  The prevalence of sexual reproduction ("sex") in eukaryotes is an enigma of
evolutionary biology. Sex increases genetic variation only tells its long-term
superiority in essence. The accumulation of harmful mutations causes an
immediate and ubiquitous pressure for organisms. Contrary to the common sense,
our theoretical model suggests that reproductive rate can influence
initiatively the accumulation of harmful mutations. The interaction of
reproductive rate and the integrated harm of mutations causes a critical
reproductive rate R*. A population will become irreversibly extinct once the
reproductive rate reduces to lower than R*. A sexual population has a R* lower
than 1 and an asexual population has a R* higher than 1. The mean reproductive
rate of a population reached to the carrying capacity has to reduce to 1. That
explains the widespread sex as well as the persistence of facultative and
asexual organisms. Computer simulations support significantly our conclusion.
