The operation of the Ratchet represents a serious threat to the integrity of germ lines which have no means of exogenous repair. In many circumstances this threat cannot be parried, and long-continued asexual propagation will eventually end in decline and extinction. The fate of isolate cultures is the vera causa of this process. This leads us to interpret sex as being maintained by most organisms – all those which do not have enormous populations or very small genomes – because of its role in containing the otherwise inexorable increase of mutational load.

The sort of evolutionary process envisaged by this argument is, roughly speaking, one of group selection. Competition between two reproductively isolated populations, one sexual and the other asexual, will be decided in favour of the sexual population, because the performance of its asexual competitor will gradually decay. It is probably necessary that they should be isolated: if asexual individuals occasionally produce reduced eggs which are fertilized by sexual males, then a gene which codes for almost completely obligate parthenogenesis will have an opportunity to become associated with lightly-loaded genomes. This is the force of Shield's (1982) arguments for the importance of partial inbreeding. I have described above how effective rather low rates of recombination will be in populations of moderate size. One example of such a process would be species selection: we are unlikely to observe asexuality, at least among relatively large organisms, because most obligately asexual species persist only for geologically brief spans of time. However, a similar process can operate even within an outcrossing sexual population, provided that genes coding for recombination are recessive to alleles which suppress recombination.