COMMENTARY Continued hope for a general explanation

Some natural phenomena may defy explanations in terms of single causes. The genetic basis for Haldane's Rule, for example, depends in part on whether the rule manifests itself in terms of hybrid inviability or sterility & varies among taxa (Orr, 1997). West et al. suggest that the widespread occurrence of sexual reproduction sim-ilarly requires multiple explanations. Currently, neither models of mutation accumulation nor those of changing environments, notably the Red Queen, can on their own fully account for the maintenance of sexual reproduction given the two-fold advantage of asexuality. The motiva-tion for West et al.'s argument makes sense: creatures typically suffer from parasites as well as from deleterious mutations and sexual reproduction may be the best way to ®ght their combined attack. This may be especiall