A synthesis of coevolution across levels of biological organization

How do parasites and predators choose their victim? A trade-off between quality and vulnerability across antagonistic interactions

From blood-sucking lice and food-stealing gulls to pandemic-inducing viruses and egg-eating snakes: parasites and predators are ubiquitous in shaping ecology and evolution. Fundamental to these interactions is the way in which parasites and predators choose their victim. Here, I argue that a trade-off between host quality and vulnerability can be generalised across systems to understand parasites' choice of hosts. This principle defines quality as the value of resources a host has, and vulnerability as the ease with which a parasite can obtain those resources. A parasite can choose a low-quality host, which is easier to attack but offers limited resources, or a high-quality host, which is more challenging to attack but offers more resources if the parasite is successful. The optimal decision for a parasite will depend on its ecology and the shape of the trade-off in a given system. The trade-off applies equally to predator-prey systems. Many studies of different types of parasitism and predation across taxa have investigated traits pertaining to quality or vulnerability, but their findings have not previously been integrated. Doing so makes it possible to draw out broad principles that determine whether quality or vulnerability has the greater impact on victim choice. It can also help explain contradictory findings, such as why the same antagonists choose high-quality victims in some studies, and low-quality victims in others. Further applications include predicting the effects of global change on host-parasite and predator-prey dynamics, and providing an integrated perspective on coevolutionary adaptations.

Behavioural vs. physiological adaptation: which contributes more to the evolution of complex traits in a warming climate?

Through behavioural adaptation, organisms can alter their environment and consequently, their exposure to selective pressures. In contrast, physiological traits adapt by accommodating environmental influences. Here, we examine how the coevolution of behavioural and physiological traits is shaped by their different relationships to the environment by modelling the adaptation of species with temperature-dependent sex determination to climate change. In these species, pivotal temperature and maternal nesting behaviour can evolve in response to rising temperatures that destabilize sex ratios. We used individual-based simulation modelling to ascertain the relative response to selection of these traits and determine how temperature-dependent embryonic survival and behavioural plasticity influence their coevolution. We found that pivotal temperature evolved to ameliorate sex ratio bias more readily than nesting behaviour, though behaviour played an important role in adaptation to extreme environments. Selection favoured increased behavioural evolution when embryonic survival depended on nest temperature, while plasticity reduced the adaptive potential of behaviour. We demonstrate that the capacity of behavioural traits to respond to multiple selective pressures has a substantial impact on the coevolution of behavioural and physiological traits. Our findings highlight the complex interactions that occur when species adapt to new environments and the potential for plasticity to shape the course of evolution.

The multidimensional spectrum of eco-evolutionary relationships between sharks and remoras

Remoras are a highly specialised group of fishes known to associate with a range of marine megafauna, including elasmobranchs, cetaceans and marine reptiles. Remoras appear to benefit from these interspecific interactions through consumption of host dermal parasites or reduced cost of transport. Shark-remora associations are widely documented, yet our understanding of the costs and benefits involved in these interactions is poor. Studies frequently make claims about mutualistic, commensalistic or parasitic relationships without providing the necessary quantitative information necessary to make these claims. Here I explain why this approach is problematic, and proceed to examine shark-remora interactions in a rigorous eco-evolutionary framework. These interactions cannot be properly classified without considering net evolutionary fitness and context dependence. In reality, shark-remora interactions are best defined by a multidimensional spectrum of fitness consequences, with net fitness outcomes shifting between mutualism and parasitism (and vice versa) through space and time.