Sex: deleterious mutations
"This is what I usually refer to as the engine-and-gearbox model. The notion is that if you buy two clapped out motor cars - one with an engine and no gearbox, the other with a gearbox and no engine - you can make one functional motor car out of them.
The notion of the effect of sex on mutational damage is that sex by recombination does enable a population to generate some viable individuals from ones that have bad mutations."
John Maynard Smith