Table 2.

Parasitologist’s guide to the ecology and evolution of life histories.

From metazoans to malaria?	Using theories developed to explain the strategies of multicellular organisms (such as plants, insects, vertebrates) to the behaviours of parasites that exist as many individual cells may intuitively seem inappropriate. However, each clone of genetically identical parasites within an infection is the evolutionary equivalent of an individual organism [113,114] because the fitness interests of close relatives are aligned. Thus, the transmission of a genotype early in an infection counts towards the fitness of their clonal progeny later in the infection, and vice-versa. In genetically diverse infections, parasites belonging to one genotype have no evolutionary interest in the fitness of unrelated genotypes and therefore each genotype is selected to maximise its own fitness, usually at the expense of competitors [115]. Thus, to understand parasite evolution, it is necessary to investigate the strategies of individual genotypes. This is particularly important if co-infecting genotypes are expected to behave differently to each other, for instance if they differ in competitive ability, or if strain-specific immune responses operate [116,117,118]. How a genotype orchestrates collective action is unknown but several pathways of parasite-parasite communication have been proposed [53,81,86,99,119] (Section 4).
Predictably plastic parasites	Adaptive phenotypic plasticity (APP) is the ability of a genotype to alter aspects of its phenotype and allows organisms to alter phenotype faster than evolutionary time scales would allow. Whilst the host provides parasites with an environment that is homeostatically controlled, the availability of resources (e.g. RBC), immune defences, and drug exposure can vary considerably throughout infections and between hosts, exposing parasites to a wide range of circumstances in a short period of time. Parasites that can plastically adjust traits, both according to the kind of host they find themselves in and as infections progress, have a fitness advantage over genotypes with inflexible (“fixed”) strategies [17,25,120] (Sections 2 and 3). However, plastic strategies come with the cost of having to maintain sensory mechanisms and mount responses, the risk of errors being made, and the possibility that the range of possible responses is more limited than if phenotypes are fixed [121]. Malaria parasite populations experience variation in multiplicity and host responses across transmission settings. Infections in low transmission settings may be less dynamic than in high transmission settings, resulting in a sufficiently stable within-host environment that parasites may longer need to pay the costs of plasticity and adopt fixed strategies. APP evolves when: environmental variation is frequently encountered and predictable; organisms can assess environmental change with reasonable accuracy; and the costs of environmental sensing are outweighed by the benefits of adjusting traits. If unpredictable environmental variation frequently occurs, a “bet-hedging” strategy may be better than APP [122,123]. Bet-hedging occurs when an individual produces diverse forms (usually offspring) that are suited to different types of environmental conditions. Only the forms that happen to be well matched to the conditions they encounter will thrive. Whereas the loss of unsuited forms decreases short-term fitness, the avoidance of extinction through ensuring some forms will maximise long-term fitness. Var gene switching in malaria is assumed to be a bet-hedging strategy [124]. However, bet-hedging is unlikely to apply to sex ratio or conversion rate because parasites adjust these traits in consistent and directional manners in response to the same change of circumstances (i.e. plasticity is repeatable rather than diversifying) [14,16,17].
The footprints of hosts and parasites on phenotypes	Parasite trait values reflect the combined impacts of parasite strategic decisions and the direct impact (by-product) of host conditions on the development and survival of asexuals, males, and females. For example, variation in sex ratio or conversion rate could result from the host causing selective death of certain stages and/or constraining the parasites’ ability to express an altered phenotype through resource limitation. Separating the relative contributions of host and parasite to infection dynamics matters because this reveals whose genes are under selection via the phenotypes they produce. For example, P. chabaudi produces consistent, genotype-specific patterns in sex ratio and conversion rate when measured in common garden conditions [14,17,78]. This suggests that genetic variation for these traits exists and is heritable (including via epigenetic modifications), which is a pre-requisite for parasite genes to be at least in part, in control of parasite phenotypes. To conclude that, e.g. parasites have increased conversion rate, it is necessary to exclude that survival of asexuals has decreased (and/or survival of gametocytes has increased) due to confounding changes in host immune responses [78]. Similarly, whether changes in the respective mortality rates of males and females masquerades as sex ratio adjustment must be discounted. Mathematical models can resolve these issues by estimating the stage- or sex-specific mortality rates required to fully explain observed changes in traits, and evaluating whether such rates are realistic ways for infections to operate. Experimental perturbations that induce a parasite response in the absence of environmental change (i.e. tricking parasites [52] or forcing phenotypes via conditional control of Ap2-G [71]) can also reveal the extent parasites control their phenotypes. Finally, the explanatory power of theories with a rich history in evolutionary ecology can also be leveraged: evaluating data against a priori predictions with a solid mathematical foundation is more compelling than generating post-hoc explanations for observations that invoke parasite adaptation.
From models to humans?	To test whether evolutionary theories for reproductive strategies apply to parasites, it is irrelevant whether parasites of humans or animals are investigated. However, to inform a medically, socially, and economically important disease such as malaria, it is necessary to verify whether evolutionary theories apply to human parasites. Details of parasite reproductive strategies are likely to vary across species [104], but if the same basic principles apply to lab models and diverse multicellular taxa, the assumption is that they also apply to human infecting Plasmodium spp. Simply observing that human parasite phenotypes correlate as predicted with variation in e.g. multiplicity of infection, is limited to revealing whether data are consistent with theory rather than testing its predictions. Elevating correlation to causation requires experimental perturbations of specific factors, demonstrating that parasites respond as expected, and that their responses return greater fitness than no response or alternative strategies. Testing how parasites respond to relevant environmental variation requires examining parasites in ecologically realistic settings. Clearly, this is more easily achieved using in vivo systems because the complexity of life inside a vertebrate host is captured. However, in vitro approaches do allow the environment to be tightly controlled to e.g. parse-out parasite and host contributions to parasite phenotypes and refine which cues parasites respond to. Natural infections of humans are defined by evolutionary and ecological realism but experimental possibilities are limited. Experiments are possible with natural infections of wild animals, including passerine birds [19,125] and lizards [43], and modest experiments might be compatible with human challenge models [126,127]. However, these systems are more challenging to work with than lab animal models, which therefore tend to be a more tractable “go-to” system for undertaking proof of principle studies.