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, us . If the value of us varies between lineages, then one of the effects of group selection is to favor groups (i.e., species) with the lowest values of us . This occurs because lines that do convert to asexuality (because of a high us ) 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 us ) 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. © 1989 The Society for the Study of Evolution.