A while back, I made a post about parasite-host coevolution.
Here’s a cartoon, because my posts are too often filled with text.
Anyway, to continue on the topic. In the last article about coevolution, I covered the idea of host and parasite coevolution leading to continual and reciprocal evolutionary changes – the hosts and parasites are constantly running, but they stay in the same place relative to each other. The (excellent) cartoon makes sense now, right?
In the article about coevolution, I also touched on the issue of multiple parasites infecting multiple hosts, and how this can complicate host-parasite coevolution. Here, I’m going to further complicate matters by talking a little about environmental heterogeneity and other species that interact with host-parasite associations.
Harrison et al. (2013) dealt with the issue of environmental heterogeneity. They conducted an evolutionary experiment with bacteria and their viral parasites, varying the bacterial habitat over time. They found that varying the habitat prevents coevolutionary dynamics between the bacteria and their viral parasite.
Conversely, Friman and Buckling (2013) experimented with bacteria, their viral parasites, and protist predators of the bacteria. These authors found that the bacteria tend to coevolve to either the viruses or the predator, not both. The presence of the bacterial predator also constrained the coevolution between the bacterial and virus.
So where does this leave us? It seems that when we consider only single host-single parasite systems, we may find the treadmill coevolutionary pattern seen in the cartoon above, but the likelihood of this situation breaks down when we consider other factors like habitat heterogeneity and multiple natural enemies (i.e., predators and parasites). However, one could reasonably argue that the discussed experiments with bacteria are not relevant for all organisms, and systems in which parasites and hosts do engage in treadmill (aka arms race) coevolution could exist. Whether such coevolution does or does not occur, though, will likely be system specific.
Friman, V. P. and A. Buckling (2013). Effects of predation on real-time host-parasite coevolutionary dynamics. Ecology Letters 16(1): 39-46.
Harrison, E., A. L. Laine, M. Hietala and M. A. Brockhurst (2013). Rapidly fluctuating environments constrain coevolutionary arms races by impeding selective sweeps. Proceedings of the Royal Society B-Biological Sciences 280(1764).