THE MAINTENANCE OF SEX BY GROUP SELECTION

The traditional group-selection model for the maintenance of sex is based upon the assumption that the long-term evolutionary benefits of sexual reproduction result in asexual lineages having a higher extinction rate than sexual species. This model is reexamined, as is a related model that incorporates the possibility that sexual and asexual lines differ in their speciation rates. In these models, the long-term advantage of sex is opposed by a strong short-term disadvantage arising from the twofold reproductive cost of producing males. It is shown that once some sexual lines become established, then group selection can act to maintain sex despite its short-term disadvantage. The short-term disadvantage is included in the model by assuming that, if asexual individuals arise by mutation within a previously completely sexual species, then the asexuals quickly displace their sexual conspecifics and the species is transformed to asexuality. The probability of this event is given by the transition rate, u_s . If the value of u_s varies between lineages, then one of the effects of group selection is to favor groups (i.e., species) with the lowest values of u_s . This occurs because lines that do convert to asexuality (because of a high u_s ) are doomed to a high rate of extinction, and in the long term only those that do not convert to asexuality (because of a low u_s ) survive. The net result of group selection is that sex is maintained because of its lower extinction rate (or higher speciation rate) and because asexual mutants only rarely arise.