So, let’s talk about coevolution. Broadly, coevolution refers to the situation in which two (or more) interacting species evolve reciprocally, when an adaptation in one species selects for a reciprocal adaptation in the other (Brockhurst et al., 2013). One can often find such a situation in parasite-host systems. For example, in bacterial (host) and viral (parasite) systems, when bacteria evolve a new mechanism of preventing viral infection, the viruses evolve a mechanism to defeat that new defense. While coevolution often occurs for parasites and their hosts, it also occurs in other systems, such as herbivore-plant, predator-prey, and mutualistic relationships. Regardless of the specifics of the scenario, coevolution can only occur where two or more species interact.
Because of this requirement for interaction, parasites and their hosts – which interact very closely – often undergo coevolution. Generally, this evolution is antagonistic, meaning a host adaptation negatively affects the parasite, and vice versa. Such reciprocal evolution, where the host and parasite are continually trying to “one up” each other, often results in one or both of two evolutionary scenarios: Fluctuating Selection Dynamics (FSD) and Arms Race Dynamics (ARD) (Brockhurst et al., 2013). Fluctuating Selection Dynamics occurs when parasites select against (by most easily infecting) hosts with the most common resistance genotypes. In this situation, rare host resistance types become favored because the parasites evolved to counter the common resistance type, which enables the rare host to spread in the population. But as the rare host becomes more common, the parasites adapt to it and the formerly common host resistance type – which is now rare – becomes beneficial again, and the process repeats. In Arms Race Dynamics, on the other hand, parasites and hosts undergo recurring evolution of new infective and resistance genes, as opposed to the oscillation of rare-common genes in FSD. Interestingly, while one gene in a host or parasite can undergo FSD, another can experience ARD. It is unclear how this can occur, or what can drive a gene from switching from FSD to ARD and vice versa (which also happens). Insight into these processes would not only further our general understanding of evolution, but could also aid human health research (e.g., the ongoing coevolution among malaria, humans, and mosquitos), so if you have any thoughts, I encourage you to pursue them – with the tenacity of a parasite searching for its host (which would be a lot…).
Now that ideas of parasite-host coevolutionary dynamics, i.e., all that good stuff about FSD and ARD, have parasitized your mind, let’s talk about some potential complications for your infection. Current understanding of host-parasite coevolution generally comes from single host-single parasite systems, but “in the wild” hosts are often infected with many parasite species, and parasites often infect many host species. Because of these added interactions, I believe that coevolution in natural ecosystems is probably more complex than the coevolution between one host and one parasite would indicate. However, even that statement can be qualified. Parasites that infect multiple host species often have one or a few host species that maintain the parasite population, which are called reservoir species. The parasites may infect other host species, but without the reservoirs, the parasites would go extinct. Because of this concentration, though the parasite infects multiple host species, coevolution may occur mostly between the parasite and one or a few host species. Thus, our one parasite-one host system may be fairly realistic. On the host side of things, hosts usually have multiple parasite species, which may or may not have different prevalences within the host by parasite species. If all the parasite species are uniformly represented within the host population, or if they all cause similar levels of damage to host survival and reproduction, it could be difficult for coevolution to occur because the host would be facing so many different evolutionary pressures (one from each parasite), which are likely to be contrary (one parasite pushes the host one way, but another pushes the host the opposite way, so the host cannot evolve to either). Although, the situation in which all parasites are equally present or cause equal harm to the host is highly unlikely, so a host could experience more pressure to coevolve with parasite A than with parasite B (either because parasite A is more common or harms the host much more than B). Furthermore, parasites could have similar modes of parasitism within the host, meaning a host could evolve in one direction for multiple parasites. Such complications – regarding both parasite and host perspectives – make the study of multiple parasite-multiple host coevolution incredibly complicated. But they also make the topic all the more interesting and open to questions. A parasite may be manipulating me to think that though…
1. Brockhurst, M.A. and B. Koskella, Experimental coevolution of species interactions. Trends Ecol Evol, 2013. 28(6): p. 367-75.