Hybrid theories of sex

West et al. (1999) propose that we should examine the situations when more than one mechanism works to protect a sexual population from invasion by asexual clones. They think this pluralistic view is better than trying to choose only one theory for the advantage of sex and drop the others. In particular, they look for the joint action of theories considering mutation accumulation (the stochastic Muller's ratchet, and the mutational deterministic hypothesis of Kondrashov) and the effect of parasites (the Red Queen). As every species must ®ght against both parasites and deleterious mutations, it is a realistic situation. Note that, taken separately, all the hypotheses have dif®culties with assigning a two-fold advantage to sex in the short run irrespective of population size. If two mechanisms act simultaneously, one does not need to assume extremely high mutation rates or severe effects and extreme transmission probabilities of parasites. West et al. argue that different mechanisms may interact not only simultaneously but synergistically, i.e. the combined effect of the two mechanisms is greater than the sum of their contributions. We would like to comment on this aspect of the pluralistic view. We think there are three main reasons why this synergism can exist: (i) the action of one mechanism supports the key assumption of the other, so the latter can work better; (ii) one mechanism slows down the spread of the clone in the short run so that the other has more time to render it a disadvantageous strategy; (iii) the selective forces assumed in the two mechanisms work against each other in the clone while in the sexual population they act independently, hence the clone cannot climb onto the adaptive peak. We comment below on some (maybe not all) possibilities for these types of synergistic interaction between theories.