What is the cost of sex? Put simply, it is the cost of males.

Consider two populations: one is asexual, and one is fully sexual. Both have a stable sex ratio of 1:1. For simplicity, suppose that each female produces two offspring. In the asexual case, the population will double every generation, since each member of the population produces two offspring. In the sexual case, the population will remain constant, since half of all offspring are males, and only contribute to the next generation by fertilizing the females.

The clone of offspring from an asexual female multiplies at twice the rate of the progeny descended from a sexual female, and a sexual female has only 50% of the fitness of an asexual female. This has been described by Maynard Smith as the two-fold cost of sex.

Fifty per cent is a large cost. The problem of explaining sex is to find a compensating advantage of sexual reproduction that is large enough to make up for its cost. The problem is not trivial. Indeed, G.C. Williams has described it as 'the outstanding puzzle in evolutionary biology'. Several possible advantages of sex have been suggested.