  Sex in higher diploids carries a two-fold cost of males that should reduce
its fitness relative to cloning and result in its extinction. Instead, sex is
widespread and it is clonal species that face early obsolescence. One possible
reason is that sex is an adaptation to resist ubiquitous parasites, which
evolve rapidly and potentially antagonistically. We use a heuristic approach to
model mutation-selection in finite populations where a parasitic haploid mounts
a negative frequency-dependent attack on a diploid host. The host evolves
reflexively to reduce parasitic load. Both host and parasite populations
generate novel alleles by mutation and have access to large allele spaces. Sex
outcompetes cloning by two overlapping mechanisms. First, sexual diploids adopt
advantageous homozygous mutations more rapidly than clonal diploids under
conditions of lag load. This rate advantage can offset the lesser fecundity of
sex. Second, a relative advantage to sex emerges under host mutation rates that
are fast enough to retain fitness in a rapidly mutating parasite environment
and increase host polymorphism. Clonal polymorphic populations
disproportionately experience interference with selection at high mutation
rates, both between and within loci. This slows clonal population adaptation to
a changing parasite environment and reduces clonal population fitness relative
to sex. The interference increases markedly with the number of loci under
independent selection. Rates of parasite mutation exist that not only allow sex
to survive despite the two-fold cost of males but which enable sexual and
clonal populations to have equal fitness and co-exist. Since all higher
organisms carry parasitic loads, the model is of general applicability.
