One of the greatest challenges for evolutionary biology is explaining the widespread occurrence of sexual reproduction, and the associated process of genetic recombination (Williams, 1975; Maynard Smith, 1978; Bell, 1982; Stearns, 1987; Michod & Levin, 1988; Hurst & Peck, 1996). Asexual females can potentially produce twice as many daughters as sexual females, so that the ratio of asexual to se...