Can group selection explain sexual reproduction?
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,
