On page 133 we discussed some of the evidence supporting the Red Queen hypothesis (the idea that parasites drive the evolution of sex by becoming more and more adept at infecting asexual clones) from New Zealand water snails studied by Curtis Lively. This remarkable work has continued. The researchers are looking at a population where both sexual and asexual types coexist, and they have used genetic techniques over a number of years to study the changing make-up of the population. This gives us one of the most complete windows we have ever had on the evolutionary dynamics of sexual reproduction competing with asexual reproduction in an otherwise similar organism (Jokela et al. 2009). They show that particular lineages of asexual snails don’t last very long in evolutionary terms. That is, lineages that were very common in 1994 had often gone extinct by 2003, and the lineages which were common in 2003 had been rare in 1994. This seems to be because the parasites are better at infecting asexual lineages which have been common for a while. Thus, a complex evolutionary dynamic is going on in these lakes. Rare asexuals can multiply rapidly, and become quite common (this is because, you will recall, that the proliferation rate for an asexual reproducer is twice that for a sexual). However, as they become common, parasites begin to evolve to infect them, and they then decline, making way for the rapid rise of other lineages. However, in addition, a more parasite-resistant sexual population is also present, expanding less rapidly but less prone to disease. Presumably, in our ancestors this battle was definitively won by the sexual, but in this snail, it is finely poised, and both sexual and asexual persist.

Jokela, J. Et al. (2009). The maintenance of sex, clonal dynamics and host-parasite coevolution in a mixed population of sexual and asexual snails. American Naturalist 174: S43-S53.