Complementation, genetic conflict, and the evolution of sex and recombination.

The existence of sexual reproduction is difficult to explain because the 2-fold cost of meiosis requires a compensatory 2-fold advantage that is difficult to prove. Here, I show that asexual reproduction has a short-term disadvantage due to the loss of complementation of recessive deleterious mutations, which can overcome the 2-fold cost of meiosis in one or few generations. This complementation hypothesis can also explain why most asexual species are polyploid, why only certain types of asexual reproduction exist, why meiosis is not one-step, and the origin of amphimixis. I also show that the promotion of variation by recombination is not necessary to explain the evolution of amphimixis. Instead, recombination can be the result of an intragenomic conflict between alleles that induce the initiation of crossing over and alleles that evolve to resist that initiation. Thus recombination does not require any advantage at the individual or population level.