Explanations of the evolution of diploidy have focused on the advantag
es gained from masking deleterious alleles. Recent theory has shown, h
owever, that masking does not always provide an advantage to diploidy
and would never favor diploidy in predominantly asexual organisms. We
explore a neglected alternative theory which posits that, by doubling
the genome size, diploids double the rate at which favorable mutations
arise. Consequently, the rate of adaptation in diploids is presumed t
o be faster than in haploids. The rate of adaptation, however, depends
not only on the rate of appearance of new favorable mutations but als
o on the rate at which these mutations are incorporated (which depends
on the population size and on the dominance of favorable mutations).
We show that, in both asexuals and sexuals, doubling the mutation rate
via diploidy often does not accelerate the rate of adaptation. Indeed
, under many conditions, diploidy slows adaptation.