Is a little bit of sex as good as a lot?

Few of the costs and benefits that have been suggested for sex can be measured quantitatively. One cost which can be measured is the cost of producing males (or the “50% loss due to meiosis”). Some animals avoid much of this cost by reproducing asexually most of the time, but occasionally reproducing sexually. Others avoid the cost through regular self-fertilization, occasionally interrupted by outcrossing. Models show that these savings may be had with little loss in benefits. In particular, for some models which show that evolution is faster with sexual than with asexual reproduction, the rate of evolution is nearly as fast when only a fraction of reproduction is sexual and the rest is asexual. We describe two such models in detail. One is the segregation model of Kirkpatrick & Jenkins (1989a), which was modified to apply to facultative sexuality by Hedrick & Whittam (1989). The other is the recombination model of Eshel & Feldman (1970), which we modify to apply to facultative sexuality. We give some examples that illustrate a variety of ways in which animals may reduce the effort devoted to producing males, or to sexual reproduction. While some examples are clear and well known, others are less clear and deserve further study.