Theories of sex

Why is sexual reproduction so important? The key advantage is thought to be the way it leads to the shuffling of genes, increasing genetic variability. Oddly, though, it is not totally clear why this is so important.

Natural selection underlies the main theories of sexual reproduction. Although in the short term asexual reproduction might be favoured, over longer periods sexual reproduction has a distinct advantage.

Why might this be so? An early model, known as Muller's ratchet, imagined a mutation-free population reproducing asexually. As individuals multiplied, mutations would gradually accumulate in a gene until eventually none would be left with the original 'pure' gene; it would have disappeared forever. The ratchet would have moved along one notch.

But selective pressures could keep a gene free of harmful mutations. So an alternative model, Kondrashov's hatchet, suggested that the accumulation of lots of mutations was important. At a certain point these would become lethal – and down would come the hatchet.

Perhaps the strongest theory is that sex helps species to evolve in response to a constantly changing environment or to pathogens. There is a constant 'arms race' between host and intruder, as each tries to gain the upper hand.

This idea is often called the Red Queen hypothesis, from Lewis Carroll's book 'Through the Looking Glass', where the Red Queen says: "It takes all the running you can do to keep in the same place".