Curiouser and Curiouser
The Red Queen Hypothesis and Its Link to Host-Parasite Coevolution
by Srija Sengupta
The Red Queen hypothesis states that a species must exist in a constant state of adaptation and evolution in order to survive. This hypothesis is linked to host-parasite relationships: in parasitic relationships, sexual reproduction is favored over asexual reproduction.
Key Words: evolution, Red Queen hypothesis, symbiotic relationships, sexual reproduction, asexual reproduction
For many of us, Alice in Wonderland exists as a lovely, if vaguely trippy, childhood memory—full of mad riddles and even madder characters. It’s so nonsensical that it seems entirely removed from the realm of science and logic. And yet, one hypothesis in evolutionary biology bears the name of one of Lewis Carroll’s famous characters: the Red Queen hypothesis, which states that a species must constantly adapt and evolve in order to survive when faced with an opposing species.
The Red Queen hypothesis was first named by Leigh Van Valen in 1973 after a quote from Carroll’s book Through the Looking Glass1. In the story, the Red Queen tells Alice “it takes all the running you can do, to keep in the same place”. According to Van Valen, a species must evolve in order to survive- “run… to keep in the same place”2. Opposing species are constantly co-evolving in order to stay existing, whether that is predator-prey or host-parasite. A host-parasite relationship is a symbiotic relationship where one half benefits at the cost of the other half.
How is the Red Queen hypothesis linked to host-parasite coevolution? To get to the bottom of this rabbit hole, we need to get comfortable with a couple ideas.
Parasites and their hosts evolve with each other, and diversity in one half of the relationship leads to an increased adaptation rate in both halves. We can best see this in one study where researchers exposed a bacterial host to different parasite groups. There were five different types of parasites, and each individual host was exposed anywhere from groups of one to five parasite types. Researchers observed the hosts and parasites as they reproduced and saw a rather interesting trend. In later generations, hosts that were exposed to more diverse parasite groups were more resistant to parasites than hosts that were exposed to less diverse groups. Parasites in more diverse groups also had a higher average infectivity than those in less diverse groups3.
What happened here? The higher diversity meant that hosts had a stronger opposition, so their rate of adaptation went up. This in turn affected the parasites, which also had an increased rate of adaptation. In order to survive, both species had to “run,” much like the White Rabbit when he realizes he’s late.
Another key idea is that some hosts and parasites can reproduce both asexually and sexually, and they favor one over the other based on certain conditions. In most conditions, a species that can reproduce both ways will usually reproduce asexually; this is because asexual reproduction is more efficient than sexual reproduction and passes on genes without mixing them. Sexual reproduction breaks up genetic combinations, and it takes longer to produce a new generation, which is why it is only favored during times of stress4. In sexual reproduction, genes will mix, and so new combinations of genotypes and phenotypes will form. This increases genetic diversity and the chance of gaining a beneficial combination. Just like how Alice needs some food to grow and some to shrink, a species needs to reproduce asexually when times are good and sexually when times are stressful.
And now we’ve come to the end of the rabbit hole. The Red Queen hypothesis predicts that, in host-parasite relationships, sexual reproduction is favored. This is because a host-parasite relationship is a stressful situation, and sexual reproduction is preferred in times of stress. Therefore, host-parasite relationships will favor sexual reproduction as per the Red Queen hypothesis. This isn’t just us going mad; there’s evidence for this hypothesis! In one study, scientists followed a species of snails, or hosts, that reproduce both sexually and asexually, as they were infected by parasites. In later generations, snails that reproduced sexually were less infected than snails that reproduced asexually, so there is a clear advantage to sexual reproduction being favored5.
The Red Queen hypothesis, while oddly named, is extremely important to our understanding of evolutionary biology. For one, it can be used to explain why sex is favored some times over asexual reproduction; for another, it can explain some of the patterns we see in host-parasite relationships. Just like how the original Red Queen is Alice’s antagonist whom she must outsmart to return home, the titular hypothesis emphasizes the importance of constant adaptation and evolution to ensure survival when pitted against an opposing species.
1. Betts, A., Gray, C., Zelek, M., Maclean, R. C., & King, K. C. High parasite diversity accelerates host adaptation and diversification. Science, 360, 907-911. doi:10.1126/science.aam9974. (2018).
2. Carroll, L. & Tenniel, J. Alice’s adventures in Wonderland & Through the looking glass. (Schocken Books, 1978).
3. Daniela Vergara, J. J. C. M. L. Infection Dynamics in Coexisting Sexual and Asexual Host Populations: Support for the Red Queen Hypothesis*. The American Naturalist, 184(S1), S22. https://doi-org.proxy-remote.galib.uga.edu/10.1086/676886 (2014).
4. Herreid, C. F. Why Sex is Good. National Center for Case Study Teaching in Science. (n.d.).
5. Van Valen, L. A New Evolutionary Law. Evol. Theory 1. 1-30. (1973).