Kondrashov, among others, has tried to show that sex reduces the effects of deleterious mutations. Slightly deleterious mutations arise each generation. Natural selection will work against the deleterious mutations, but does sex make any difference?

Maynard Smith describes the theory by analogy. Suppose you have two cars. One car has a faulty clutch, the other has faulty brakes. What do you do? One obvious way to deal with the problem is to swap the good clutch from the car with bad brakes with the faulty clutch from the car with good brakes. Now you have one good car with working clutch and brakes, and one pretty useless car.

In the same way, the theory goes, recombination may concentrate deleterious mutations into certain individuals, leaving others free from mutation.

Given the high cost of sex, this is advantageous only if

• the mutation rate is high enough. A female gains the advantage whatever the deleterious mutation rate, but the advantage increases with mutation rate, and

• an individual with two mutations is significantly worse off than an individual with only one. In other words, mutations have combined deleterious effects on an organism's fitness.

When is sex advantageous?