Sex is thought to facilitate accumulation of initially rare beneficial muta
tions by allowing simultaneous allele replacements at many loci. However, t
his advantage of sex depends on a restrictive assumption that the fitness o
f a genotype is determined by fitness potential, a single intermediate vari
able to which all loci contribute additively, so that new alleles can accum
ulate in any order. Individual-based simulations of sexual and asexual popu
lations reveal that under generic selection, sex often retards adaptive evo
lution. When new alleles are beneficial only if they accumulate in a prescr
ibed order, a sexual population may evolve two or more times slower than an
asexual population because only asexual reproduction allows some overlap o
f successive allele replacements. Many other fitness surfaces lead to an ev
en greater disadvantage of sex. Thus, either sex exists in spite of its imp
act on the rate of adaptive allele replacements, or natural fitness surface
s have rather specific properties, at least at the scale of intrapopulation
genetic variability.