How exploratory and selective developmental mechanisms generate facilitated variation and influence evolution, with a focus on the evolution of adaptive immunity

Phenotypic plasticity is often conceptualized in two contrasting ways: as reaction norms that map specific environmental cues to phenotypic changes (a highly deterministic notion of plasticity) and more variable and experience-dependent forms of plasticity such as behavioural and cognitive adaptation (a.k.a. ‘exploratory mechanisms’). In practice, many plastic traits are likely to incorporate both inherited components that produce structured responses to environmental cues and flexible components that rely on environmental feedback, and these components of plasticity will frequently interact. For instance, innate immunity biases clonal selection in vertebrate adaptive immunity to ensure that common microbial threats are rapidly targeted and suppressed. Despite functioning in concert, innate and feedback-dependent forms of plasticity significantly differ in their inheritance and their impact on fitness, leaving their joint evolution subject to non-trivial trade-offs. Here I use a computational approach to investigate the evolution of forms of plasticity based on the iterative generation of superfluous phenotypic variation and subsequent selective reinforcement using environmental feedback. After a brief introduction to the topic (chapter 1) and discussion of a framework of assumptions (chapter 2), I go on to focus on the idea that variation and selection, when occurring within the lifetime of an organism rather than at the scale of generations and populations, may facilitate the evolution of an inherited bias in the variability of phenotypes (chapter 3). In chapter 4, I explore this idea further in the context of negative frequency-dependent selection. Finally, in chapter 5 and 6 I confine my analysis to adaptive immunity, conceptualised as a form of within-lifetime variation and selection. I find support for the hypothesis that adaptive immunity could have evolved to facilitate interactions with a diversity of beneficial and threatening microbes across co-evolutionary timescales. Together, this work illustrates how variability in the formation and reinforcement of phenotypes within lifetimes may influence long-term evolution.