Welcome to the Ashby Group at the University of Bath! We are based in the Department of Mathematical Sciences, but we are also affiliated with the Milner Centre for Evolution.

The goal of our research is to better understand the ecology and evolution of hosts and their parasites (“parasite” is taken in the broadest sense of the word to include bacteria and viruses as well as helminths and ticks). We’re especially interested in coevolution, which is a process of reciprocal adaptations by hosts to defend themselves against parasites, and counter-adaptations by parasites to overcome or avoid host defences.

We use mathematical models and simulations to understand how parasites and the infectious diseases they cause evolve, for example to infect a broader or narrower range of hosts, or to become more or less virulent, and in turn, how hosts evolve to defend themselves through traits such as resistance, tolerance, and mate choice. We also collaborate with some fantastic experimentalists to combine theoretical and empirical research. Read more about our research below, or view our publications.

1. How do environmental and genetic factors affect host-parasite coevolution?

We’re interested in how environmental (e.g. spatial structure, fitness costs) and genetic (e.g. specificity, epistasis) factors affect the coevolution of traits such as resistance and infectivity. Using relatively simple eco-evolutionary models, we can study how these factors lead to qualitatively and quantitatively different outcomes, such as optimal strategies, fluctuating selection, arms races, and diversification (simulations below).


Examples of different types of coevolutionary dynamics: optimal strategy (left); fluctuating selection/ cycling (middle); diversification leading to polymorphism (right)

Using a variety of mathematical and numerical techniques, we’ve shown how:


Individual-based simulation of coevolution in a spatially-structured population (black=susceptible, green=low resistance, blue=high resistance, white=empty). From Ashby et al. (2014) Am Nat.

Return to top

2. How do parasites influence the evolution of host reproductive strategies?

The second key focus of our research seeks to understand the role that parasites play in the evolution of sex and mating strategies, and in turn, the role that host mating strategies play in the epidemiology and evolution of sexually transmitted infections.

Why does sex exist?

Sex is something of an evolutionary enigma. Surely it would be much more efficient to reproduce asexually, especially when we consider that males do not bear their own offspring? This so-called “twofold cost of sex” was first identified by Maynard Smith in 1978 and has remained a problem for evolutionary biologists ever since. A prominent theory for the near-ubiquity of sex is that it allows populations to rapidly adapt in the presence of a constantly changing antagonist, such as a parasite or predator. By shuffling their genes with another individual, sexual females are able to produce diverse offspring which are more likely to resist contemporaneous antagonists than the offspring of an asexual female. Thus, antagonistic co-evolution could help to explain the predominance of sex among eukaryotes (this is commonly known as the Red Queen Hypothesis, named after the Red Queen in Lewis Carroll’s Through the Looking-Glass).

“Now, here, you see, it takes all the running you can do, to keep in the same place” – The Red Queen to Alice, from Lewis Carroll’s Through the Looking-Glass. This is commonly used as an analogy for coevolution, as hosts and parasites have to rapidly adapt to each other.

Our research into the evolution of sex has shown that ecological dynamics play a really important role in mediating selection for sex.


Selection for sex should peak at intermediate levels of host diversity due to ecological feedbacks. At low diversity, the sexual population is unable to avoid disease much better than clonal individuals. At high diversity, the sexual population is very good at resisting parasites, but this reduces disease prevalence to low levels and allows faster-growing clonal lineages to invade. From Ashby & King (2015, J Evol Biol).

How do parasites affect mating behaviour, and vice versa?

Parasites are not only thought to play an important role in why sex exists, but also in the evolution of mating behaviour and secondary sex traits (Hamilton and Zuk 1982, Science). This will be especially true for sexually transmitted infections (STIs), as they are inherently closely linked to host reproduction.

Our research has shown that:


Coevolutionary cycling of host mate choosiness (blue) and the virulence of a sexually transmitted infection (red). From Ashby & Boots (2015) PNAS.

Return to top

3. How do hosts and parasites (co)evolve in complex communities?

Much of our understanding of host and parasite evolution is based on theoretical and empirical work on two-species systems. But in reality hosts and parasites exist in complex communities interacting with a wider range of other species. How do these interactions affect how hosts and parasites (co)evolve?

Working with Kayla King at the University of Oxford, we have shown how microbes can evolve along the parasitism mutualism continuum to provide context dependence against other parasites, in a “the enemy of my enemy is my friend” scenario (see Ben’s article in The Conversation about this phenomenon!) We have also shown both theoretically and experimentally (using the nematode C. elegans and two bacterial species) how the host can coevolve with a protective microbe to form a mutualistic relationship.

Screenshot 2019-06-24 at 16.00.55

From the host’s point of view (top), is the enemy (left) of my enemy (red) my friend?

Return to top


I am currently funded by a NERC Independent Research Fellowship at the University of Bath.