Sex and the Red Queen

There are benefits of sexual reproduction. In 1932, Muller along with Fisher (1930) proposed that sex might purge deleterious mutations, and increase fitness over time. The idea was further expanded upon by Kondrashov (1988). This process has been shown in a homothallic Ascomycete, Aspergillus nidulans, where sexual and asexual lineages have very low level of genetic differences (Bruggeman et al. 2003). Mutations accumulate in both sexual and asexual lineages, but the sexual lineage suffers less fitness decrease over time grown under the same conditions. Another benefit of sex is thought to arise from the evolutionary arms race with competitors, pathogens and predators. This idea was originally proposed by Leigh van Valen (1973) and termed the Red Queen hypothesis, after Lewis Carroll’s "Through the Looking Glass." The Red Queen says to Alice, “it takes all the running you can do to stay in the same place,” meaning in an evolutionary context that for each novel beneficial mutation a species acquires, a competitor or pathogen will acquire a mutation to counter it, and so on, ad infinitum. Sexual reproduction can bring together genetic material from two different sources with unique successful mutations. Recombination potentially leads to novel resistance to a pathogen in a rare individual. Work by Lively, Craddock and Vrijenhoek (1990) demonstrates that populations of Mexican poecillid fish can reproduce sexually and asexually. In pools where there is predominantly asexual reproduction, the fish suffer more infection by parasites. In pools where sexual reproduction is the dominant mode, the population switches between two main genotypes, providing a “moving target” to which the parasite continuously adapts. The sexual population has greater fitness overall, but cannot repopulate pools as rapidly, and hence the maintenance of asexuality. Sex is though to benefit organisms because it increases the rate at which selection can fix beneficial mutations within the population because recombination over time breaks linkage between genes (Maynard Smith 1958, 1993; Fisher 1937). The ability to tolerate or adapt to changing environments, pathogens, predators and competitors is necessary for a species to continue to exist.