literature that tests these predictions. Our results show that there have been some notable successes in integrating theory and data but also that theory and empiricism in this field do not ‘speak’ to each other very well. We offer a few suggestions for how the connection between the two might be improved.

How social traits such as altruism and spite evolve remains an open question in evolutionary biology. One factor thought to be potentially important is demographic stochasticity. Here we provide a general theoretical analysis of the role of demographic stochasticity in social evolution. We show that the evolutionary impact of stochasticity depends on how the social action alters the recipient’s life cycle. If the action alters the recipient’s death rate, then demographic stochasticity always favours altruism and disfavours spite. On the other hand, if the action alters the recipient’s birth rate, then stochasticity can either favour or disfavour both altruism and spite depending on the ratio of the rate of population turnover to the population size. Finally, we also show that this ratio is critical to determining if demographic stochasticity can reverse the direction of selection upon social traits. Our analysis thus provides a general understanding of the role of demographic stochasticity in social evolution.

Sexual conflict is the divergence of evolutionary interests between the sexes. A neglected aspect of sexual conflict theory is that the conflict often occurs within the female's body, which can lead to a power asymmetry between the sexes. In particular, the female may often be able to respond flexibly to the actions of the male, and so exhibits plasticity. Here, we consider the implications of female plasticity, and find that it tends to result in lower levels of sexual conflict. We then relate our results to a comparison of pre- versus post-copulatory sexual conflict, and we also show that this asymmetry between males and females reduces the likelihood of runaway selection, preventing co-evolutionary arms races. Finally, we discuss our results in the context of the evolution of adaptive harm and sexual conflict when there are direct benefits.

The selective forces shaping mating systems have long been of interest to biologists. One particular selective pressure that has received comparatively little attention is sexually transmitted infections (STIs). While it has been hypothesized that STIs could drive the evolutionary emergence of monogamy, there is little theoretical support. Here we use an evolutionary invasion analysis to determine what aspects of pathogen virulence and transmission are necessary for serial monogamy to evolve in a promiscuous population. We derive a biologically intuitive invasion condition in terms of population-specific quantities. From this condition, we obtain two main results. First, when pathogen virulence causes mortality rather than sterility, monogamy is more likely to evolve. Second, we find that at intermediate pathogen transmission rates, monogamy is the most selectively advantageous, whereas at high-and lowtransmission rates, monogamy is generally selected against. As a result, it is possible for a pathogen to be highly virulent, yet for promiscuity to persist.

Host resistance consists of defences that limit pathogen burden, and can be classified as either adaptations targeting recovery from infection or those focused upon infection avoidance. Conventional theory treats avoidance as a fixed strategy which does not vary from one interaction to the next. However, there is increasing empirical evidence that many avoidance strategies are triggered by external stimuli, and thus should be treated as phenotypically plastic responses. Here, we consider the implications of avoidance plasticity for host-pathogen coevolution. We uncover a number of predictions challenging current theory. First, in the absence of pathogen trade-offs, plasticity can restrain pathogen evolution; moreover, the pathogen exploits conditions in which the host would otherwise invest less in resistance, causing resistance escalation. Second, when transmission trades off with pathogen-induced mortality, plasticity encourages avirulence, resulting in a superior fitness outcome for both host and pathogen. Third, plasticity ensures the sterilizing effect of pathogens has consequences for pathogen evolution. When pathogens castrate hosts, selection forces them to minimize mortality virulence; moreover, when transmission trades off with sterility alone, resistance plasticity is sufficient to prevent pathogens from evolving to fully castrate.

Evolutionary conflicts arise when the fitness interests of interacting individuals differ. Well-known examples include sexual conflict between males and females and antagonistic coevolution between hosts and parasites. A common feature of such conflicts is that compensating evolutionary change in each of the parties can lead to little overt change in the interaction itself. As a result, evolutionary conflict is expected to persist even if the evolutionary dynamic between the parties reaches an equilibrium. In these cases, it is of interest to know whether certain kinds of interactions are expected to lead to greater or lesser evolutionary conflict at such evolutionary stalemates. Here we present a theoretical analysis showing that when one of the interacting parties can respond to the other through adaptive phenotypic plasticity, evolutionary conflict is reduced. Paradoxically, however, it is the party that does not express adaptive plasticity that experiences less conflict. Conflict for the party displaying adaptive plasticity can increase or decrease, depending on the situation.

Cooperative breeding is a system in which certain individuals facilitate the production of offspring by others. The ecological constraints hypothesis states that ecological conditions deter individuals from breeding independently, and so individuals breed cooperatively to make the best of a bad situation. Current theoretical support for the ecological constraints hypothesis is lacking. We formulate a mathematical model that emphasizes the underlying ecology of cooperative breeders. Our goal is to derive theoretical support for the ecological constraints hypothesis using an ecological model of population dynamics. We consider a population composed of two kinds of individuals, nonbreeders (auxiliaries) and breeders. We suppose that help provided by an auxiliary increases breeder fecundity, but reduces the probability with which the auxiliary becomes a breeder. Our main result is a condition that guarantees success of auxiliary help. We predict that increasing the cost of dispersal promotes helping, in agreement with verbal theory. We also predict that increasing breeder mortality can either hinder helping (at high population densities), or promote it (at low population densities). We conclude that ecological constraints can exert influence over the evolution of auxiliary help when population dynamics are considered; moreover, that influence need not coincide with direct fitness benefits as previously found. K E Y W O R D S :Alloparental care, helpers-at-the-nest, inclusive fitness, kin selection, mathematical model, sociality.In many cooperatively breeding species, nonbreeding individuals (i.e., auxiliary individuals) postpone or forgo their own reproduction to increase the fecundity of others. Cooperative breeders include a taxonomically broad set of species that occur in a wide range of environments (Jennions and MacDonald 1994;Cockburn 1998;Leadbeater et al. 2011; Wong and Balshine 2011). Thus, the help provided by nonbreeding auxiliaries represents one of the most diverse forms of cooperation in nature.The adaptive significance of auxiliary help is often explained in terms of its direct and indirect fitness benefits (Wiley and Rabenold 1984;Brown 1987;Heinsohn and Legge 1999; CluttonBrock 2002;Griffin and West 2002). Direct benefits of help accrue through the production of descendant offspring. These are realized, for example, when a helpful auxiliary gains valuable parenting experience (Skutch 1961), or when a helpful auxiliary contributes to the formation of a larger, more productive group that it will inherit at some later date (Wiley and Rabenold 1984;Kokko et al. 2001; Clutton-Brock 2002). Indirect benefits of help accrue through increased production of related, nondescendant offspring. For indirect benefits to play a role in the emergence and maintenance of auxiliary help, helpers must be able to associate with breeding kin (Griffin and West 2002). While both direct and indirect fitness benefits likely promote the evolution of helpful behavior in cooperatively breeding species (Clutton-Brock 2009), we ch...

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