Question

The Red Queen hypothesis is considered to be a special example of balancing selection. Which kind of balancing selection below do you think that is?

(a) overdominance

(b) spatially or temporally variable selection

(c) negative frequency-dependent selection

Given your answer for above, what would we expect in terms of long-term evolution of genotypes in a snail population? Briefly describe the pattern we would observe across many generations.

Answer 1

(c) negative frequency-dependent selection

Over long periods of evolutionary time, we would expect each of the genotypes to cycle
between periods of being common and rare, and we would expect for there to be a high level of genetic diversity at genes encoding parasite resistance. It is important to note that this cycling of genotypes is very different from an arms race, which might be expected between predator and prey. There is no net increase in resistance over time, but rather a cycling of previous used strategies.

Changes in genotypic composition of clonal host populations in experimental populations evolving either with or without parasites for six generations. The Red Queen model of coevolution, the initially most common host genotype decreased in frequency in the presence, but not the absence, of parasitism. The host genotype became more susceptible to infection by the coevolving parasite population. These results are consistent with parasite‐meditated selection leading to a rare advantage, and they indicate rapid coevolution at the genotypic level between a host and its parasite.

Coevolving parasites select for rare host genotypes, and thus preserve genetic polymorphism, both within and between populations. Specifically, the Red Queen model of coevolution posits that rare host genotypes will have an advantage as parasites target the common host genotype(s) in a population. Assuming that some degree of genetic matching is required for infection, obligate parasites will be under strong natural selection to infect common host genotypes. If infection reduces host fitness, these common host genotypes should decrease in frequency over time and be replaced by previously rare and uninfected host genotypes. This change in the host population will then result in selection on the parasite population to infect newly common host genotypes; but a time lag is expected before these newly common host genotypes become infected.