Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Master of Science


Applied Mathematics

Collaborative Specialization

Scientific Computing


Wild, Geoff


We investigate how natural selection shapes the coevolution of parasitism. We discuss the antagonism fuelled by parasites’ necessity to transmit to novel hosts, and host’s desire to minimise virulence. In support, we build a mathematical model which considers the epidemiology and life-history trade-offs faced by an obligate microparasite and its host. Our model allows parasites to be transmitted to new hosts via direct contact (horizontally) or from parent to offspring during birth (vertically). We test the hypothesis that vertical transmission causes virulence to diminish in the long run, and contrary to widely accepted views, find that vertical transmission need not result in benign coevolutionary outcomes in general. However, vertical transmission does promotes benign parasitism whenever: it is cheap for the host to retaliate; horizontal transmission saturates quickly; or the intrinsic growth rate of the host population is low.

Summary for Lay Audience

Parasites might seem like the villains, but they are not always necessarily out to get us. Often the damage that they do is a consequence of trying to complete their lifecycle, instead of pure malice. In this thesis we will explore how hosts and parasites coexist, despite their conflicting interests. And how, after many generations, their descendants might have changed to adapt to one another. We use a mathematical model which lets us keep a track of how many hosts are infected, and how many are not. This information is useful for predicting which way natural selection will push the relationship. It is commonly thought that if a parasite can transmit from a mother to her offspring, then the host and parasite will share mutual interests and de-escalate their attack on each other. However, in this paper we will show that the situation is more complicated, and depends on the compromises that both parties face, as well as what each has to gain.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.