Many bacteria, viruses and other parasites cause severe morbidity or mortality in their host populations, creating strong selection for physiological or behavioural mechanisms to avoid disease. Likewise, changes in host susceptibility and contact patterns can dramatically influence the spread of infectious diseases, and hence selection for traits such as virulence and infectivity range in parasites. Understanding how ecological and evolutionary changes in one population affect selection in another represents a key challenge for theoreticians and empiricists alike, and is essential for gaining further insights into host-parasite relationships. This thesis contains theoretical models that explore how genetic and environmental factors shape the evolutionary and coevolutionary dynamics of hosts and parasites. In particular, the roles of genetic specificity (i.e. genotype-by-genotype interactions) and population mixing patterns are investigated, using both mathematical models and computer simulations. A broad range of scenarios are covered, including the coevolution of broad resistance and infectivity ranges (generalism), the persistence of coevolutionary cycling and the maintenance of sex, the effects of mating behaviour on disease prevalence and evolution, and the evolution of sexual and social behaviour. The models presented herein develop our understanding of host-parasite relationships and highlight the importance of genetic interactions and ecological feedbacks.

Proponents of moral particularism have often sought support for their negative claim about moral principles in a doctrine called holism in the theory of reasons. According to holism, a feature that is a reason in one case may be no reason at all, or even the opposite reason, in another. The aim of this thesis is to investigate and elucidate the supposed connection between holism and particularism. This is done by considering the particularistic position embraced by Jonathan Dancy in his book Ethics without Principles and the arguments against the argument from holism recently put forth by Sean McKeever and Michael Ridge in their book Principled Ethics: generalism as a regulative ideal. In conclusion it is argued that holism does provide at least some support for the form of particularism which Dancy defends.

This thesis will revisit the debate between moral particularists and moral generalists in the field of meta-ethics. The general aim of this project will be to come to a better understanding of the status and role of moral principles in a reasons-holistic moral landscape. The specific aim will be to develop a viable position within the particularism-generalism debate that will combine elements from both theories. My central argument will be threefold: (a) I will argue that, in a reasons-holistic moral landscape, exceptionless moral principles are not sufficient to ground the possibility of moral thought and judgement; (b) that the possibility of moral thought and judgement depends in part on a determinate set of exceptionless moral principles and an indeterminate set of defeasible moral principles; and (c) that moral principles are insufficient to codify all or most moral truths in finite and manageable terms. My position therefore is a modified version of Principled Particularism. In this thesis, despite defending a version of particularism, I will not shy away from employing generalist terminology or from accepting certain generalist assumptions. Furthermore, unlike some particularists, I will stress the necessity and utility of moral principles. This thesis will also incorporate research about moral thought and judgement from the fields of moral psychology, cognitive science, and neuroscience.