Coevolution is a process of reciprocal adaptations by two or more species. This process can be mutualistic (e.g., when plants and pollinators become better adapted to one another and both benefit), or antagonistic (e.g., when hosts adapt defences against parasites, and parasites counter-adapt to overcome host defences). Coevolution plays a crucial role in the natural world, leading to lots of interesting biological phenomena. It can help us to explain why certain traits have evolved, from immune responses to virulence (and even the peacock’s tail!), to patterns of biodiversity and local adaptation, and even the evolution of sex.

One of the reasons that coevolution is so interesting is because it can lead to all sorts of interesting dynamics, including “arms races” and “fluctuating selection”, as illustrated in the graphics on this page. We use mathematical models to study coevolution, and the fascinating dynamics that it produces!

Arms races

Coevolutionary “arms races” occur when there is directional selection for increased host defences against parasites and greater parasite counter-adaptations to overcome those defences. This is similar to arms races that can occur between countries (e.g. the USA and the Soviet Union during the Cold War). An example of an arms race between hosts and parasites is illustrated in the image below (click on the image to enlarge).

Suppose that a host is initially susceptible to infection by a particular parasite (stage 1). If resistance is not too “costly” to the host (e.g. it does not reduce the host’s growth rate or amount of offspring it produces), selection will favour the evolution of host resistance to infection. Parasites may now struggle to infect their hosts (stage 2), leading to selection for more infective parasites that can overcome host defences (stage 3). This process can repeat, with hosts evolving increasing resistance to infection (stage 4), and parasites becoming increasingly more infective (stage 5).

Click the image below to enlarge it.

Coevolutionary arms race
Coevolutionary arms race

​Fluctuating selection

Unlike arms races, where the direction of selection does not change (i.e. always increasing), fluctuating selection occurs when the direction of selection changes. This is sometimes also called Red Queen Dynamics (after the Red Queen from Lewis Carroll’s Through the Looking Glass) or coevolutionary cycling. Two types of fluctuating selection are illustrated below.

The first type of fluctuating selection is driven by fitness costs. It starts off similar to an arms race: a host that is initially susceptible to infection (stage 1) may experience selection for resistance (stage 2), followed by selection for greater infectivity among parasites to overcome host resistance (stage 3). However, it may be too costly for the host to keep increasing resistance (e.g. it may have to divert limited resources from other functions, like reproduction), and so it may lose resistance that is ineffective (stage 4). If being more infective also carries a cost for the parasite (e.g. producing enzymes that are no longer necessary to overcome host defences), then selection may favour less infectious parasites (stage 1). We are now back at the beginning, and the cycle can repeat indefinitely.

The second type of fluctuating selection is driven by negative frequency-dependent selection. It occurs when the parasite must match its host to infect it. You can think of the parasite as having a key, which can only open a matching lock on host cells. If the key doesn’t fit, the parasite can’t infect the host.

Suppose two types of locks and keys are possible, A and B, and that Type A hosts and parasites are initially abundant in the population (stage 1). Since Type A parasites can only infect Type A hosts, Type B hosts will become more common in the population (stage 2). Now there is a mismatch, with mostly Type B hosts and Type A parasites, which will select for Type B parasites to become more common (stage 3). Like stage 1, the most common host type (Type B) is now at a disadvantage because it can be infected by the most common parasite type (also Type B), and so there will be selection for the rare host type (Type A). Once again, there is a mismatch (stage 4), with Type A hosts and Type B parasites, which will select for Type A parasites to become more common (stage 1). We are now back at the beginning, and the cycle can repeat indefinitely.