The balance argument

Williams has put forward a general argument to cast doubt upon the importance of group selection in the maintenance of sex. It is his balance argument.

Some species, such as strawberry plants (pictured opposite), aphids, sponges, rotifers, and water fleas (Cladocera), can reproduce both sexually or asexually according to the conditions. These species are called heterogonic. Many heterogonic species time their sexual reproduction for periods of environmental uncertainty, and reproduce asexually when conditions are more stable; but that is not the important point here. What matters is that an individual can reproduce in either way. Therefore, when an aphid reproduces sexually, it must be advantageous to the individual, because if it was not the aphid could have reproduced asexually. Both sexual and asexual reproduction must have 'balanced' advantages to maintain them in the species' life cycle: otherwise the inferior one would be lost.

The group selectionist, recall, believes that sex is disadvantageous to the individual, and only advantageous to the group. But in aphids and other heterogonic species in which individuals have a 'choice', sex must have an individual advantage. The argument can be extended. If sex is advantageous in aphids, it is probably also advantageous to the individual in non-heterogonic species too. We have no good reason to think that sex is exceptional in aphids, or that special factors favor sex in heterogonic species. If we must find an individual advantage for sex in aphids, that same advantage will probably also exist in other species. If group selection can be ruled out for aphids, it can probably also be ruled out for other species.

Williams's argument is powerful, but not decisive. In most heterogonic species, the asexual and sexual propagules differ in other respects besides being asexual and sexual. For example, the cladoceran sexual offspring form special winter eggs which are adapted for winter survival. Any cladoceran that gave up sex would also lose its over-wintering stage: in practice, the loss of sex while retaining the winter egg would need two mutations, one for the loss of sex and the other for transferring the winter egg phenotype to asexual eggs. So the balance argument is not perfectly clear-cut.