Why Sex? And why only in Pairs?
We thank Elizabeth Elle, Alan Grafen, Sally Otto, the late Rob Seymour, Jasna Strus and Dan Weil for very helpful comments and discussions, Dagan Eshar for expert Matlab advice, Hua Jiang for programming assistance, and the editor and an anonymous referee for very helpful suggestions. We gratefully acknowledge financial support as follows: Perry, from the Israel Science Foundation (0321548) and from the ESRC (ES/K006347/1); Reny, from the National Science Foundation (SES-1227506); and Robson, from the Canada Research Chairs Program, the Human Evolutionary Studies Program, and the Social Sciences and Humanities Research Council of Canada.
Understanding the purpose of sex is a fundamental unresolved problem in evolutionary biology. The difficulty is not that there are too few theories of sex, the difficulty is that there are too many and none stand out. To distinguish between theories, we ask: Why are there no triparental species with offspring composed of the genetic material of three individuals? A successful theory should confer an advantage to biparental sex over asexual reproduction without conferring an even greater advantage to triparental sex. Of two leading theories (red queen and mutational), we show that only one is successful in this sense.