The problem of sex

In sexual reproduction, a new organism is formed by the fusion of two gametes. It is to be contrasted with asexual reproduction, in which females produce offspring without any male contribution; the female's gametes develop directly into female offspring.

• In sexual reproduction, the sex cells - eggs and sperm or pollen - are produced by a reduction division and the gametic fusion restores the original chromosome complement.

• In asexual reproduction, there is either no reduction division (apomixis) or the product cells of one individual's reduction division fuse together again (automixis). Asexual reproduction is rare, but occurs in species such as dandelions (pictured opposite).

It is easy for a mutation to convert meiosis into mitosis, and thus a sexually reproducing into an asexually reproducing female.

If the loss of sex would be relatively easy, why is sex maintained?

Although some species do reproduce asexually, the majority are sexual. So there is probably a selective advantage to sex in most species.

This advantage, moreover, must be large, because it has to overcome an automatic two-fold cost of sex, relative to asexual reproduction.