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From Guy Cox, University of Sydney

Christopher Wills displays a very anthropocentric (or at least mammalocentric) view when he writes: “The numerical disadvantage that sexual organisms have in competition with equivalent asexual ones is known as the twofold cost of sex.” (6 December, p 44).

There is no such cost. There is a twofold cost of being dioecious but that is not the same thing at all. In the case of the simplest organisms two cells fuse in sexual congress and the two divisions of meiosis then give four offspring – exactly the same number as if each parent cell had divided asexually.

Once we become multicellular it gets a bit more complex, but so long as we are hermaphrodite (like most plants and many animals, including the humble earthworm) sexual reproduction has no “twofold cost”.

The real question is why we are not hermaphrodite. It is a puzzle, especially considering how many of our sexual organs are duplicated (ovaries, fallopian tubes, testes). Why not have one of each instead of two the same?

Sydney, Australia