41 Testing the Red Queen Hypothesis
The Red Queen hypothesis—that sex evolved to combat our coevolving pathogens—can be tested by analyzing a few key predictions of this hypothesis:
- Sex is most beneficial where there is a high risk of infection
- Pathogens are more likely to attack common phenotypes (for example, clones) in a population, as opposed to the less-common counterparts (such as those that resulted from sex)
- In sexually reproducing populations, individuals choose mates that maximize diversity in their offspring
Note that all of these predictions implicitly rely on the heritability of being healthy (in this case, the ability to combat pathogens); specifically, parents must be able to pass along to their offspring genes for avoiding pathogens. Testing these predictions has resulted in several lines of evidence supporting the Red Queen hypothesis.
Prediction 1: Sex is most beneficial where there is a high risk of infection
An excellent system for testing this prediction involves a flatworm parasite in the genus Microphallus, a duck, and a small mud snail (Potamopyrgus antipodarum; Figure 7.6). This species of snail is able to reproduce sexually or asexually. The extent of sexual reproduction in a population of snails can be quantified by counting the number of males—asexual snails are all female.
The flatworm’s life cycle begins inside of the snail, where the worm emerges from its egg. Infected snails are consumed by ducks. Once in the duck’s intestine, adult worms have sex and produce eggs. Flatworm eggs are released, with duck feces, into the water, where they are ingested by snails and the cycle continues (Figure 2). Snails are harmed by this flatworm, largely because a symptom of infection is sterilization (the flatworm’s scientific name, Microphallus, translates to “small penis”).
Observations of this system in two New Zealand lakes (Alexandrina and Kaniere) revealed that snails are more likely to be sexual (measured by frequency of males) in shallow waters, where ducks feed, than in deeper waters, where ducks do not feed (Figure 3).
These results suggest that coevolutionary pressure is greater on the snails in the shallows, presumably because the feeding ducks effectively “close the circle” on the worm’s life cycle. Finally, higher infection rates in the shallows indicate that, in support of Prediction 1, above, sex is most beneficial where there is a high risk of infection.
Prediction 2: Pathogens are more likely to attack common phenotypes in a population, as opposed to the less-common counterparts
In the Mexican desert there are isolated pools inhabited by a species of minnow. Within these pools, populations of asexually reproducing individuals exist alongside sexually reproducing individuals. Fish in these ponds exhibit “black spot disease,” which is caused by a parasitic flatworm. Investigators have observed the frequency of sexual and asexual fish and the number of black spots in each type of fish in these ponds. Clonal fis